Mips implementation

University of Ulster at Jordanstown
University of Applied Sciences, Augsburg

Master of Engineering
VLSI Design Project Report

Processor
Implementation
in VHDL
According to Computer Organisation & Design
by David A. Patterson and John L. Hennessy
Author(s):

M. Linder
M. Schmid

Supervisor(s): J. Färber
A. Eder

Submitted: 06/07/07

Document Revision History, Designers
Department of Electrical Engineering

Document Revision History
Rev.

Date

Author

Description

0.1

15/05/2007

M. Schmid

First draft release

0.2

15/05/2007

M. Linder

Features of the project

0.3

29/05/2007

M. Linder

Target Spec. (2.1, 2.2)

0.4

10/06/2007

M. Linder

Target Spec. (2.3)

0.5

30/06/2007

M. Linder

- include jump instruction to Target Spec.

0.6

02/07/2007

M. Linder

Module Spec. of Data

0.6.1

02/07/2007

M. Schmid

Module Spec. of ALU and Memory

0.6.2

03/07/2007

M. Schmid

Design Tasks

0.7

04/07/2007

M. Linder

- Module Spec. of Datapath

- Module Spec. of Control

- Synthesis Results
- References
0.8

05/07/2007

M. Linder, M. Schmid

- Synthesis Results
- Source Code
- Conclusion

1.0

05/07/2007


Final release

Designer(s)
M. Linder

michael-linder@web.de

M. Schmid

martin-werner.schmid@gmx.de

Contact
Michael Linder
Angerstraße 8a
86356 Neusäß, Germany
Phone: +49 (0) 176 22 93 58 30
Mail: michael-linder@web.de

Martin Schmid
Fichtenstraße 2
86500 Kutzenhausen, Germany
Phone: +49 (0) 160 92 94 91 54
Mail: martin-werner.schmid@gmx.de


II

Contents

Contents
1

Introduction................................................................................ 1
1.1
1.2

Using Multicycle Implementations.........................................................2

1.3

2

Starting from a Simple Implementation Scheme...................................1
Enhancing Performance with Pipelining................................................2

Target Specification................................................................... 3
2.1

Building a Datapath............................................................................... 3

2.1.1

Major Components.................................................................................... 3

2.1.2

Components for Arithmetic and Logic Functions....................................... 4

2.1.3

Load word (lw) and store word (sw) instructions........................................ 5

2.1.4

Branch on equal instruction....................................................................... 6

2.1.5

Jump Instruction........................................................................................ 6

2.2

Simple Implementation Scheme............................................................7

2.2.1

Creating a Single Datapath....................................................................... 7

2.2.2

ALU Control............................................................................................... 8

2.2.3

Main Control.............................................................................................. 9

2.2.4

Disadvantages of a Single-Cycle Implementation................................... 10

2.3

Multicycle Implementation...................................................................11

2.3.1

Additions and Changes in the Scheme.................................................... 11

2.3.2

Execution of Instructions in Clock Cycles................................................ 14

2.3.3

Defining the Control by a Finite State Machine........................................ 18

3

Design Tasks............................................................................ 21

4

Module Specification............................................................... 22
4.1

ALU......................................................................................................22

4.1.1

Functional Description............................................................................. 22

4.1.2

Block Diagram......................................................................................... 23

4.1.3

Simulation Results................................................................................... 26

4.1.4

Design Files............................................................................................. 26

4.2

Memory................................................................................................27

4.2.1


4.2.2

Block Diagram......................................................................................... 28

4.2.3


4.2.4

Design Files............................................................................................. 29


III

Contents

4.3

Control..................................................................................................30

4.3.1


4.3.2

State Diagram......................................................................................... 31

4.3.3

Block Diagram......................................................................................... 32

4.3.4


4.3.5

Design Files............................................................................................. 33

4.4

Data Path.............................................................................................34

4.4.1

Instruction Fetch...................................................................................... 34

4.4.1.1

Functional Description.................................................................................... 34

4.4.1.2

Block Diagram................................................................................................ 34

4.4.1.3

Design Files.................................................................................................... 35

4.4.2

Instruction Decode.................................................................................. 35

4.4.2.1


4.4.2.2

Block Diagram................................................................................................ 35

4.4.2.3

Design Files.................................................................................................... 36

4.4.3

Execution................................................................................................. 36

4.4.3.1


4.4.3.2

Block Diagram................................................................................................ 37

4.4.3.3

Design Files.................................................................................................... 38

4.4.4

Memory Writeback.................................................................................. 39

4.4.4.1


4.4.4.2

Block Diagram................................................................................................ 40

4.4.4.3

Design Files.................................................................................................... 41

4.4.5

Data Path................................................................................................ 42

4.4.5.1

Block Diagram................................................................................................ 42

4.4.5.2

Design Files.................................................................................................... 42

4.5

Processor and Memroy........................................................................43

4.5.1


4.5.2

Block Diagram......................................................................................... 43

4.5.3

Design Files............................................................................................. 44

5

Synthesis Results.................................................................... 45

6

Results of Prototype Testing.................................................. 47
6.1
6.2

7

Description...........................................................................................47
Simulation Result.................................................................................49

Conclusion................................................................................ 50
7.1

Our own experiences...........................................................................50


IV

Contents

7.2
7.3

8

Annotations to “Computer Organization & Design” [PaHe98].............50
Further work on the project..................................................................51

Appendix................................................................................... 52
8.1

Design files..........................................................................................52

8.1.1

Project Entities........................................................................................ 52

8.1.2

Project Architectures............................................................................... 58

8.1.3

Package.................................................................................................. 79

8.1.4

Testbenches............................................................................................ 80

8.2

References...........................................................................................91


V

Contents

List of Figures
Figure 1.1: Simple block diagram with datapaths [PaHe98] p. 352....................... 1
Figure 1.2: Multicycle Datapath [PaHe98] p. 414...................................................2
Figure 1.3: Pipelined Version of the Datapath [PaHe98], p. 452........................... 2
Figure 2.1: Instruction Memory, Program Counter and Adder [PaHe98], p 344....3
Figure 2.2: Datapath for fetching instructions and incrementing the PC
[PaHe98] p. 345......................................................................................................3
Figure 2.3: Register and ALU [PaHe98] p. 346......................................................4
Figure 2.4: Datapath for R-type Instructions [PaHe98] p. 347...............................4
Figure 2.5: Data Memory and Sign extension unit [PaHe98] p. 348......................5
Figure 2.6: Load or Store Word instruction field.....................................................5
Figure 2.7: Datapath for Load Word and Store Word [PaHe98] p. 348.................5
Figure 2.8: Datapath for a branch instruction [PaHe98] p. 350..............................6
Figure 2.9: Completed Simple Datapath [PaHe98] p. 353.....................................7
Figure 2.10: MIPS field...........................................................................................8
Figure 2.11: Table for ALU Control.........................................................................8
Figure 2.12: Datapath with ALU Control Unit [PaHe98] p. 358..............................9
Figure 2.13: Meaning of the main control signals [PaHe98] p. 359....................... 9
Figure 2.14: The simple datapath with the control unit [PaHe98] p. 360.............10
Figure 2.15: Truth table of the main control unit [PaHe98] p. 361.......................10
Figure 2.16: Abstract view of a multicycle desing [PaHe98] p. 378..................... 11
Figure 2.17: Complete Datapath for multicycle design [PaHe98] p. 383............. 13
Figure 2.18: Actions of 1-bit control signals [PaHe98] p. 384..............................14
Figure 2.19: Actions of 2-bit control signals [PaHe98] p. 384..............................14
Figure 2.20: Summary of the multicycle steps [PaHe98] p. 389..........................18
Figure 2.21: Complete finite state machine control [PaHe98] p. 396.................. 19
Figure 2.22: Setting of Control Signals.................................................................20
Figure 4.1: ALU 1/3...............................................................................................23
Figure 4.2: ALU 2/3...............................................................................................24
Figure 4.3: ALU 3/3...............................................................................................25
Figure 4.4: Simulation Results of ALU..................................................................26
Figure 4.5: Memory...............................................................................................28

VI

Contents

Figure 4.6: Simulation Results of Memory (registered outputs)...........................28
Figure 4.7: Simulation Results of Memory (unregistered outputs).......................29
Figure 4.8: Control Finite State Machine..............................................................31
Figure 4.9: Control FSM........................................................................................32
Figure 4.10: ALU Control......................................................................................32
Figure 4.11: Control..............................................................................................33
Figure 4.12: Simulation Results of the Control FSM............................................33
Figure 4.13: Instruction Fetch...............................................................................34
Figure 4.14: Instruction Decode............................................................................35
Figure 4.15: Execution..........................................................................................37
Figure 4.16: Memory Writeback........................................................................... 40
Figure 4.17: Processing Unit (Datapath & Controlpath).......................................43
Figure 4.18: Processing Unit & Memory...............................................................43
Figure 5.1: Analysis & Synthesis Summary..........................................................45
Figure 5.2: Analysis & Synthesis Settings............................................................46
Figure 5.3: Compilation History............................................................................ 46
Figure 6.1: Simulation Results of MIPS and Memory.......................................... 49


VII

Contents

List of VHDL-Source
VHDLSource 8.1: e_control_ControlFSM.vhd......................................................52
VHDLSource 8.2: e_control_ALUControl.vhd...................................................... 52
VHDLSource 8.3: e_control.vhd...........................................................................52
VHDLSource 8.4: e_tempreg.vhd.........................................................................53
VHDLSource 8.5: e_pc.vhd..................................................................................53
VHDLSource 8.6: e_instreg.vhd...........................................................................53
VHDLSource 8.7: e_regfile.vhd............................................................................54
VHDLSource 8.8: e_alu_vhd................................................................................54
VHDLSource 8.9: e_data_fetch.vhd.....................................................................54
VHDLSource 8.10: e_data_decode.vhd...............................................................55
VHDLSource 8.11: e_data_execution.vhd...........................................................55
VHDLSource 8.12: e_data_memwriteback.vhd................................................... 56
VHDLSource 8.13: e_data.vhd.............................................................................56
VHDLSource 8.14: e_ram.vhd..............................................................................56
VHDLSource 8.15: e_memory.vhd.......................................................................57
VHDLSource 8.16: e_mips.vhd............................................................................57
VHDLSource 8.17: e_procmem.vhd.....................................................................57
VHDLSource 8.18: a_control_ControlFSM.vhd....................................................60
VHDLSource 8.19: a_control_ALUControl.vhd.................................................... 61
VHDLSource 8.20: a_control.vhd.........................................................................62
VHDLSource 8.21: a_tempreg_behave.vhd.........................................................63
VHDLSource 8.22: a_pc_behave.vhd..................................................................63
VHDLSource 8.23: a_instreg_behave.vhd...........................................................64
VHDLSource 8.24: a_regfile_behave.vhd............................................................64
VHDLSource 8.25: a_alu_behave.vhd.................................................................65
VHDLSource 8.26: a_data_fetch.vhd...................................................................67
VHDLSource 8.27: a_data_decode.vhd...............................................................69
VHDLSource 8.28: a_data_execution.vhd...........................................................69
VHDLSource 8.29: a_data_memwriteback.vhd................................................... 70
VHDLSource 8.30: a_data.vhd.............................................................................73
VHDLSource 8.31: a_ram_rtl.vhd.........................................................................73

VIII

Contents

VHDLSource 8.32: a_memory_behave.vhd.........................................................75
VHDLSource 8.33: a_mips.vhd............................................................................77
VHDLSource 8.34: a_procmem.vhd.....................................................................78
VHDLSource 8.35: p_procmem_definitions.vhd.................................................. 79
VHDLSource 8.36: t_alu_fileio.vhd.......................................................................83
VHDLSource 8.37: t_memory.vhd........................................................................86
VHDLSource 8.38: t_procmem.vhd......................................................................87


IX

1 Introduction

1 Introduction
“The performance of software systems is dramatically affected by how well software designers understand the basic hardware technologies at work in a system.” According to the book “Computer Organization & Design” written by David
A. Patterson and John L. Hennessy the hardware and behaviour of a microprocessor is implemented in VHDL.

1.1 Starting from a Simple Implementation Scheme
In the first section starting from a simple implementation scheme of a MIPS subset the basic hardware of the microcontroller´s datapath and its control is developed step by step and implemented in VHDL. Testbenches will verify the correct
implementation of the arithmetic-logical instructions (add, sub, and, or and slt),
the memory-reference instructions (load word and store word) and the branch instructions (beq and jump).

Figure 1.1: Simple block diagram with datapaths [PaHe98] p. 352

M. Linder, M. Schmid, 07/07

1

1.2 Using Multicycle Implementations

1.2 Using Multicycle Implementations

Figure 1.2: Multicycle Datapath [PaHe98] p. 414
Establishing that the efficiency of a long single-cycle implementation is not likely
to be very good the processor´s speed is improved by using multicycle implementations. Then, instructions are allowed to take different numbers of clock cycles and functional units can be shared within the execution of single instructions.

1.3 Enhancing Performance with Pipelining
In order to enhance the performance and to get very fast processors another implementation technique called pipelining is introduced. Multiple instructions are
overlapped in execution so that some stages are working in parallel.

Figure 1.3: Pipelined Version of the Datapath [PaHe98], p. 452
2


2 Target Specification

2 Target Specification
2.1 Building a Datapath
2.1.1 Major Components
At first we look at the elements required to execute the MIPS instructions and
their connection.
The first element needed is a place to store the program instructions. This Instruction Memory is used to hold and supply instructions given an address.
The address must be kept in the Program Counter (PC), and in order to increment the PC to the address of the next instruction, we also need an Adder.
All these elements are shown in figure 2.1.

Figure 2.1: Instruction Memory, Program Counter and Adder [PaHe98], p 344

After fetching one instruction from the instruction memory, the program counter
has to be incremented so that it points to the address of the next instruction 4
bytes later.
This is realised by the datapath shown in figure 2.2.

Figure 2.2: Datapath for fetching instructions and incrementing the PC
[PaHe98] p. 345


3

2.1 Components for Arithmetic and Logic Functions

2.1.2 Components for Arithmetic and Logic Functions
The instructions we use all read two registers, perform an ALU operation and
write back the result.
These arithmetic-logical instructions are also called R-type instructions
([PaHe98] p. 154). This instruction class considers add, sub, slt, and and or.
The 32 registers of the processor are stored in a Register File. To read a dataword two inputs and two outputs are needed. The inputs are 5 bits wide and
specify the register number to be read, the outputs are 32 bits wide and carry the
value of the register.
To write the result back two inputs are needed: one to specify the register number and one to supply the data to be written. The Register is shown in Figure 2.3.

Figure 2.3: Register and ALU [PaHe98] p. 346
To process the data from the Register, an ALU with two data inputs is used.
Figure 2.4 shows the combination of Register and ALU to operate on R-type instructions.

Figure 2.4: Datapath for R-type Instructions [PaHe98] p. 347

4


2.1 Load word (lw) and store word (sw) instructions

2.1.3 Load word (lw) and store word (sw) instructions
Two more elements are needed to implement the sw- and lw-instructions: the
Data Memory and the Sign Extension Unit.

Figure 2.5: Data Memory and Sign extension unit [PaHe98] p. 348
The sw- and lw-instructions compute a memory address by adding a register value to the 16-bit signed offset field contained in the instruction.
Because the ALU has 32-bit values, the instruction offset field must be sign extended from 16 to 32 bits simply by concatenating the sign-bit 16 times to the
original value.
The instruction field for a lw- or sw-instruction is shown in figure 2.6:
op
6 bits

rs
5 bits

rt
5 bits

address
16 bits

Figure 2.6: Load or Store Word instruction field

Figure 2.7: Datapath for Load Word and Store Word [PaHe98] p. 348


5

2.1 Branch on equal instruction

2.1.4 Branch on equal instruction
The beq instruction has three operands, two registers that are compared for
equality, and a 16-bit offset used to compute the branch target address relative
to the branch instruction address.

Figure 2.8: Datapath for a branch instruction [PaHe98] p. 350
Figure 2.8 shows the datapath for a branch on equal instruction. This datapath
must do two operations: compare the register contents and compute the branch
target.
Therefore two things must be done: The address field of the branch instruction
must be sign extended from 16 bits to 32 bits and must be shifted left 2 bits so
that it is a word offset.
The branch target address is computed by adding the address of the next instruction (PC + 4) to the before computed offset.

2.1.5 Jump Instruction
The jump instruction is similar to the branch instruction, but computes the target
PC differently and not conditional.
The destination address for a jump is formed by concatenating the upper 4 bits
of the current PC + 4 to the 26-bit address field in the jump instruction (see figure
2.10 on page 8) and adding “00” as the last two bits.

6


2.2 Simple Implementation Scheme

2.2 Simple Implementation Scheme
The simplest possible implementation of the MISP Processor contains the datapath segments explained above added by the required control lines.
2.2.1 Creating a Single Datapath
The simplest datapath might attempt to execute all instructions in one clock cycle. This means that any element can be used only once per instruction. So
these elements have to be duplicated.
If possible datapath elements can be shared by different instruction flows. Therefore multiple connections to the input must be realised. This is commonly done
by a multiplexer.
Figure 2.9 shows the combined datapath including a memory of instructions and
one for data, the ALU, the PC-unit and the mentioned multiplexers.

Figure 2.9: Completed Simple Datapath [PaHe98] p. 353


7

2.2 ALU Control

2.2.2 ALU Control
The MIPS field that contains the information about the instruction has the following structure:
op
6 bits

rs
5 bits

rt
5 bits

rd
5 bits

shamt
5 bits

funct
6 bits

Desired
ALU action

ALU control
input

Figure 2.10: MIPS field
The meaning of the fields are:
•

op:

basic operation

•

rs:

first register source

•

rt:

second register source

•

rd:

register destination

•

shamt:

shift amount

•

funct:

function

Instruction
opcode

ALUOp

Instruction
operation

Funct field

LW

00

load word

XXXXXX

add

010

SW

00

store word

XXXXXX

add

010

Branch equal

01

branch equal

XXXXXX

subtract

110

R-type

10

add

100000

add

010

R-type

10

subtract

100010

subtract

110

R-type

10

AND

100100

and

000

R-type

10

OR

100101

or

001

R-type

10

set on less than

101010

set on less than

111

Figure 2.11: Table for ALU Control
Figure 2.11 shows in the last column the 3-bit ALU control input.
It depends on the 6-bit funct field of the MIPS instruction and the 2-bit ALUOp
signal generated form the Main Control Unit (see Chapter 2.2.3).
Figure 2.12 shows the datapath including the ALU Control Unit.

8


2.2 ALU Control

Figure 2.12: Datapath with ALU Control Unit [PaHe98] p. 358
2.2.3 Main Control
The main control unit generates the control bits for the multiplexers, the data
memory and the ALU control unit.
The input of the main control unit is the 6-bit op-field of the MIPS instruction field
(see figure 2.9 on page 7).
Figure 2.13 shows the meaning of the several control signals.
Signal name

Effect when deasserted

Effect when asserted

RegDst

The register destination number for the Write register comes from the rt field (bits 20-16).

The register destination number for the Write register comes from the rd field (bits 15-11).

RegWrite

None

The register on the Write register input is written
with the value on the Write data input.

ALUSrc

The second ALU operand comes from the second register file output (Read data 2).

The second ALU operand is the sign-extended,
lower 16 bits of the instruction.

PCSrc

The PC is replaced by the output of the adder
that computes the value of PC + 4.

The PC is replaced by the output of the adder
that computes the branch target.

MemRead

None

Data memory contents designated by the address input are put on the Read data output.

MemWrite

None

Data memory contents designated by the address input are replaced by the value on the Write data input.

MemtoReg

The value fed to the register Write data input comes from the ALU.

The value fed to the register Write data input comes from the data memory.

Figure 2.13: Meaning of the main control signals [PaHe98] p. 359


9

2.2 Main Control

The connection of the main control unit is shown in figure 2.14. This and the
meaning of the signals described in figure 2.13 leads directly to the truth table for
the main control unit shown in figure 2.15.

Figure 2.14: The simple datapath with the control unit [PaHe98] p. 360

RegDst

ALUSrc

MemtoReg

Reg
Write

Mem
Read

Mem
Write

Branch

ALUOp1

ALUOp2

1

0

0

1

0

0

0

1

0

lw

0

1

1

1

1

0

0

0

0

sw

X

1

X

0

0

1

0

0

0

beq

X

0

X

0

0

0

1

0

1

Instruction
R-format

Figure 2.15: Truth table of the main control unit [PaHe98] p. 361

2.2.4 Disadvantages of a Single-Cycle Implementation
In modern designs a single cycle implementation of a processor is not used, because it is inefficient.
A clock cycle must have the same length for every instruction and therefore it is
determined by the longest possible path. Almost this is the path of the load word
instruction which uses five functional units in series: the instruction memory, the
register file, the ALU, the data memory and the register file again.
However a single cycle implementation can be used for a small instruction set.
But if the machine gets more powerful there can be used thousands of functional
units and then the longest path causes the cycle time.
10


2.3 Multicycle Implementation

2.3 Multicycle Implementation
To avoid the disadvantages of the single cycle implementation described in the
section before, a multicycle implementation is used.
This technique divides each instruction into steps and each step is executed in
one clock cycle.
The multicycle implementation allows a functional unit to be used more than
once in a instruction, so that the number of functional units can be reduced.
The major advantage of a multicycle design is the ability to share functional units
within an execution.

2.3.1 Additions and Changes in the Scheme
Figure 2.16 shows a abstract design of a multicycle datapath.

Figure 2.16: Abstract view of a multicycle desing [PaHe98] p. 378
Comparing to the single-cycle datapath the differences are that only one memory
unit is used for instructions and data, there is only one ALU instead of an ALU
and two adders and several output registers are added to hold the output value
of a unit until it is used in a later clock cycle.
The instruction register (IR) and the memory data register (MDR) are added to
save the output of the memory. The registers A and B hold the register operands
read form the register file and the ALUOut holds the output of the ALU.
With exception of the IR all these registers hold data only between a pair of
adjacent clock cycles.


11

2.3 Additions and Changes in the Scheme

Because the IR holds the value during the whole time of the execution of a
instruction, it requires a write control signal.
The reduction from former three ALUs to one causes also the following changes
in the datapath:
An additional multiplexer is added for the first ALU input to choose between the
A register and the PC.
The multiplexer at the second ALU input is changed from a two-way to a fourway multiplexer. The two new inputs are a constant 4 to increment the PC and
the sign-extended and shifted offset field for the branch instruction.
In order to handle branches and jumps more additions in the datapath are
required.
The three cases of R-type instructions, branch instruction and jump instruction
cause three different values to be written into the PC:
•

The output of the ALU which is PC + 4 should be stored directly to the PC.

•

The register ALUOut after computing the branch target address.

•

The lower 26 bits of the IR shifted left by two and concatenated with the
upper 4 bits of the incremented PC, when the instruction is jump.

If the instruction is branch, the write signal for the PC is conditional. Only if the
the two compared registers are equal, the computed branch address has to be
written to the PC.
Therefore the PC needs two write signals, which are PCWrite if the write is
unconditional (value is PC + 4 or jump instruction) and PCWriteCond if the write
is conditional.

12


2.3 Additions and Changes in the Scheme

Figure 2.17 shows the completed datapath for a multicycle implementation
including the whole control.
It also shows that the write signal for the PC is combined form the ALU zero bit
and the two write signals PCWrite and PCWriteCond by an AND gate and OR
gate.

Figure 2.17: Complete Datapath for multicycle design [PaHe98] p. 383


13

2.3 Execution of Instructions in Clock Cycles

2.3.2 Execution of Instructions in Clock Cycles
The execution of an instruction is broken into clock cycles, that means that each
instruction is divided into a series of steps.
Therefore the setting of the control signals are shown in figures 2.18 and 2.19.

Signal name

Effect when deasserted

Effect when asserted

RegDst

The register file destination number for the Write
register comes from the rt field

The register file destination for the Write register
comes from the rd field

RegWrite

None

The general-purpose register selected by the Write register number is written with the value of the
Write data input.

ALUSrcA

The first ALU operand is the PC

The first ALU operand comes from the A register

MemRead

None

Content of memory at the location specified by the
Address input is put on Memory data output.

MemWrite

None

Memory contents at the location specified by the
Address input is replaced by value on Write data
input.

MemtoReg

The value fed to the register file Write data input
comes from ALUOut.

The value fed to the register file Write data input
comes from the MDR.

IorD

The PC is used to supply the address to the memory unit.

ALUOut is used to supply the address to the memory unit.

IRWrite

None

The output of the memory is written into the IR.

PCWrite

None

The PC is written; the source is controlled by
PCSource.

PCWriteCond

None

The PC is written if the Zero output from the ALU
is also active.

Figure 2.18: Actions of 1-bit control signals [PaHe98] p. 384
Signal name
ALUOp

Value

Effect

The ALU performs an subtract operation.

10

The function field of the instruction determines the ALU operation.

00

The second input to the ALU comes from the B register.

01

The second input to the ALU is the constant 4.

10

The second input to the ALU is the sign-extended, lower 16 bits of the IR.

11

The second input to the ALU is the sign-extended, lower 16 bits of the IR shifted left
2 bits.

00

Output of the ALU (PC + 4) is sent to the PC for writing.

01

The contents of ALUOut (the branch target address) are sent to the PC for writing.

10

PCSource

The ALU performs an add operation.

01

ALUSrcB

00

The jump target address (IR[25-0] shifted left 2 bits and concatenated with
PC +4[31-28]) is sent to the PC for writing.

Figure 2.19: Actions of 2-bit control signals [PaHe98] p. 384

14



The execution of an instruction is divided into maximal five steps.
Different elements of the datapath can work in parallel during one clock cycle,
whereas others can only be used in series.
So there must be sure, that after one step the values computed are stored either
in the memory or in one of the registers.
The operation steps are:
1. Instruction fetch step
Fetch the instruction from the memory and computed the address of the
sequential instruction:
IR = Memory[PC]
PC = PC + 4

Control signal setting:
MemRead = 1
IRWrite = 1
IorD = 0
ALUSrcA = 1
ALUSrcB = 01
ALUOp = 00
PCSource = 00
PCWrite = 1

2. Instruction decode and register fetch step
It is still unknown what the instruction is, so there can only be performed
actions that are applicable for all instructions or are not harmful.
The registers indicated by the rs and rd field of the instruction are read
and store into the A and B register, and the potential branch target is
computed and stored into the ALUOut register.
A = Reg[IR[25-21]]
B = Reg[IR[20-16]]
ALUOut = PC + (sign-extend (IR[15-0]) << 2)

ALUSrcA = 0
ALUSrcB = 11
ALUOp = 00


15


3. Execution, memory address computation or branch completion
In this step the instruction is known and the operation depends on what
the instruction is. One of these four functions is executed:
1. Memory reference:
ALUOut = A + sign-extend(IR[15-0])

ALUSrcA = 1
ALUSrcB = 10
ALUOp = 00

2. Arithmetic-logical instruction:
ALUOut = A op B

ALUSrcA = 1
ALUSrcB = 00
ALUOp = 10

3. Branch:
if (A == B) PC = ALUOut

ALUSrcA = 1
ALUSrcB = 00
ALUOp = 01
PCWriteCond = 1
PCSource = 01

4. Jump:
PC = PC[31-28] & (IR[25-0] << 2)

PCWrite = 1

16



4. Memory access or R-type instruction completion step
In this step a load or store instruction accesses memory or a arithmeticlogical instruction writes its result.
1. Memory reference:
MDR = Memory [ALUOut]

or
Memory [ALUOut] = B

MemRead = 1

or MemWrite = 1

IorD = 1

2. Arithmetic-logical instruction:
Reg[IR[15-11]] = ALUOut

RegDst = 1
RegWrite = 1
MemtoReg = 0

5. Memory read completion step
The load instruction is completed by writing back the value from the
memory:
Reg[IR[20-16]] = MDR

MemtoReg = 1
RegWrite = 1
RegDst = 0

These five steps are summarised in figure 2.20.


17


Step name

Action for R-type
instructions

Action for memoryreference instructions

Instruction fetch

Action for
branches

Action for
jumps

IR = Memory[PC]
PC = PC + 4

Instruction decode
register fetch

A = Reg[IR[25-21]]
B = Reg[IR[20-16]]
ALUOut = PC + (sign-extend(IR[15-0] << 2)

Execution, address
computation,
branch/jump completion

ALUOut = A op B

ALUOut = A + sign-extend
(IR[15-0])

Memory access or Rtype completion

Reg[IR[15-11]] =
ALUOut

Load: MDR = Memory[ALUOut]
or
Store: Memory[ALUOut] = B

Memory read completion

if (A == B) then
PC = ALUOut

PC = PC[31-28] ||
(IR[25-0] << 2)

Load: Reg[IR[20-16]] = MDR

Figure 2.20: Summary of the multicycle steps [PaHe98] p. 389

2.3.3 Defining the Control by a Finite State Machine
In the single step implementation the control was defined by simple truth tables
that set the control signals depending on the instruction.
This does not work for a mulitcycle datapath.
The control is more complex, because it must specify both the signals to be set
in any step and the next step in the sequence.
Therefore a finite state machine is used.
Figure 2.21 shows the finite state machine for the control of the multicycle
datapath implementation.

18


2.3 Defining the Control by a Finite State Machine

Figure 2.21: Complete finite state machine control [PaHe98] p. 396

The setting of the control signals is also shown in figure 2.21.
All unused signals have to be deasserted or keep their value during the next
states until they are set again.
All signal settings in all states is shown in figure 2.22.


19

2.3 Defining the Control by a Finite State Machine

Signal name

State
0

1

2

3

4

5

6

7

8

9

RegDst

0

0

0

0

0

0

0

1

0

0

RegWrite

0

0

0

0

1

0

0

1

0

0

ALUSrcA

0

0

1

1

1

1

1

1

1

0

MemRead

1

0

0

1

0

0

0

0

0

0

MemWrite

0

0

0

0

0

1

0

0

0

0

MemtoReg

0

0

0

0

1

0

0

0

0

0

IorD

0

0

0

1

0

1

0

0

0

0

IRWrite

1

0

0

0

0

0

0

0

0

0

PCWrite

1

0

0

0

0

0

0

0

0

1

PCWriteCond

0

0

0

0

0

0

0

0

1

0

ALUOp

00

00

00

00

00

00

10

10

01

00

ALUSrcB

01

11

10

10

10

10

00

00

00

11

PCSource

00

00

00

00

00

00

00

00

01

10

Figure 2.22: Setting of Control Signals

20


3 Design Tasks

3 Design Tasks
•

Block Diagram of first hierarchy levels

•

Register Transfer Level Models implemented in pure VHDL

•

VHDL Testbench of important RTL Models

•

Implementation in Altera Target Technology

•

Prototype Testing

•

Simulation Tool: ModelSim

•

Synthesis Tool: Altera Quartus

•

Milestone Presentations

•

Design Project Report in OpenOffice Document Format

•

Design Directory Structure is mandatory according to the following table:
Object

Description

toplevel

Root directory for a VHDL design project

toplevel/src

directory for VHDL source code

toplevel/work

directory for VHDL working library, contains compiled object code of
ModelSim VHDL compiler

toplevel/simulation

simulation results

toplevel/stimuli

stimuli files of extended simulation runs should be stored in
this directory

toplevel/pnr

data produced after a place&route run can be found in this directory

toplevel/scripts

scriptfiles for automated batch processing of the design steps
should be placed here

toplevel/log

log files of the different design steps

toplevel/doc

directory for project documentation, data sheets, etc.


21

4 Module Specification

4 Module Specification
4.1 ALU
4.1.1 Functional Description
The arithmetic-logic unit (ALU) performs basic arithmetic and logic operations
which are controlled by the opcode. The result of the instruction is written to the
output. An additional zero-bit signalizes an high output if the result equals zero.
At the present time, the basic arithmetic operations add and sub and the logic
operations and, or and slt can be applied to inputs. The inputs are 32 bit wide
with type unsigned. A detection of overflow or borrow is not supported at the moment.

22


4.1 Block Diagram

4.1.2 Block Diagram

Figure 4.1: ALU 1/3


23

4.1 Block Diagram

Figure 4.2: ALU 2/3

24


4.1 Block Diagram

Figure 4.3: ALU 3/3


25

4.1 Simulation Results

4.1.3 Simulation Results

Figure 4.4: Simulation Results of ALU

4.1.4 Design Files
File Name

File Type

Description

e_alu.vhd
a_alu_behave.vhd

VHDL Source Files

Arithmetic-logic unit

t_alu.vhd

VHDL Testbench File

Testbench for single operations

t_alu_fileio.vhd

VHDL Testbench File

Testbench using file I/O

26


4.2 Memory

4.2 Memory
Data is synchronously written to or read from the memory with a data bus width
of 32 bit. The memory consists of four ram blocks with 8 bit data width each.
A control signal enables the memory to be written, otherwise data is only read. In
order to store data to the memory the data word is subdivided into four bytes
which are separately written to the ram blocks. Vice versa, the single bytes are
concatenated to get the data word back again.
At the moment, it is only possible to read and write data words. An addressing of
half-words or single bytes is not allowed. In order to write or read data words, all
ram blocks have to be selected. Hence, the lowest two bit are not examined for
chip-select logic.
Data is addressed by the MIPS-processor with an address width of 32 bit, while
the address width of a ram block is 8 bit each. All ram blocks are connected to
the same address, namely from mem_address(9 downto 2). Since we do not use
the full address width for addressing and chip selects, data words are addressed
by multiple addresses.

Unfortunately, some problems occurred during simulation of the memory unit.
According to the MIPS design shown in literature [PaHe98], there should be implemented a memory unit with an unregistered output. The Altera Quartus
MegaWizard Plug-In Manager yielded a ram block with a synchronous output
(a_ram_syn.vhd) , although the output was defined as unregistered.
In order to get an unregistered memory output, another ram block was defined in
VHDL code (a_ram_rtl.vhd). There, the output directly yields the data being addressed by the unregistered input address. Unfortunately, the synthesizer does
not support memory initialisation files in the RTL-code for setting data to the
memory. Hence, it was not possible to implement the memory in real hardware.


27

4.2 Block Diagram

4.2.2 Block Diagram

Figure 4.5: Memory

Figure 4.6 shows the simulation results with registered data output.

Figure 4.6: Simulation Results of Memory (registered outputs)

28


4.2 Simulation Results

Figure 4.7 shows the simulation results with unregistered output. Note that the
simulation contains unknown values, because the memory initialisation files are
not supported.

Figure 4.7: Simulation Results of Memory (unregistered outputs)

4.2.4 Design Files
File Name

File Type

Description

e_ram.vhd
a_ram_rtl.vhd
a_ram_syn.vhd
a_ram_lpm.vhd

VHDL Source Files

Ram block used as component for
memory instantiation

e_memory.vhd
a_memory_behave.vhd

VHDL Source Files

Instantiation and connection of ram
blocks

t_memory.vhd

VHDL Testbench Files

Test memory read. write and address

./simulation/ram0_256x8.hex

Intel Hex Format Files

Used for memory initialisation
(a_ram_syn.vhd)


29

4.3 Control

4.3 Control
The control of the processor is realised by a Finite State Machine described in
section 2.3.3.
The input to the State Machine are the upper 6 bits of the function field containing the instruction.
The outputs of the state machine are the control signals of the single functional
units of the processor implementation especially the multiplexers of the datapath.
The Operation Code of the ALU is stored in a truth table and the corresponding
Opcode is produced depending on the ALUOp signal of the state machine and
the lower 6 bits of the function field containing the information which of the arithmetic or logic instruction is to use.

30


4.3 State Diagram

4.3.2 State Diagram

ErrorState

Figure 4.8: Control Finite State Machine

An additional Error State is inserted which is a deadlock. If any unknown instruction occurs the Error State is entered.


31

4.3 Block Diagram

4.3.3 Block Diagram

Figure 4.9: Control FSM

Figure 4.10: ALU Control

32


4.3 Block Diagram

Figure 4.11: Control


Figure 4.12: Simulation Results of the Control FSM

4.3.5 Design Files
File Name

File Type

Description

e_control_ControlFSM.vhd
a_control_ControlFSM.vhd

VHDL Source Files

Finite State Machine for Control

e_control_ALUControl.vhd
a_control_ALUControl.vhd

VHDL Source Files

Truth Tabel for ALU Control

e_control.vhd
a_control.vhd

VHDL Source Files

Controlpath


33

4.4 Data Path

4.4 Data Path
The datapath is divided into four sections with respect to the pipelining structure
of a processor. The four parts are the Instruction Fetch, Instruction Decode, Execution and Memory Writeback.
These sections are synthesized of their own and then combined to the Data
Block.

4.4.1 Instruction Fetch
4.4.1.1 Functional Description
The Instruction Fetch Block contains the PC the Instruction Register and the
Memory Data Register.
This part provides the data and instruction form the memory.

4.4.1.2 Block Diagram

Figure 4.13: Instruction Fetch

34


4.4 Instruction Fetch

4.4.1.3 Design Files
File Name

File Type

Description

e_pc.vhd
a_pc_behave.vhd

VHDL Source Files

Program Counter

e_tempreg.vhd
a_tempreg_behave.vhd

VHDL Source Files

Temporary Memory Data Register

e_instreg.vhd
a_instreg_behave.vhd

VHDL Source Files

Instruction Register

e_data_fetch.vhd
a_data_fetch_behave_vhd

VHDL Source Files

Instruction Fetch Block

4.4.2 Instruction Decode
The Instruction Decode Block writes the instruction of the Instruction Register to
the Register File and computes the second operand for a Branch Instruction or a
sw- or lw-instruction.


Figure 4.14: Instruction Decode


35

4.4 Instruction Decode

File Name

File Type

Description

e_regfile.vhd
a_regfile_behave.vhd

VHDL Source Files

Register File

e_tempreg.vhd

VHDL Source Files

Temporary Memory Data Register

e_data_decode.vhd
a_data_decode_behave.vhd

VHDL Source Files

Data Decode Block

4.4.3 Execution
The Execution contains the ALU as main element and computes the desired result of the instruction.
It also computes the jump target address and provides it for the Memory Writeback Block.
The operands loaded to the ALU are chosen by two multiplexers which are sensible to the signals ALUSrcA and ALUSrcB.

36


4.4 Execution


Figure 4.15: Execution


37

4.4 Execution

File Name

File Type

Description

e_alu.vhd
a_alu_behave.vhd

VHDL Source Files

ALU

e_data_execution.vhd
a_data_execution.vhd

VHDL Source Files

Execution Block

38


4.4 Memory Writeback

4.4.4 Memory Writeback
The Memory Writeback Block consists of the ALUOut register and a multiplexer
with source signal PCSource.
This block leads the result of the computation either back to memory or to the
register file.
The multiplexer leads back the next PC value depending on the PCSource signal.


39



Figure 4.16: Memory Writeback

40



File Name

File Type

Description

e_tempreg.vhd

VHDL Source Files

Temporary ALUOut Register

e_data_memwriteback.vhd
a_data_memwriteback.vhd

VHDL Source Files

Memory Writeback Block


41

4.4 Data Path

4.4.5 Data Path

File Name

File Type

Description

e_data.vhd
a_data_vhd

VHDL Source Files

Datapath

e_data_fetch.vhd
a_data_fetch.vhd

VHDL Source Files

Data Fetch Block

e_data_decode.vhd
a_data_decode.vhd

VHDL Source Files

Data Decode Block


VHDL Source Files

Data Execution Block


VHDL Source Files


e_tempreg.vhd

VHDL Source Files


e_alu.vhd
a_alu_behave.vhd

VHDL Source Files

ALU

e_regfile.vhd

VHDL Source Files

Register File

e_pc.vhd
a_pc_behave.vhd

VHDL Source Files

Program Counter

e_instreg.vhd

VHDL Source Files


42


4.5 Processor and Memroy

4.5 Processor and Memroy
The both parts Datapath and Controlpath are combined to the processing unit.
Together with the Memory the whole processor is completed.

4.5.2 Block Diagram

Figure 4.17: Processing Unit (Datapath & Controlpath)

Figure 4.18: Processing Unit & Memory


43

4.5 Design Files

4.5.3 Design Files
File Name

File Type

Description

e_control_ControlFSM.vhd
a_control_ControlFSM.vhd

VHDL Source Files

Finite State Machine for Control

e_control_ALUControl.vhd
a_control_ALUControl.vhd

VHDL Source Files

Truth Tabel for ALU Control

e_control.vhd
a_control.vhd

VHDL Source Files

Controlpath

e_data.vhd
a_data.vhd

VHDL Source Files

Datapath

e_data_fetch.vhd
a_data_fetch.vhd

VHDL Source Files

Data Fetch Block

e_data_decode.vhd
a_data_decode.vhd

VHDL Source Files

Data Decode Block


VHDL Source Files

Data Execution Block


VHDL Source Files


e_tempreg.vhd

VHDL Source Files


e_alu.vhd
a_alu_behave.vhd

VHDL Source Files

ALU

e_regfile.vhd

VHDL Source Files

Register File

e_pc.vhd
a_pc_behave.vhd

VHDL Source Files

Program Counter

e_instreg.vhd

VHDL Source Files


e_ram.vhd
a_ram_rtl.vhd
a_ram_syn.vhd
a_ram_lpm.vhd

VHDL Source Files

Ram block used as component for
memory instantiation

t_procmem.vhd
t_procmem_init.vhd

VHDL Testbench Files

Testbench for testing the processor

44


5 Synthesis Results

5 Synthesis Results
+------------------------------------------------------------------------------+
; Analysis & Synthesis Summary
;
+------------------------------------+-----------------------------------------+
; Analysis & Synthesis Status
; Successful - Thu Jul 05 11:15:33 2007
;
; Quartus II Version
; 7.0 Build 33 02/05/2007 SJ Full Version ;
; Revision Name
; procmem
;
; Top-level Entity Name
; procmem
;
; Family
; Cyclone II
;
; Total logic elements
; 0
;
;
Total combinational functions ; 0
;
;
Dedicated logic registers
; 0
;
; Total registers
; 0
;
; Total pins
; 2
;
; Total virtual pins
; 0
;
; Total memory bits
; 0
;
; Embedded Multiplier 9-bit elements ; 0
;
; Total PLLs
; 0
;
+------------------------------------+-----------------------------------------+

Figure 5.1: Analysis & Synthesis Summary

+--------------------------------------------------------------------------------------------------------------+
; Analysis & Synthesis Settings
;
+--------------------------------------------------------------------+--------------------+--------------------+
; Option
; Setting
; Default Value
;
+--------------------------------------------------------------------+--------------------+--------------------+
; Device
; EP2C20F484C7
;
;
; Top-level entity name
; procmem
; procmem
;
; Family name
; Cyclone II
; Stratix
;
; Restructure Multiplexers
; Auto
; Auto
;
; Create Debugging Nodes for IP Cores
; Off
; Off
;
; Preserve fewer node names
; On
; On
;
; Disable OpenCore Plus hardware evaluation
; Off
; Off
;
; Verilog Version
; Verilog_2001
; Verilog_2001
;
; VHDL Version
; VHDL93
; VHDL93
;
; State Machine Processing
; Auto
; Auto
;
; Safe State Machine
; Off
; Off
;
; Extract Verilog State Machines
; On
; On
;
; Extract VHDL State Machines
; On
; On
;
; Ignore Verilog initial constructs
; Off
; Off
;
; Add Pass-Through Logic to Inferred RAMs
; On
; On
;
; DSP Block Balancing
; Auto
; Auto
;
; NOT Gate Push-Back
; On
; On
;
; Power-Up Don't Care
; On
; On
;
; Remove Redundant Logic Cells
; Off
; Off
;
; Remove Duplicate Registers
; On
; On
;
; Ignore CARRY Buffers
; Off
; Off
;
; Ignore CASCADE Buffers
; Off
; Off
;
; Ignore GLOBAL Buffers
; Off
; Off
;
; Ignore ROW GLOBAL Buffers
; Off
; Off
;
; Ignore LCELL Buffers
; Off
; Off
;
; Ignore SOFT Buffers
; On
; On
;
; Limit AHDL Integers to 32 Bits
; Off
; Off
;
; Optimization Technique -- Cyclone II
; Balanced
; Balanced
;
; Carry Chain Length -- Stratix/Stratix GX/Cyclone/MAX II/Cyclone II ; 70
; 70
;
; Auto Carry Chains
; On
; On
;
; Auto Open-Drain Pins
; On
; On
;
; Perform WYSIWYG Primitive Resynthesis
; Off
; Off
;
; Perform gate-level register retiming
; Off
; Off
;
; Allow register retiming to trade off Tsu/Tco with Fmax
; On
; On
;
; Auto ROM Replacement
; On
; On
;
; Auto RAM Replacement
; On
; On
;
; Auto Shift Register Replacement
; On
; On
;
; Auto Clock Enable Replacement
; On
; On
;
; Allow Synchronous Control Signals
; On
; On
;
; Force Use of Synchronous Clear Signals
; Off
; Off
;
; Auto RAM to Logic Cell Conversion
; Off
; Off
;
; Auto Resource Sharing
; Off
; Off
;
; Allow Any RAM Size For Recognition
; Off
; Off
;
; Allow Any ROM Size For Recognition
; Off
; Off
;
; Allow Any Shift Register Size For Recognition
; Off
; Off
;
; Ignore translate_off and synthesis_off directives
; Off
; Off
;
; Show Parameter Settings Tables in Synthesis Report
; On
; On
;
; Ignore Maximum Fan-Out Assignments
; Off
; Off
;


45

5 Synthesis Results

; Retiming Meta-Stability Register Sequence Length
; 2
; 2
;
; PowerPlay Power Optimization
; Normal compilation ; Normal compilation ;
; HDL message level
; Level2
; Level2
;
; Suppress Register Optimization Related Messages
; Off
; Off
;
; Number of Removed Registers Reported in Synthesis Report
; 100
; 100
;
; Use smart compilation
; Off
; Off
;
+--------------------------------------------------------------------+--------------------+--------------------+

Figure 5.2: Analysis & Synthesis Settings

+-----------------------------------------------------+
; Compilation Hierarchy
;
+-----------------------------------------------------+
; |procmem
;
;
|mips
;
;
|control:inst_control|
;
;
|ALUControl:inst_ALUControl|
;
;
|ControlFSM:inst_ControlFSM|
;
;
|data:inst_data|
;
;
|data_decode:inst_data_decode|
;
;
|regfile:inst_regfile|
;
;
|tempreg:A|
;
;
|tempreg:B|
;
;
|data_execution:inst_data_execution|
;
;
|alu:alu_inst|
;
;
|data_fetch:inst_data_fetch|
;
;
|instreg:instr_reg|
;
;
|pc:proc_cnt|
;
;
|tempreg:mem_data_reg|
;
;
|data_memwriteback:inst_data_memwriteback| ;
;
|tempreg:tempreg_inst|
;
;
|memory
;
;
|ram:mem_block0|
;
;
|altsyncram:ram_block_rtl_0|
;
;
|altsyncram_ia61:auto_generated|
;
;
|ram:mem_block1|
;
;
;
;
;
;
|ram:mem_block2|
;
;
;
;
;
;
|ram:mem_block3|
;
;
;
;
;
+-----------------------------------------------------+

Figure 5.3: Compilation History

46


6 Results of Prototype Testing

6 Results of Prototype Testing
6.1 Description
For the first test of the completed processor and the memory a simple addition of
two numbers was done.
Therefore at first the memory has to be loaded with the instructions and data by
using *.mif-files to write the information into the memory blocks before starting
the simulation.
The instructions written into the memory are:
Memory

Instruction Field

Address

Instruction

op

rs

rt

000

lw $s0, 128($zero)

100011

00000

10000

004

lw $s1, 132($zero)

100011

00000

10001

008

add $s2, $s0, $s1

000000

10000

10001

012

sw $s2, 136($zero)

101011

00000

10010

016

sub $s3, $s1, $s0

000000

10001

10000

020

sw $s3, 140($zero)

101011

00000

10011

024

and $s4, $s1, $s0

000000

10001

10000

028

sw $s4, 144($zero)

101011

00000

10100

032

or $s5, $s1, $s0

000000

10001

10000

036

sw $s5, 148($zero)

101011

00000

10101

040

slt $s6, $s1, $s0

000000

10000

10001

044

sw $s6, 152($zero)

101011

00000

10110

048

beq $s0, $s4, 56

000100

10000

10100

052

UNDEFINED

UUUUUU

UUUUU

UUUUU

056

j8

000010

rd

shamt

funct

0000000010000000
0000000010000100
10010

00000

100000

0000000010001000
10011

00000

100010

0000000010001100
10100

00000

100100

0000000010010000
10101

00000

100101

0000000010010100
10110

00000

101010

0000000010011000
0000000000000001
UUUUU

UUUUU

UUUUUU

00000000000000000000000010

The data written to the memory are:
Memory
Address

Data (dec)

Data (bin)

128

379

00000000

00000000

00000001

01111011

132

383

00000000

00000000

00000001

01111111


47

6.1 Description

The expected values stored back into the memory are:
Memory
Address

Data (dec)

Data (bin)

136

762

00000000

00000000

00000010

11111010

140

4

00000000

00000000

00000000

00000100

144

379

00000000

00000000

00000001

01111011

148

383

00000000

00000000

00000001

01111111

152

1

00000000

00000000

00000000

00000001

The simulation starts at memory address 000 with a load word instruction. The
value of memory address 128 is written into register $s0. The PC is incremented
and the next instruction of memory address 004 is executed. It is also an load
word instruction which loads the value of memory address 132 to register $s1.
Then an add instruction follows which adds the two operands written into the registers $s0 and $s1 and writes the result to register $s2.
Then a store word instruction writes the content of register $s2 to the memory at
address 136.
The following instructions are for subtract, add, or, slt, beq and jump. The result
of a computation is always stored to the memory by a store word instruction.

Note:
For description of the register numbers and names used for the test see Figure
3.13 of [PaHe98] p. 140.
The used assembler instructions are not completely declared in this report.
For information on the machine language see [PaHe98] Chapter 3, especially figure 3.14 on page 141.

48


6.2 Simulation Result

6.2 Simulation Result

Figure 6.1: Simulation Results of MIPS and Memory


49

7 Conclusion

7 Conclusion
7.1 Our own experiences
While working on our miniproject, we applied a lot of knowledge learned in the
lecture VHDL. Furthermore, we gained a lot of experience in using the simulation
and synthesis tools. It was very interesting and exciting to describe real hardware
and to see the expected results in simulation and the block diagrams after synthesis.
Our miniproject implementing a processor in VHDL has been a real challenge.
The complexity was not located in the single components, but rather in the implementation of the synchronous operation of the whole control and datapath. Due
to an intensive preparation of the desired hardware according to the literature
“Computer Organization & Design” [PaHe98], we prevented unintended design
errors. Since our project transcends a pure implementation of VHDL code, we
were able to gain experience in hierarchical design with component instantiation
and package design.
Additionally, while implementing a microprocessor, we could refresh our knowledge in processor operations, memory addressing and MIPS instruction coding.

7.2 Annotations to “Computer Organization & Design” [PaHe98]
Since the design of our MIPS processor is closely connected to the literature
[PaHe98], we read the chapter 5 in detail. Overall, we adjusted the design as
conform as possible to the description in [PaHe98]. There are some passages
which do not provide a full description, e.g. the output signals of the control FSM
are not listed completely for each state. Nevertheless, “Computer Organization &
Design” by Patterson and Hennessy provides a brilliant composition describing
the control and datapath of a processor implementation.

50


7.3 Further work on the project

7.3 Further work on the project
Although we spent much more time than scheduled, we did not reach all our
aims. For further work on the project, we recommend our successors to continue
the following tasks:
– Verify the synthesis results (with VHDL code created by Quartus) with desired
behavior implemented in RTL and seen in testbench simulation in order to
obtain the desired unregistered memory output mentioned in chapter 4.2.
– Realise an hardware implementation of processor and memory in order to
verify the behavior of the desired hardware on the Cyclone II Development &
Education Board, e.g. debug the memory data.
– Introduce the pipelining of instructions described in chapter 6 [PaHe98] to improve the performance of the MIPS processor.


51

8 Appendix

8 Appendix
8.1 Design files
8.1.1 Project Entities
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY ControlFSM IS
PORT (clk, rst_n : IN std_ulogic;
instr_31_26 : IN std_ulogic_vector(5 downto 0);
RegDst, RegWrite, ALUSrcA, MemRead, MemWrite, MemtoReg, IorD, IRWrite, PCWrite,
PCWriteCond : OUT std_ulogic;
ALUOp, ALUSrcB, PCSource : OUT std_ulogic_vector(1 downto 0)
);
END ControlFSM;

VHDLSource 8.1: e_control_ControlFSM.vhd

LIBRARY IEEE;
ENTITY ALUControl IS
PORT (instr_15_0 : IN std_ulogic_vector(15 downto 0);
ALUOp : IN std_ulogic_vector(1 downto 0);
ALUopcode : OUT std_ulogic_vector(2 downto 0)
);
END ALUControl;

VHDLSource 8.2: e_control_ALUControl.vhd

LIBRARY IEEE;
ENTITY control IS
zero
: IN std_ulogic;
ALUopcode
: OUT std_ulogic_vector(2 downto 0);
RegDst, RegWrite, ALUSrcA, MemRead, MemWrite, MemtoReg, IorD, IRWrite : OUT
std_ulogic;
ALUSrcB, PCSource : OUT std_ulogic_vector(1 downto 0);
PC_en
: OUT std_ulogic
);
END control

VHDLSource 8.3: e_control.vhd

52


8.1 Project Entities

LIBRARY IEEE;
USE IEEE.numeric_std.ALL;
-- use package
USE work.procmem_definitions.ALL;
ENTITY tempreg IS
PORT (
clk
: IN STD_ULOGIC;
rst_n
: IN STD_ULOGIC;
reg_in
reg_out

: IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
: OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0) );

END tempreg

VHDLSource 8.4: e_tempreg.vhd

LIBRARY IEEE;
-- use package
ENTITY pc IS
PORT (
clk
rst_n
pc_in
PC_en
pc_out
END pc;

:
:
:
:
:

IN
IN
IN
IN
OUT

STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
STD_ULOGIC;
STD_ULOGIC_VECTOR(width-1 DOWNTO 0) );

VHDLSource 8.5: e_pc.vhd

LIBRARY IEEE;
-- use package
ENTITY instreg IS
PORT (
clk
: IN STD_ULOGIC;
rst_n
: IN STD_ULOGIC;
memdata
IRWrite
: IN STD_ULOGIC;
instr_31_26
instr_25_21
instr_20_16
instr_15_0

:
:
:
:

OUT
OUT
OUT
OUT

STD_ULOGIC_VECTOR(5 DOWNTO 0);
STD_ULOGIC_VECTOR(15 DOWNTO 0) );

END instreg;

VHDLSource 8.6: e_instreg.vhd


53


LIBRARY IEEE;
-- use package
ENTITY regfile IS
PORT (clk,rst_n
wen
writeport
adrwport
adrport0
adrport1
readport0
readport1
);
END regfile;

:
:
:
:
:
:
:
:

IN
IN
IN
IN
IN
IN
OUT
OUT

std_ulogic;
std_ulogic;
-- write control
std_ulogic_vector(width-1 DOWNTO 0); -- register input
std_ulogic_vector(regfile_adrsize-1 DOWNTO 0);-- address write
std_ulogic_vector(regfile_adrsize-1 DOWNTO 0);-- address port 0
std_ulogic_vector(width-1 DOWNTO 0); -- output port 0
std_ulogic_vector(width-1 DOWNTO 0)
-- output port 1

VHDLSource 8.7: e_regfile.vhd

LIBRARY IEEE;
-- use package
ENTITY alu IS
PORT (
a, b
: IN
opcode : IN


result : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
zero
: OUT STD_ULOGIC);
END alu;

VHDLSource 8.8: e_alu_vhd

LIBRARY IEEE;
-- use package
ENTITY data_fetch IS
PORT (
-- inputs
clk
: IN STD_ULOGIC;
rst_n
: IN STD_ULOGIC;
pc_in
: IN STD_ULOGIC_VECTOR(width-1 DOWNTO
alu_out
: IN STD_ULOGIC_VECTOR(width-1 DOWNTO
mem_data
: IN std_ulogic_vector(width-1 DOWNTO
-- control signals
PC_en
: IN STD_ULOGIC;
IorD
: IN STD_ULOGIC;
IRWrite
: IN STD_ULOGIC;
-- outputs
reg_memdata : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO
instr_31_26 : OUT STD_ULOGIC_VECTOR(5 DOWNTO 0);
mem_address : OUT std_ulogic_vector(width-1 DOWNTO
pc_out
: OUT std_ulogic_vector(width-1 DOWNTO
);

0);
0);
0);

0);

0);
0)

END data_fetch;

VHDLSource 8.9: e_data_fetch.vhd

54



LIBRARY IEEE;
-- use package
ENTITY data_decode IS
PORT (
-- inputs
clk
: IN STD_ULOGIC;
rst_n
: IN STD_ULOGIC;
instr_25_21 : IN STD_ULOGIC_VECTOR(4 DOWNTO 0);
reg_memdata : IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
alu_out
-- control signals
RegDst
: IN STD_ULOGIC;
RegWrite
: IN STD_ULOGIC;
MemtoReg
: IN STD_ULOGIC;
-- outputs
reg_A
: OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
reg_B
: OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
instr_15_0_se : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
instr_15_0_se_sl : OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0)
);
END data_decode;

VHDLSource 8.10: e_data_decode.vhd

LIBRARY IEEE;
-- use package
ENTITY data_execution IS
PORT (instr_25_21 : IN
instr_20_16 : IN
instr_15_0 : IN
ALUSrcA
: IN
ALUSrcB
: IN
ALUopcode
: IN
reg_A, reg_B
pc_out
instr_15_0_se
instr_15_0_se_sl

std_ulogic_vector(4 downto 0);
std_ulogic;
: IN std_ulogic_vector(width-1

downto
downto
downto
downto

0);
0);
0);
0);

jump_addr
: OUT std_ulogic_vector(width-1 downto 0);
alu_result : OUT std_ulogic_vector(width-1 downto 0);
zero
: OUT std_ulogic
);
END data_execution;

VHDLSource 8.11: e_data_execution.vhd


55


LIBRARY IEEE;
-- use package
ENTITY data_memwriteback IS
jump_addr
: IN std_ulogic_vector(width-1 downto 0);
alu_result : IN std_ulogic_vector(width-1 downto 0);
PCSource
: IN std_ulogic_vector(1 downto 0);
pc_in
alu_out
);

: OUT std_ulogic_vector(width-1 downto 0);
: OUT std_ulogic_vector(width-1 downto 0)

END data_memwriteback;

VHDLSource 8.12: e_data_memwriteback.vhd

LIBRARY IEEE;
-- use package
ENTITY data IS
PC_en, IorD, MemtoReg, IRWrite, ALUSrcA, RegWrite, RegDst : IN std_ulogic;
PCSource, ALUSrcB : IN std_ulogic_vector(1 downto 0);
ALUopcode : IN std_ulogic_vector(2 downto 0);
mem_data : IN std_ulogic_vector(width-1 downto 0);
reg_B, mem_address : OUT std_ulogic_vector(width-1 downto 0);
instr_31_26 : OUT std_ulogic_vector(5 downto 0);
instr_15_0 : OUT std_ulogic_vector(15 downto 0);
zero : OUT std_ulogic
);
END data;

VHDLSource 8.13: e_data.vhd

LIBRARY IEEE;
-- use altera_mf library for RAM block
LIBRARY altera_mf;
USE altera_mf.ALL;
-- use package
ENTITY ram IS
GENERIC (adrwidth : positive := ram_adrwidth;
datwidth : positive := ram_datwidth;
ramfile : string
:= ramfile_std
-- initial RAM content
-- in IntelHEX Format
);
PORT (address : IN std_logic_vector(ram_adrwidth-1 DOWNTO 0);
data
: IN std_logic_vector(ram_datwidth-1 DOWNTO 0);
inclock : IN std_logic;
-- used to write data in RAM cells
wren_p : IN std_logic;
q
: OUT std_logic_vector(ram_datwidth-1 DOWNTO 0));
END ram;

VHDLSource 8.14: e_ram.vhd

56



LIBRARY IEEE;
-- use package
ENTITY memory IS
PORT (
clk
:
rst_n
:
MemRead
:
MemWrite
:
mem_address :
data_in
:
data_out
:
END memory;

IN
IN
IN
IN
IN
IN
OUT

STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;

VHDLSource 8.15: e_memory.vhd

LIBRARY IEEE;
-- use package
ENTITY mips IS
mem_data
: IN std_ulogic_vector(width-1 downto 0);
reg_B, mem_address : OUT std_ulogic_vector(width-1 downto 0);
MemRead, MemWrite : OUT std_ulogic
);
END mips;

VHDLSource 8.16: e_mips.vhd

LIBRARY IEEE;
-- use package
ENTITY procmem IS
PORT (clk, rst_n : IN std_ulogic
);
END procmem;

VHDLSource 8.17: e_procmem.vhd


57

8.1 Project Architectures

8.1.2 Project Architectures

58



LIBRARY IEEE;
ARCHITECTURE behave OF ControlFSM IS
-------------------------------------------------------------------------------- Definition of the state names
TYPE state_type IS (InstDec, MemAddComp, MemAccL, MemReadCompl, MemAccS, Exec, RCompl, BranchCompl, JumpCompl, ErrState, InstFetch);
SIGNAL state, next_state : state_type;
BEGIN
-------------------------------------------------------------------------------- State process
state_reg : PROCESS(clk, rst_n)
BEGIN
IF rst_n = '0' THEN
state <= InstFetch;
ELSIF RISING_EDGE(clk) THEN
state <= next_state;
END IF;
END PROCESS;
-------------------------------------------------------------------------------- Logic Process
logic_process : PROCESS(state, instr_31_26)
-- RegDst RegWrite ALUSrcA MemRead MemWrite MemtoReg IorD IRWrite PCWrite PCWriteCond
10x1bit
-ALUOp
ALUSrcB
PCSource
3x2bit
VARIABLE control_signals : std_ulogic_vector(15 downto 0);
-- Defintion of Constants for the value of the Inst_Funct_Field
Constant LOADWORD : std_ulogic_vector(5 Downto 0) := "100011";
Constant STOREWORD : std_ulogic_vector(5 Downto 0) := "101011";
Constant RTYPE : std_ulogic_vector(5 Downto 0) := "000000";
Constant BEQ : std_ulogic_vector(5 Downto 0) := "000100";
Constant JMP : std_ulogic_vector(5 Downto 0) := "000010";
BEGIN
CASE state IS
-- Instruction Fetch
WHEN InstFetch =>
control_signals := "0001000110000100";
next_state <= InstDec;
-- Instruction Decode and Register Fetch
WHEN InstDec =>
control_signals := "0000000000001100";
IF instr_31_26 = LOADWORD OR instr_31_26 = STOREWORD THEN
next_state <= MemAddComp;
ELSIF instr_31_26 = RTYPE THEN
next_state <= Exec;
ELSIF instr_31_26 = BEQ
THEN
next_state <= BranchCompl;
ELSIF instr_31_26 = JMP THEN
next_state <= JumpCompl;
ELSE
next_state <= ErrState;
END IF;
-- Memory Address Computation
WHEN MemAddComp =>
control_signals := "0010000000001000";
if instr_31_26 = LOADWORD THEN
next_state <= MemAccL;
ELSIF instr_31_26 = STOREWORD THEN
next_state <= MemAccS;
ELSE
END IF;


59


-- Memory Access Load Word
WHEN MemAccL =>
control_signals := "0011001000001000";
next_state <= MemReadCompl;
-- Memory Read Completion
WHEN MemReadCompl =>
control_signals := "0110010000001000";
next_state <= InstFetch;
-- Memory Access Store Word
WHEN MemAccS =>
control_signals := "0010101000001000";
-- Execution
WHEN Exec =>
control_signals := "0010000000100000";
next_state <= RCompl;
-- R-type Completion
WHEN RCompl =>
control_signals := "1110000000100000";
-- Branch Completion
WHEN BranchCompl =>
control_signals := "0010000001010001";
-- Jump Completion
WHEN JumpCompl =>
control_signals := "0000000010001110";
WHEN OTHERS =>
control_signals := (others => 'X');
END case;
RegDst <= control_signals(15);
RegWrite <= control_signals(14);
ALUSrcA <= control_signals(13);
MemRead <= control_signals(12);
MemWrite <= control_signals(11);
MemtoReg <= control_signals(10);
IorD <= control_signals(9);
IRWrite <= control_signals(8);
PCWrite <= control_signals(7);
PCWriteCond <= control_signals(6);
ALUOp <= control_signals(5 downto 4);
ALUSrcB <= control_signals(3 downto 2);
PCSource <= control_signals(1 downto 0);
END process;
END behave;

VHDLSource 8.18: a_control_ControlFSM.vhd

60



LIBRARY IEEE;
ARCHITECTURE behave OF ALUControl IS
BEGIN
Alu_Control : PROCESS(instr_15_0, ALUOp)
CONSTANT
CONSTANT
CONSTANT
CONSTANT
CONSTANT

cADD
cSUB
cAND
cOR
cSLT

:
:
:
:
:

std_ulogic_vector(5
std_ulogic_vector(5
std_ulogic_vector(5
std_ulogic_vector(5
std_ulogic_vector(5

downto
downto
downto
downto
downto

0)
0)
0)
0)
0)

:=
:=
:=
:=
:=

"100000";
"100010";
"100100";
"100101";
"101010";

BEGIN
case ALUOp is
when "00" => ALUopcode <= "010";
-- add
when "01" => ALUopcode <= "110";
-- subtract
when "10" =>
-- operation depends on function field
case instr_15_0(5 downto 0) is
when cADD => ALUopcode <= "010";
-- add
when cSUB => ALUopcode <= "110";
-- subtract
when cAND => ALUopcode <= "000";
-- AND
when cOR => ALUopcode <= "001";
-- OR
when cSLT => ALUopcode <= "111";
-- slt
when others => ALUopcode <= "000";
end case;
when others => ALUopcode <= "000";
end case;
END PROCESS;
END behave;

VHDLSource 8.19: a_control_ALUControl.vhd


61


LIBRARY IEEE;
ARCHITECTURE behave OF control IS
COMPONENT ControlFSM
PORT (
clk, rst_n
: IN std_ulogic;
instr_31_26
RegDst, RegWrite, ALUSrcA, MemRead, MemWrite, MemtoReg, IorD, IRWrite, PCWrite,
PCWriteCond : OUT std_ulogic;
ALUOp, ALUSrcB, PCSource
: OUT std_ulogic_vector(1 downto 0)
);
END COMPONENT;
COMPONENT ALUControl
PORT (
instr_15_0
ALUOp
ALUopcode
: OUT std_ulogic_vector(2 downto 0)
);
END COMPONENT;
SIGNAL ALUOp_intern : std_ulogic_vector(1 downto 0);
SIGNAL PCWrite_intern : std_ulogic;
SIGNAL PCWriteCond_intern : std_ulogic;
BEGIN
inst_ControlFSM : ControlFSM
PORT MAP (
clk
=> clk,
rst_n
=> rst_n,
instr_31_26
=> instr_31_26,
RegDst
=> RegDst,
RegWrite
=> RegWrite,
ALUSrcA
=> ALUSrcA,
MemRead
=> MemRead,
MemWrite
=> MemWrite,
MemtoReg
=> MemtoReg,
IorD
=> IorD,
IRWrite
=> IRWrite,
PCWrite
=> PCWrite_intern,
PCWriteCond
=> PCWriteCond_intern,
ALUOp
=> ALUOp_intern,
ALUSrcB
=> ALUSrcB,
PCSource
=> PCSource
);
inst_ALUControl : ALUControl
PORT MAP (
instr_15_0
=> instr_15_0,
ALUOp
=> ALUOp_intern,
ALUopcode
=> ALUopcode
);
PC_en <= PCWrite_intern OR (PCWriteCond_intern AND zero);
END behave;

VHDLSource 8.20: a_control.vhd

62



LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF tempreg IS
BEGIN
temp_reg: PROCESS(clk, rst_n)
BEGIN
IF rst_n = '0' THEN
reg_out <= (OTHERS => '0');
-- write register input to output at rising edge
reg_out <= reg_in;
END IF;
END PROCESS;
END behave;

VHDLSource 8.21: a_tempreg_behave.vhd

LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF pc IS
BEGIN
proc_pc : PROCESS(clk, rst_n)
VARIABLE pc_temp : STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
BEGIN
IF rst_n = '0' THEN
pc_temp := (OTHERS => '0');
IF PC_en = '1' THEN
pc_temp := pc_in;
END IF;
END IF;
pc_out <= pc_temp;
END PROCESS;
END behave;

VHDLSource 8.22: a_pc_behave.vhd


63


LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF instreg IS
BEGIN
proc_instreg : PROCESS(clk, rst_n)
BEGIN
IF rst_n = '0' THEN
instr_31_26 <= (OTHERS => '0');
instr_25_21 <= (OTHERS => '0');
instr_20_16 <= (OTHERS => '0');
instr_15_0 <= (OTHERS => '0');
-- write the output of the memory into the instruction register
IF(IRWrite = '1') THEN
instr_31_26 <= memdata(31 DOWNTO 26);
END IF;
END IF;
END PROCESS;
END behave;

VHDLSource 8.23: a_instreg_behave.vhd

LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF regfile IS
SUBTYPE WordT IS std_ulogic_vector(width-1 DOWNTO 0); -- reg word TYPE
TYPE
StorageT IS ARRAY(0 TO regfile_depth-1) OF WordT;
-- reg array TYPE
SIGNAL registerfile : StorageT;
-- reg file contents
BEGIN
-- perform write operation
PROCESS(rst_n, clk)
BEGIN
IF rst_n = '0' THEN
FOR i IN 0 TO regfile_depth-1 LOOP
registerfile(i) <= (OTHERS => '0');
END LOOP;
ELSIF rising_edge(clk) THEN
IF wen = '1' THEN
registerfile(to_integer(unsigned(adrwport))) <= writeport;
END IF;
END IF;
END PROCESS;
-- perform reading ports
readport0 <= registerfile(to_integer(unsigned(adrport0)));
readport1 <= registerfile(to_integer(unsigned(adrport1)));
END behave;

VHDLSource 8.24: a_regfile_behave.vhd

64



LIBRARY IEEE;
ARCHITECTURE behave OF alu IS
BEGIN
PROCESS(a, b, opcode)
-- declaration
VARIABLE a_uns
VARIABLE b_uns
VARIABLE r_uns
VARIABLE z_uns

of variables
: UNSIGNED(width-1 DOWNTO 0);
: UNSIGNED(0 DOWNTO 0);

BEGIN
-- initialize values
a_uns := UNSIGNED(a);
b_uns := UNSIGNED(b);
r_uns := (OTHERS => '0');
z_uns(0) := '0';
-- select desired operation
CASE opcode IS
-- add
WHEN "010" =>
r_uns := a_uns + b_uns;
-- sub
WHEN "110" =>
r_uns := a_uns - b_uns;
-- and
WHEN "000" =>
r_uns := a_uns AND b_uns;
-- or
WHEN "001" =>
r_uns := a_uns OR b_uns;
-- slt
WHEN "111" =>
r_uns := a_uns - b_uns;
IF SIGNED(r_uns) < 0 THEN
r_uns := TO_UNSIGNED(1, r_uns'LENGTH);
ELSE
r_uns := (OTHERS => '0');
END IF;
-- others
WHEN OTHERS => r_uns := (OTHERS => 'X');
END CASE;
-- set zero bit if result equals zero
IF TO_INTEGER(r_uns) = 0 THEN
z_uns(0) := '1';
ELSE
z_uns(0) := '0';
END IF;
-- assign variables to output signals
result <= STD_ULOGIC_VECTOR(r_uns);
zero <= z_uns(0);
END PROCESS;
END behave;

VHDLSource 8.25: a_alu_behave.vhd


65


LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF data_fetch IS
COMPONENT instreg
PORT (
clk
:
rst_n
:
memdata
:
IRWrite
:
instr_31_26 :
instr_25_21 :
instr_20_16 :
instr_15_0 :
END COMPONENT;

IS

COMPONENT tempreg
PORT (
clk
:
rst_n
:
reg_in
:
reg_out
:
END COMPONENT;

IS

COMPONENT pc IS
PORT (
clk
rst_n
pc_in
PC_en
pc_out
END COMPONENT;

IN STD_ULOGIC;
IN STD_ULOGIC;
IN STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
IN STD_ULOGIC;
OUT STD_ULOGIC_VECTOR(5 DOWNTO 0);
OUT STD_ULOGIC_VECTOR(15 DOWNTO 0) );

:
:
:
:
:

IN STD_ULOGIC;
IN STD_ULOGIC;
OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0) );

IN
IN
IN
IN
OUT

STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;

-- signals for components
SIGNAL pc_out_intern
: STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
BEGIN
-- instances of components
proc_cnt: pc
PORT MAP (
clk
=> clk,
rst_n
=> rst_n,
pc_in
=> pc_in,
PC_en
=> PC_en,
pc_out => pc_out_intern);
instr_reg : instreg
PORT MAP (
clk
=> clk,
rst_n
=> rst_n,
memdata => mem_data,
IRWrite => IRWrite,
instr_31_26 => instr_31_26,
instr_25_21 => instr_25_21,
instr_20_16 => instr_20_16,
instr_15_0 => instr_15_0 );
mem_data_reg :
PORT MAP (
clk
=>
rst_n
=>
reg_in =>
reg_out =>

tempreg
clk,
rst_n,
mem_data,
reg_memdata );

-- multiplexer
addr_mux : PROCESS(IorD, pc_out_intern, alu_out)
VARIABLE mem_address_temp : STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
BEGIN
IF IorD = '0' THEN
mem_address_temp := pc_out_intern;
ELSIF IorD = '1' THEN
mem_address_temp := alu_out;
ELSE

66



mem_address_temp := (OTHERS => 'X');
END IF;
mem_address <= mem_address_temp;
END PROCESS;
pc_out <= pc_out_intern;
END behave;

VHDLSource 8.26: a_data_fetch.vhd


67


LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF data_decode IS
COMPONENT regfile
PORT (clk,rst_n
wen
writeport
adrwport
adrport0
adrport1
readport0
readport1
);
END COMPONENT;

IS
: IN
: IN
: IN
: IN
: IN
: IN
: OUT
: OUT

COMPONENT tempreg
PORT (
clk
:
rst_n
:
reg_in
:
reg_out
:
END COMPONENT;

IS

std_ulogic;
std_ulogic;
-- write control
std_ulogic_vector(width-1 DOWNTO 0); -- register input
std_ulogic_vector(regfile_adrsize-1 DOWNTO 0);-- address write
std_ulogic_vector(width-1 DOWNTO 0); -- output port 0
std_ulogic_vector(width-1 DOWNTO 0)
-- output port 1

IN STD_ULOGIC;
IN STD_ULOGIC;
OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0) );

-- internal signals
SIGNAL write_reg
SIGNAL write_data
SIGNAL data_1
SIGNAL data_2

:
:
:
:

STD_ULOGIC_VECTOR(regfile_adrsize-1 DOWNTO 0);

BEGIN
A : tempreg
PORT MAP (
clk
=>
rst_n
=>
reg_in =>
reg_out =>

clk,
rst_n,
data_1,
reg_A );

B : tempreg
PORT MAP (
clk
=>
rst_n
=>
reg_in =>
reg_out =>

clk,
rst_n,
data_2,
reg_B );

inst_regfile :
PORT MAP (
clk
rst_n
wen
writeport
adrwport
adrport0
adrport1
readport0
readport1

regfile
=>
=>
=>
=>
=>
=>
=>
=>
=>

clk,
rst_n,
RegWrite,
write_data,
write_reg,
instr_25_21,
instr_20_16,
data_1,
data_2 );

-- multiplexer for write register
write_reg <= instr_20_16 WHEN RegDst = '0' ELSE
instr_15_0(15 DOWNTO 11) WHEN RegDst = '1' ELSE
(OTHERS => 'X');
-- multiplexer for write data
write_data <= alu_out WHEN MemtoReg = '0' ELSE
reg_memdata WHEN MemtoReg = '1' ELSE
(OTHERS => 'X');
-- sign extension and shift
proc_sign_ext : PROCESS(instr_15_0)
-- variables needed for reading result of sign extension
VARIABLE temp_instr_15_0_se : STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
VARIABLE temp_instr_15_0_se_sl : STD_ULOGIC_VECTOR(width-1 DOWNTO 0);
BEGIN

68



-- sign extend instr_15_0 to 32 bits
temp_instr_15_0_se := STD_ULOGIC_VECTOR(RESIZE(SIGNED(instr_15_0),
instr_15_0_se'LENGTH));
-- shift left 2
temp_instr_15_0_se_sl := temp_instr_15_0_se(width-3 DOWNTO 0) & "00";
instr_15_0_se <= temp_instr_15_0_se;
instr_15_0_se_sl <= temp_instr_15_0_se_sl;
END PROCESS;
END behave;

VHDLSource 8.27: a_data_decode.vhd

LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF data_execution IS
COMPONENT alu
PORT (
a, b
: IN
opcode : IN
result : OUT
zero
: OUT
);
END COMPONENT;
SIGNAL mux_A_out
SIGNAL mux_B_out

STD_ULOGIC

: std_ulogic_vector(width-1 downto 0);
: std_ulogic_vector(width-1 downto 0);

BEGIN
alu_inst: alu
PORT MAP (
a
b
opcode
result
zero
);

=>
=>
=>
=>
=>

mux_A_out,
mux_B_out,
ALUopcode,
alu_result,
zero

-- Multiplexor for ALU input A:
mux_A : PROCESS (ALUSrcA, PC_out, reg_A)
BEGIN
CASE ALUSrcA IS
WHEN '0' => mux_A_out <= PC_out;
WHEN '1' => mux_A_out <= reg_A;
WHEN OTHERS => mux_A_out <= (OTHERS => 'X');
END CASE;
END PROCESS;
-- Multiplexor for AlU input B:
mux_B : PROCESS (ALUSrcB, reg_B, instr_15_0_se, instr_15_0_se_sl)
BEGIN
CASE ALUSrcB IS
WHEN "00" => mux_B_out <= reg_B;
WHEN "01" => mux_B_out <= STD_ULOGIC_VECTOR(TO_UNSIGNED(4, width));
constant 4
WHEN "10" => mux_B_out <= instr_15_0_se;
WHEN "11" => mux_B_out <= instr_15_0_se_sl;
WHEN OTHERS => mux_B_out <= (OTHERS => 'X');
END CASE;
END PROCESS;

--

-- Computation of Jump Address:
jump_addr <= PC_out(width-1 downto width-4) & instr_25_21 & instr_20_16 & instr_15_0 &
"00";
END behave;

VHDLSource 8.28: a_data_execution.vhd


69


LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF data_memwriteback IS
COMPONENT tempreg
PORT (
clk
:
rst_n
:
reg_in
:
reg_out
:
);
END COMPONENT;

IN STD_ULOGIC;
IN STD_ULOGIC;
OUT STD_ULOGIC_VECTOR(width-1 DOWNTO 0)

SIGNAL alu_out_internal : std_ulogic_vector(width-1 downto 0);
BEGIN
tempreg_inst: tempreg
PORT MAP (
clk
rst_n
reg_in
reg_out
);

=>
=>
=>
=>

clk,
rst_n,
alu_result,
alu_out_internal

-- Multiplexor for ALU input A:
mux : PROCESS (PCSource, ALU_result, ALU_out_internal, jump_addr)
BEGIN
CASE PCSource IS
WHEN "00" => pc_in <= alu_result;
WHEN "01" => pc_in <= alu_out_internal;
WHEN "10" => pc_in <= jump_addr;
WHEN OTHERS => pc_in <= (OTHERS => 'X');
END CASE;
END PROCESS;
alu_out <= alu_out_internal;
END behave;

VHDLSource 8.29: a_data_memwriteback.vhd

70



LIBRARY IEEE;
-- use package
ARCHITECTURE behave OF data IS
COMPONENT data_fetch
PORT (
clk
: IN
rst_n
: IN
pc_in
: IN
alu_out
: IN
mem_data
: IN
PC_en
: IN
IorD
: IN
IRWrite
: IN
reg_memdata : OUT
instr_31_26 : OUT
instr_25_21 : OUT
instr_20_16 : OUT
instr_15_0 : OUT
mem_address : OUT
pc_out
: OUT
END COMPONENT;
COMPONENT data_decode
PORT (
clk
:
rst_n
:
instr_25_21
:
instr_20_16
:
instr_15_0
:
reg_memdata
:
alu_out
:
RegDst
:
RegWrite
:
MemtoReg
:
reg_A
:
reg_B
:
instr_15_0_se
:
instr_15_0_se_sl :
END COMPONENT;

STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC_VECTOR(width-1 DOWNTO
std_ulogic_vector(width-1 DOWNTO
STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;

IN
IN
IN
IN
IN
IN
IN
IN
IN
IN
OUT
OUT
OUT
OUT

COMPONENT data_execution
PORT (
instr_25_21
: IN
instr_20_16
: IN
instr_15_0
: IN
ALUSrcA
: IN
ALUSrcB
: IN
ALUopcode
: IN
reg_A, reg_B
: IN
pc_out
: IN
instr_15_0_se
: IN
instr_15_0_se_sl : IN
jump_addr
: OUT
alu_result
: OUT
zero
: OUT
END COMPONENT;

0);

0);
0));

STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;
STD_ULOGIC;

std_ulogic;
std_ulogic_vector(width-1 downto
std_ulogic);

COMPONENT data_memwriteback
PORT (
clk, rst_n : IN std_ulogic;
jump_addr : IN std_ulogic_vector(width-1 downto
alu_result : IN std_ulogic_vector(width-1 downto
PCSource
pc_in
: OUT std_ulogic_vector(width-1 downto
alu_out
: OUT std_ulogic_vector(width-1 downto
END COMPONENT;
SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL

0);
0);
0);

0);
0);

0);
0);
0);
0));

0);
0);
0);
0);
0);
0);

0);
0);
0);
0));

pc_in_intern : std_ulogic_vector(width-1 downto 0);
alu_out_intern : std_ulogic_vector(width-1 downto 0);
reg_memdata_intern : std_ulogic_vector(width-1 downto 0);
instr_25_21_intern : std_ulogic_vector(4 downto 0);


71


SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL
SIGNAL

pc_out_intern : std_ulogic_vector(width-1 downto 0);
reg_A_intern : std_ulogic_vector(width-1 downto 0);
reg_B_intern : std_ulogic_vector(width-1 downto 0);
instr_15_0_se_intern : std_ulogic_vector(width-1 downto 0);
instr_15_0_se_sl_intern : std_ulogic_vector(width-1 downto 0);
jump_addr_intern : std_ulogic_vector(width-1 downto 0);
alu_result_intern : std_ulogic_vector(width-1 downto 0);

BEGIN
inst_data_fetch: data_fetch
PORT MAP (
clk
=> clk,
rst_n
=> rst_n,
pc_in
=> pc_in_intern,
alu_out
=> alu_out_intern,
mem_data
=> mem_data,
PC_en
=> PC_en,
IorD
=> IorD,
IRWrite
=> IRWrite,
reg_memdata => reg_memdata_intern,
instr_31_26 => instr_31_26,
instr_25_21 => instr_25_21_intern,
mem_address => mem_address,
pc_out
=> pc_out_intern);
inst_data_decode : data_decode
PORT MAP (
clk
=> clk,
rst_n
=> rst_n,
instr_25_21
=> instr_25_21_intern,
instr_20_16
instr_15_0
reg_memdata
=> reg_memdata_intern,
alu_out
=> alu_out_intern,
RegDst
=> RegDst,
RegWrite
=> RegWrite,
MemtoReg
=> MemtoReg,
reg_A
=> reg_A_intern,
reg_B
=> reg_B_intern,
instr_15_0_se
=> instr_15_0_se_intern,
instr_15_0_se_sl => instr_15_0_se_sl_intern
);
inst_data_execution:
PORT MAP (
instr_25_21
instr_20_16
instr_15_0
ALUSrcA
ALUSrcB
ALUopcode
reg_A
reg_B
pc_out
instr_15_0_se
instr_15_0_se_sl
jump_addr
alu_result
zero
);

data_execution
=>
=>
=>
=>
=>
=>
=>
=>
=>
=>
=>
=>
=>
=>

instr_25_21_intern,
instr_20_16_intern,
instr_15_0_intern,
ALUSrcA,
ALUSrcB,
ALUopcode,
reg_A_intern,
reg_B_intern,
pc_out_intern,
instr_15_0_se_intern,
instr_15_0_se_sl_intern,
jump_addr_intern,
alu_result_intern,
zero

inst_data_memwriteback : data_memwriteback
PORT MAP (
clk
=> clk,
rst_n
=> rst_n,
jump_addr => jump_addr_intern,
alu_result => alu_result_intern,
PCSource
=> PCSource,
pc_in
=> pc_in_intern,
alu_out
=> alu_out_intern
);
reg_B <= reg_B_intern;

72


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