CISC
(Complex Instruction Set Computer)
Veeraraghavan Ramamurthy
ELEC 6200 Computer Architecture and Design
Fall 2005
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What is CISC?
A complex instruction set computer is a microprocessor instruction set
architecture (ISA) in which each instruction can execute several lowlevel operations such as a load from memory, an arithmetic operation,
and a memory store, all in a single instruction.
Most common microprocessor designs --- including the Intel 80x86 and
Motorola 68K series --- also follow the CISC philosophy.
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CISC philosophy
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use microcode
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build rich instruction sets
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build high-level instruction sets
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Characteristics of a CISC design
Instruction sets
The design constraints that led to the development of CISC (small
amounts of slow memory, and the fact that most early machines
were programmed in assembly language) give CISC instruction sets
some common characteristics:
A 2-operand format, where instructions have a source and a
destination. For example, consider the add instruction "add #5,
D0“
Register to register, register to memory, and memory to register
commands.
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Characteristics of a CISC design
Multiple addressing modes for memory, including specialized modes
for indexing through arrays
Variable length instructions where the length often varies
according to the addressing mode
Instructions which require multiple clock cycles to execute.
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Hardware architectures
Most CISC hardware architectures have several characteristics in
common:
Complex instruction-decoding logic.
A small number of general purpose registers.
Several special purpose registers. Many CISC designs set aside
special registers for the stack pointer, interrupt handling, and so on.
This can simplify the hardware design somewhat, at the expense of
making the instruction set more complex.
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Hardware architectures
A "Condition code" register which is set as a side-effect
of most instructions. This register reflects whether the
result of the last operation is less than, equal to, or
greater than zero, and records if certain error conditions
occur.
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The ideal CISC machine
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CISC processors were designed to execute each instruction
completely before beginning the next instruction.
An instruction is fetched from main memory .
The instruction is decoded.
The instruction is executed.
The results are written to memory.
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A realistic CISC machine
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In reality, some instructions may require more than one clock per
stage. However, a CISC design can tolerate this slowdown since the
idea behind CISC is to keep the total number of cycles small by
having complicated things happen within each cycle.
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CISC and the Classic Performance
Equation
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The usual equation for determining performance is the sum for all
instructions of (the number of cycles per instruction * instruction
cycle time) = execution time.
This allows you to speed up a processor in 3 different ways --- use
fewer instructions for a given task, reduce the number of cycles for
some instructions, or speed up the clock (decrease the cycle time.)
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CISC tries to reduce the number of instructions for a program, and
RISC tries to reduce the cycles per instruction.
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The advantages of CISC
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supports higher-level languages
smaller program sizes
fewer calls to main memory
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less RAM needed
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The disadvantages of CISC
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greater overhead in decoding instructions, therefore slowdown of
execution
designing the chips requires more work
harder to use registers efficiently
higher power consumption
many specialized instructions aren't used frequently enough to
justify their existence --- approximately 20% of the available
instructions are used in a typical program.
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