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UNIT I : BASIC STRUCTURE OF COMPUTERS
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Functional units – Basic operational concepts – Bus structures – Performance and metrics – Instructions and instruction sequencing – Hardware – Software Interface – Instruction set architecture – Addressing modes - RISC-CISC. ALU design – Fixed point and floating point operations.
Problem Sets
Two marks
An aid to my Dazzling Students
Thursday, January 17, 2013
RISC - CISC
RISC – CISC
Introduction
Introduction
The Instruction Set Architecture of a processor defines the
various functions that the processor can perform. The design of a processor and its instruction
set architecture are closely related. Depending on the design of the processor,
its instruction set may have different opcodes, addressing modes, instruction
formats, instruction length and so on.
It is based on the Instruction Set Architecture of a
particular processor, the high level language instructions are converted into its
corresponding machine instructions.
The term RISC and CISC refers to the design principles and techniques of a processor.
Defining RISC and
CISC :
The term RISC and CISC refers to the design principles and techniques of a processor.
- Defining RISC-CISC in terms of processor design
The RISC processors have simple design (usually
LOAD/STORE architecture) whereas CISC processors are complicated design(Memory-Memory
or Register-Memory Architecture). RISC is hardwired controlled whereas CISC is microprogram controlled.
- Defining RISC-CISC in terms of Instruction Set Architecture
RISC(Reduced Instruction Set Computer) have simple
instructions, so the average cycles per instruction (CPI) is less usually 1.
However, the compiled program (object code) has more lines of code, hence
requires more RAM. Since CPI is less, RISC enables efficient pipelining.
RISC
CISC(Complex Instruction Set Computer) have complex
instructions, so the average CPI is high(usually 2 to 15). However, the
compiled program has few lines of code, hence requires less RAM. Pipelining cannot
be efficiently implemented since CPI is high.
Performance Equation
The performance equation is given by
Execution time= Instruction count x CPI x Clock period
(or)
Execution time= ( N x S ) / R
For RISC, instruction count (N) is high and CPI is less. So
pipelining can be done efficiently.
For CISC, instruction count (N) is less and CPI is high.
Pipelining is not as efficient as RISC.
RISC and CISC : A
Comparison
The architectural distinction between RISC and CISC is shown
below:
RISC architecture uses separate instruction and data caches.
Their access paths are also different. CISC processor uses a unified cache for
both data and instructions; hence uses the same path.
Mostly the RISC processors are hardwired controlled whereas
the CISC processors are microprogammed control. Hence control memory is needed
in these processors.
A summarized comparison of RISC and CISC
RISC
|
CISC
|
Simple Processor design
|
Complicated Processor design
|
Uses Load-Store architecture(also called the Register- Register
architecture )
|
Uses Memory-Memory or Register-Memory architectures
|
RISC is hardwired controlled
|
CISC is microprogram-controlled
|
Uses simple instructions
|
Uses powerful, complex instructions
|
More lines of code in the object code
|
Less number of lines in the object code
|
Requires more memory and more instruction fetch
|
Requires less memory and less instruction fetch compared to RISC
|
The average cycles per instruction is less (usually 1)
|
The average cycles per instruction is high (usually 2 to 15)
|
Highly Pipelined
|
Pipelining is not as efficient as CISC
|
Uses fixed length instructions
|
Uses variable length instructions
|
Adressing mode is simple
|
Large variety of addressing modes
|
Uses large number of register sets
|
Less number of registers
|
Memory reference is less. Only LOAD, STORE instructions refer memory
|
Most of the instructions refer memory (LOAD and STORE incorporated in
the instruction itself)
|
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