This document discusses CPU organization, instruction formats, addressing modes, and characteristics of RISC and CISC architectures. It covers general register and stack organizations, different instruction formats like zero, one, two, and three address formats. It also covers various addressing modes and describes key differences between RISC and CISC designs like instruction complexity, number of registers, and use of microprogramming versus hardwired control units. The document discusses performance considerations for RISC versus CISC in terms of instructions per program and cycles per instruction.
7. Instruction Formats
Depend on the organization of the computer. There
are mainly following types of organization:
Accumulator Based Organization
Register Based Organization
Stack Based Organization
8. Instruction Formats
Zero Address Instruction
One Address Instruction
Two Address Instruction
Three Address Instruction
RISC Instruction
9. Addressing Modes
Implied Mode
Immediate Mode
Register Mode
Register Indirect Mode
Auto increment or Auto decrement Mode
Direct Addressing Mode
Indirect Addressing Mode
Relative Addressing Mode
Indexed Addressing Mode
Base Register Addressing Mode
10. CISC
CISC is a philosophy for designing chips that are easy to
program and which make efficient use of memory.
It stands for - Complex Instruction Set Computer
Each instruction in a CISC instruction set might perform a
series of operations inside the processor.
This reduces the number of instructions required to
implement a given program. In general terms, the
instruction sets are designed for the convenience of the
assembly-language programmer
11. CISC…
The CISC philosophy made more sense, since the earliest
machines were programmed in assembly language and
memory was slow and expensive
The design constraints that led to the development of CISC
are
• small amounts of slow memory and
• the fact that most early machines were programmed in
assembly language
12. CISC: Microprogramming
The earliest processor designs used dedicated (hardwire)
logic to decode and execute each instruction in the
processor's instruction set.
This worked well for simple designs with few registers, but
made more complex architectures hard to build, as control
path logic can be hard to implement.
So, designers switched tactics - they built some simple logic
to control the data paths between the various elements of
the processor, and used a simplified microcode instruction
set to control the data path logic. This type of
implementation is known as a microprogrammed
implementation.
13. Ideal CISC machine
CISC processors were designed to execute each instruction
completely before beginning the next instruction. (Similar
to Run To Completion (RTC) model in Co-operative
Schedulers)
Even so, most processors break the execution of an
instruction into several definite stages; as soon as one stage
is finished, the processor passes the result to the next stage
14. Ideal CISC machine…
Four stages in a typical CISC machine:
• An instruction is fetched from main memory.
• The instruction is decoded: the controlling code from the
microprogram identifies the type of operation to be performed,
where to find the data on which to perform the operation, and
where to put the result. If necessary, the processor reads in
additional information from memory.
• The instruction is executed: the controlling code from the
microprogram determines the circuitry/hardware that will
perform the operation.
• The results are written to memory.
15. CISC advantages:
As each instruction is more capable, fewer instructions
could be used to implement a given task. This made more
efficient use of the relatively slow main memory.
Microprogramming is as easy as assembly language to
implement, and much less expensive than hardwiring a
control unit.
The ease of micro coding new instructions allowed
designers to make CISC machines upwardly compatible.
Because microprogram instruction sets can be written to
match the constructs of high-level languages, the compiler
does not have to be as complicated.
16. CISC disadvantages:
Many specialized instructions aren't used frequently
enough to justify their existence - approximately 20% of the
available instructions are used in a typical program.
Earlier generations of a processor family generally were
contained as a subset in every new version - this made the
machines compatible - but the instruction set & chip
hardware become more complex with each generation of
computers.
Different instructions take different amount of clock time to
execute, due to their variable length, slowing down the
overall performance of the machine.
17. RISC
RISC is a microprocessor that is designed to perform a
smaller number computer instructions so that it can
operate at a higher speed.
It stands for - Reduced Instruction Set Computer
John Cocke of IBM Research in Yorktown, New York,
originated the RISC concept in 1974 by proving that about
20% of the instructions in a computer did 80% of the work.
The term itself (RISC) is credited to David Patterson, a
teacher at the University of California in Berkeley
19. RISC’s advantages
Speed.
Since it uses pipelining concept, its speed is faster
Simpler hardware.
Because the instruction set of a RISC processor is so simple,
it uses up much less chip space
Shorter design cycle.
Since RISC processors are simpler than corresponding
CISC processors, they can be designed more quickly
20. RISC’s disadvantages
Code Quality
If the programmer (or compiler) does a poor job of instruction
scheduling, the processor can spend quite a bit stalling:
waiting for the result of one instruction before it can proceed
with a subsequent instruction.
Since the scheduling rules can be complicated, most
programmers use a high level language (such as C or C++) and
leave the instruction scheduling to the compiler.
This makes the performance of a RISC application depend
critically on the quality of the code generated by the compiler.
Therefore, developers (and development tool suppliers such as
Apple) have to choose their compiler carefully based on the
quality of the generated code.
21. RISC’s disadvantages…
Code expansion
Code expansion refers to the increase in size that you get
when you take a program that had been compiled for a
CISC machine and re-compile it for a RISC machine. The
exact expansion depends primarily on the quality of the
compiler and the nature of the machine's instruction set.
Since CISC machines perform complex actions with a
single instruction, when RISC machines may require
multiple instructions for the same action, code expansion
can be a problem.
22. RISC’s disadvantages…
System Design
They require more instructions, and hence memory, than
CISCs to implement applications.
Another problem the RISC machines faces is that they
require very fast memory systems to feed them
instructions.
23. Classic Performance Equation
The Performance Equation
The following equation is commonly used for expressing a
computer's performance ability:
time time cycles instructions
Program cycle instruction program
24. CISC’s Performance Equation
CISC approach attempts to minimize the number of
instructions per program, sacrificing the number of cycles
per instruction.
time time cycles instructions
Program cycle instruction program
25. RISC’s Performance Equation
RISC does the opposite, reducing the cycles per instruction
at the cost of the number of instructions per program.
time time cycles instructions
Program cycle instruction program
26. RISC vs CISC
CISC RISC
Complex instructions require
multiple cycles
Reduced instructions take 1 cycle
Many instructions can reference
memory
Only Load and Store instructions
can reference memory
Instructions are executed one at
a time
Uses pipelining to execute
instructions
Few general registers Many general registers
Use micro programmed Control
Unit
Use hardwired Control unit
Large Number of instructions
and many addressing modes
Few instructions and less
addressing modes
27. Why CISC still lives?
Why are there still CISC CPUs being developed?
Why is Intel spending time and money to manufacture the Pentium III and
Pentium 4?
Answer
The answer is simple, backward compatibility.
The IBM compatible PC is the most common computer in the world. Intel
wanted a CPU that would run all the applications that are in the hands of more
than 100 million users.
