cisc processor by tuan tuan

8/11/2019 CISC processor by Tuan Tuan

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What is CISC?

CISC stands for Complex Instruction Set Computer.

CISC are chips that are easy to program and whichmake efficient use of memory.

Examples of CISC processors are:

PDP-11

Motorola 68000 family

Intel x86/Pentium CPUs


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CISC HISTORY

The first PC microprocessors developed were CISC chips,because all the instructions the processor could execute

were built into the chip.

Memory was expensive in the early days of PCs, and CISCchips saved memory because their programming could befed directly into the processor.

CISC was developed to make compiler development

simpler. It shifts most of the burden of generating machineinstructions to the processor. For example, instead ofhaving to make a compiler write long machine instructionsto calculate a square-root, a CISC processor would have abuilt-in ability to do this.


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CIS PHILOSOPHY

The three decisions that led to the CISC philosophy, whichdrove all computer designs until the late 1980s, and is stillin major use today are the use Microcode, build richinstruction sets, and build high-level instruction sets.

Use Microcode: simple logic to control the data pathsbetween the various elements of the processor.

In a micro programmed system, the main processor hassome built-in memory (typically ROM) that containsgroups of microcode instructions which correspond with

each machine-language instruction. Since the microcode memory can be much faster than

main memory, an instruction set can be implemented inmicrocode without losing much speed over a purely hard-

wired implementation.


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CIS PHILOSOPHY

Build rich instruction sets: By using a microprogrammed design, designers could build morefunctionality into each instruction.

This design cut down on the total number of

instructions required to implement a program, so itmade more efficient use of a slow main memory.

Made the job for assembly-language programmersimpler

The enhancements included string manipulationoperations, special looping constructs, and specialaddressing modes for indexing through tables inmemory.


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CIS PHILOSOPHY

Build high-level instruction sets :After theprogrammer-friendly instruction sets were built, designersstarted to build instruction sets which map directly fromhigh-level languages.

Because microprogram instruction sets can be written tomatch the constructs of high-level languages, the compilerdoes not have to be as complicated.

Allows compilers to emit fewer instructions per line ofsource


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CHARATERICTICS OF CISC DESIGN

Instruction sets : CISC instruction sets have somecommon characteristics:

A 2-operand format, where instructions have a source and adestination.

Register to register, register to memory, and memory toregister commands.

Multiple addressing modes for memory, includingspecialized modes for indexing through arrays

Variable length instructions where the length often variesaccording to the addressing mode

Instructions which require multiple clock cycles toexecute.


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CHARATERICTICS OF CISC DESIGN

CISC and the Classic Performance EquationThe equation for determining performance is the sum for allinstructions of (the number of cycles per instruction * instruction cycletime) = 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 someinstructions, or speed up the clock (decrease the cycle time.)

CISC tries to reduce the number of instructions for a program


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THE ADVANTAGES OF CISC Microprogramming is as easy as assembly language to implement, and

much less expensive than hardwiring a control unit.

The ease of microcoding new instructions allowed designers to makeCISC machines upwardly compatible: a new computer could run thesame programs as earlier computers because the new computer would

contain a superset of the instructions of the earlier computers.

As each instruction became more capable, fewer instructions could beused to implement a given task. This made more efficient use of therelatively slow main memory.

Because microprogram instruction sets can be written to match theconstructs of high-level languages, the compiler does not have to be ascomplicated.


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THE DISADVANTAGES OF CISC As many instructions as possible could be stored in memory with the

least possible wasted space, individual instructions could be of almostany length this means that different instructions will take differentamounts of clock time to execute, slowing down the overallperformance of the machine.

Many specialized instructions aren't used frequently enough to justifytheir existence --- approximately 20% of the available instructions areused in a typical program.

CISC instructions typically set the condition codes as a side effect of theinstruction. Setting the condition codes take time, and programmers

have to remember to examine the condition code bits before asubsequent instruction changes them.


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cisc processor by tuan tuan

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