1. Prepared By: Kind Kishor
Tribhuvan University

2. Topics to be Included
1.
2.
3.
4.
5.
6.
Description of basic computer.
Common bus system.
Instruction formats and their execution.
Instruction cycle.
Hardwired control unit.
IO configuration and IO handling.

3. Basic Concepts
Computer Architecture:
 It is those attributes of a system that have a direct impact on the
logical execution of a program.
 It is concerned with the structure and behavior of the computer as
seen by the user.
 It includes:
 The instruction set.
 The number of bits used to represent various data types.
 I/O mechanisms.
 Memory addressing techniques.

4. Basic Concepts
Computer Organization:
 It refers to the operational units and their interconnections that
realize the architectural specifications.
 It is concerned with the way the hardware components operate
and the way they are connected together to form the computer
system.
 Examples are things that are transparent to the programmer:
 control signals.
 interfaces between computer and peripherals.
 the memory technology being used.

5. Description of Basic Computer

6. Description of Basic Computer
The central processing unit(CPU) for manipulating the
data, registers for storing data and instructions, and
control circuits for fetching and executing instructions.
The memory of a computer contains storage for
instructions and data.
The input-output processor contains electronic circuits
for communicating and controlling the transfer of
information between the computer and user.

7. Memory Hierarchy

8. Instruction Codes
Program:
A program is a set of instructions that specify the operations,
operands and the sequence by which processing has to
occur.
Computer Instruction:
A computer instruction is a binary code that specifies a
sequence of microoperations for the computer.
Instruction Code:
An instruction code is a group of bits that instruct the
computer to perform a specific operation.

9. Instruction Codes
Operation Code:
The operation code of an instruction is a group of bits that
define such operations as add, subtract, multiply, shift, and
complement.
Micro-operation:
Micro-operation is the elementry operation performed with
the data stored in the registers.
Types of Micro-Operation:
a)Register transfer.
b)Arithmetic transfer.
c)Logic.
d)Shift.

10. Instruction Format
An instruction format or instruction code is a group of
bits used to perform a particular operation on the data
stored in computer.
Processor fetches an instruction from memory and
decodes the bits to execute the instruction.
Instruction code is divided into two parts namely
operation code and address of data.
Operation code consisting group of bits to define an
operation such as add, subtract, multiply etc.

11. Instruction Format
In an instruction format:
First 12 bits (0-11) specify an address.
Next 3 bits specify operation code (opcode), or type of
operation.
Left most bit specify the addressing mode I
I = 0 for direct address
I = 1 for indirect address

12. Addressing Modes

13. Types of Instructions
On the basis of opcode and addressing mode, the
basic computer has three 16-bit instruction code
formats:
1. Memory Reference Instructions.
2. Register Reference Instructions.
3. Input/Output Instructions.

14. Memory Reference Instructions
First 12 bits (0-11) specify an address.
3 bits of opcode are used to specify the types of instruction.
Value of opcode ranges from 000 to 110.
If I=0, it is direct addressing mode and if I=1, it is indirect
addressing mode.

15. Register Reference Instructions
First 12 bits (0-11) specify the register operation.
The next three bits equals to 111 specify opcode.
The last mode bit of the instruction is 0 for register reference
instruction.
Therefore, left most 4 bits are always 0111 which is equal to
hexadecimal 7.

16. Input/Output Instructions
First 12 bits (0-11) specify the I/O operation.
The next three bits equals to 111 specify opcode.
The last mode bit of the instruction is 1.
Therefore, left most 4 bits are always 1111 which is equal to
hexadecimal F.

17. Input/Output and Interrupt

18. Input/Output and Interrupt




The terminal sends and receives serial information.
The serial info. from the keyboard is shifted into INPR.
The serial info. for the printer is stored in the OUTR.
INPR and OUTR communicate with the terminal serially and with
the AC in parallel.
 The flags are needed to synchronize the timing difference between
I/O device and the computer

19. Registers in Basic Computer

20. Common Bus System
• A wire or a collection of wires that carry some multi-bit
information is known as bus. Main purpose of bus is to transfer
information form one system to another.
• It is also known as communication path way which connects all the
internal components of the computer to the CPU, main memory
and I/O devices.

21. Common Bus System
• Three functional groups of communication lines:
A. Data lines (data bus) - move data between system modules. Width
is a key factor in determining overall system performance.
B. Address lines - designate source or destination of data on the data
bus. Width determines the maximum possible memory capacity
of the system.
C. Control lines - control access to and use of the data and address
lines.

22. C
O
M
M
O
N
B
U
S
S
Y
S
T
E
M

23. Control Unit
CPU is divided into Arithmetic Logic Unit (ALU) and Control Unit
(CU).
The function of control unit is to generate relevant timing and
control signals to all operations in the computer.
It controls the flow of data between the processor and memory
and peripherals.
The control unit directs the entire computer system to carry out
stored program instructions.
The control unit co-ordinates the activities of the other two
units(ALU and main memory) as well as all peripherals and
auxiliary storage devices linked to the computer.

24. Control Unit
Control units are implemented using one of the two
organizations:
i.
ii.
Hardwired Control Unit: In this, the control logic is
implemented with gates, flip flops, decoders and other
digital circuits.
Micro-programmed Control Unit: A control memory on the
processor contains microprograms that activate the
necessary control signals.

25. Control Unit of Basic Computer
(Hardwired)
Fig:-Control unit of basic computer

26. Hardwired Control Unit
Control unit consist of:
o
o
o
o
Instruction Register
Number of Control Logic Gates,
Two Decoders
4-bit Sequence Counter
An instruction read from memory is placed in the instruction
register (IR).
The instruction register is divided into three parts: the I bit,
operation code, and address part.
First 12-bits (0-11) to specify an address, next 3-bits specify the
operation code (opcode) field of the instruction and last left most
bit specify the addressing mode I.
I = 0 for direct address
I = 1 for indirect address

27. Hardwired Control Unit
First 12-bits (0-11) are applied to the control logic gates.
The operation code bits (12 – 14) are decoded with a 3 x 8
decoder.
The eight outputs ( D0 through D7) from a decoder goes to the
control logic gates to perform specific operation.
Last bit 15 is transferred to a I flip-flop designated by symbol I.
The 4-bit sequence counter SC can count in binary from 0
through 15.
The counter output is decoded into 16 timing pulses T0 through
T15.
The sequence counter can be incremented by INR input or clear
by CLR input synchronously.

28. Instruction Cycle
Processing required for a single instruction is called an instruction
cycle.
Basic instruction cycle consists of the following phases:
a.
b.
c.
d.
Fetch and instruction from memory.
Decode the instruction.
Read the effective address from the memory, if the instruction has an
indirect address.
Execute the instruction.

29. Instruction Cycle
The instruction cycle can be broken down into two major phases:
 Fetch - CPU reads an instruction from a location in memory
 Program counter (PC) register keeps track of which instruction executes
next.
 Normally, CPU increments PC after each fetch.
 Fetched instruction is loaded into the instruction register (IR).
 Execute - CPU executes the instruction
 The instruction is held in IR is decoded.
 Then, the instruction is executed.
 May utilize previously changed state of CPU and (indirectly) other devices.

30. Instruction Cycle
Fig:- register transfers for the fetch phase

31. Interrupt Cycle
Fig:- flowchart for interrupt cycle

32. Interrupt Cycle
The interrupt cycle is a HW implementation of a branch
and save return address operation.
At the beginning of the next instruction cycle, the
instruction that is read from memory is in address 1.
At memory address 1, the programmer must store a
branch instruction that sends the control to an interrupt
service routine.
The instruction that returns the control to the original
program is "indirect BUN 0“.

33. References
1. M. Morris Mano, “Computer System Architecture”, third
edition.
2. William Stallings, “Computer Organization and Architecture”
3. http://www.edunep.com,
4. http://www.eazynotes.com/notes/computer-systemarchitecture/slides/registers-and-common-bus.pdf
5. http://www.eazynotes.com/notes/computer-systemarchitecture/slides/timing-and-control-unit-handouts.pdf
6. http://www.dauniv.ac.in/downloads/CArch_PPTs/CompArchC
h05L13HardwiredControl.pdf
7. http://www.slideshare.net/anujmodi555/computer-instructions