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Computer ArchitectureComputer Architecture

Part II-B: CPU Instruction Set

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Levels of RepresentationLevels of Representation

High Level Language Program

Assembly Language Program

Machine Language Program

Control Signal Specification

Compiler

Assembler

Machine Interpretation

temp = v[k];

v[k] = v[k+1];

v[k+1] = temp;

mov BL, 02hmov AX,0A56hmul BL

0000 1001 1100 0110 1010 1111 0101 10001010 1111 0101 1000 0000 1001 1100 0110 1100 0110 1010 1111 0101 1000 0000 1001 0101 1000 0000 1001 1100 0110 1010 1111

ALUOP[0:3] <= InstReg[9:11] & MASK

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Instruction Set DesignInstruction Set Design

instruction set

software

hardware

Which is easier to change?

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Instruction SetInstruction Set

The set of machine level instructions for a particular type of computer or hardware

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Instruction CycleInstruction Cycle

The sequence of operations involved in processing an instruction

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Instruction CycleInstruction CycleInstruction

Fetch

Instruction

Decode

Operand

Fetch

Execute

Result

Store

Next

Instruction

Obtain instruction from program storage

Determine required actions and instruction size

Locate and obtain operand data

Compute result, value or status

Deposit results in storage for later use

Determine successor instruction

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CPU-Memory HandshakeCPU-Memory Handshake

The CPU operates, then memory There is a need for synchronization

between CPU and memory Can be done through handshaking

Memory can inform the CPU that it has completed its read/write request through a Memory Function Complete (MFC) signal

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Types of ControlTypes of Control

Hardwired Microprogrammed

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Hardwired ControlHardwired Control

Pure hardware mode of control The control unit tells the rest of the CPU

what is to be done every clock cycle during the execution of instructions

Involves complex circuit design

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Hardwired Control ConsiderationsHardwired Control Considerations

Fast Everything is done through hardware

Permanent Difficult and costly to do modifications

Difficult to implement in a large instruction set

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Microprogrammed ControlMicroprogrammed Control

Each instruction is broken down into a series of steps, i.e. microinstructions (or microcode) The control unit becomes a miniature

computer The complete set of steps for each

instruction is stored in a ROM called the control store, and is executed sequentially

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Microprogrammed ControlMicroprogrammed Control

Decoding involves obtaining the starting address in the control store of the first control word (or microinstruction) for the instruction

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Microprogrammed Control Microprogrammed Control Considerations Considerations

Flexible Only the control store needs

modifications to implement/modify instruction execution

Slower May require more hardware

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Microprogrammed Control UnitsMicroprogrammed Control Units

Introduced by IBM in 1964 Microprogrammed control units have

seen widespread use for several reasons: Easy to implement complex instructions Cost of microprogramming is far less than

the cost of implementing a conventional hardwired control unit

Easy to add instructions and/or modify existing ones

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CISC and RISCCISC and RISC

There are two schools of thought on instruction set design: Complex instruction set computers

(CISC) Reduced instruction set computers

(RISC)

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Complex Instruction Set Complex Instruction Set Computers (CISC)Computers (CISC)

Upward compatibility New processor designs had to provide

more power over older models while maintaining compatibility

More complex and powerful instructions! Generally requires the use of

microcode Easily benefits from improved

technology

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CISCCISC

Richer or more complex instruction sets simplify compiler design Reduced software development and

learning costs More powerful instructions means

fewer instructions are needed to implement software routines Reduced system memory and bandwidth

requirements

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CISC IssuesCISC Issues

The more complex the instruction set, the larger the required control store

Compilers tend to make use of few of the available instructions

80/20 rule: Analysis of the instruction mix generated by CISC machines shows that 80% of the instructions executed used only 20% of the instructions set

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CISC IssuesCISC Issues

CISC designers may have introduced a lot of instructions simply because it was easy to do OR because its competitors did so

Manufacturing cost is increased because of the need to test all operations and combinations of operations

Design cost is increased for future systems because of the need to maintain upward or forward compatibility

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Reduced Instruction Set Reduced Instruction Set Computers (RISC)Computers (RISC)

80/20 rule: If the most often used instructions (the 20%) was speeded up then the performance benefits would be greater

Studies showed that these instructions tend to perform simpler operations and would use only the simpler addressing modes

Complex operations can still be done by using several simpler operations

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Why RISC?Why RISC?

Basic hardware is simpler to implement Easier to adopt hardwired control.

Easier to compile for a RISC than a CISC architecture

Design effort and development cost is less for RISC than for a CISC processor

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RISC CPU CharacteristicsRISC CPU Characteristics

Fewer instructions More CPU registers More instructions needed to execute

routines May mean more memory space and

increased bandwidth requirements