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RecapRecap

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Instruction Set Architecture (ISA)• The ISA is the interface between hardware and software.

• The ISA serves as an abstraction layer between the HW and SW– Software doesn’t need to know how the processor is implemented– Any processor that implements the ISA appears equivalent

• An ISA enables processor innovation without changing software– This is how Intel has made billions of dollars.

• Before ISAs, software was re-written/re-compiled for each new machine.

Software

Proc #1

ISA

Proc #2

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What is instruction set architecture (ISA)?

• Instruction set of a computer: the portion of the computer visible to the assembly level programmer or to the compiler writer ISA– Defines registers

– Defines data transfer modes (instructions) between registers, memory and I/O

– There should be sufficient instructions to efficiently translate any program

• Next, define instruction set format – binary representation used by the hardware– Variable-length vs. fixed-length instructions

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Growth/Variations of Processors1400

1300

1200

1100

1000

900

800

700

600

500

400

300

200

100

01998 2000 2001 20021999

Other

SPARC

Hitachi SH

PowerPC

Motorola 68K

MIPS

IA-32

ARM

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Instruction Set Instruction Set ArchitecturesArchitectures

• There are 4 classes of ISA

– Stack

• Embedded processors

– Accumulator

• Embedded processors

– Register-memory

• Pentium

– Register-Register (Load-store)

• Sparc

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Instruction Set Instruction Set ArchitectureArchitecture

• For a general purpose high-performance computer

– Register-Register (Load-store) is the preferred choice

• CPI uniform for most instructions

– Better pipelining

• Fixed length instructions

– Simpler encoding

• The complier is more complicated

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Important issues to Important issues to consider for designing an consider for designing an

ISAISA

• What are the different addressing modes that should be used

• What is the length of instructions

• What are the different instructions used

• What is the type and size of operands

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Addressing Modes

Addressing mode Example Meaning

Register Add R4,R3 R4 R4+R3

Immediate Add R4,#3 R4 R4+3

Displacement Add R4,100(R1) R4 R4+Mem[100+R1]

Register indirect Add R4,(R1) R4 R4+Mem[R1]

Indexed / Base Add R3,(R1+R2) R3 R3+Mem[R1+R2]

Direct or absolute Add R1,(1001) R1 R1+Mem[1001]

Memory indirect Add R1,@(R3) R1 R1+Mem[Mem[R3]]

Auto-increment Add R1,(R2)+ R1 R1+Mem[R2]; R2 R2+d

Auto-decrement Add R1,–(R2) R2 R2-d; R1 R1+Mem[R2]

Scaled Add R1,100(R2)[R3] R1 R1+Mem[100+R2+R3*d]

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Addressing Modes

0 10 20 30 40 50 60

Displacement

I mmediate

Register indirect

Scaled

Memory indirect

Frequency of addressing mode

avggccspiceTeX

75%

85%

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Addressing Modes

Register (direct) mode

Example: ADD $r1, $r2, $r3

op: opcode

rs: first source register

rt: second source register

rd: destination register

op rs rt rd

register

op 1 2 3

100

$r1 = 100

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Addressing Modes

Immediate mode

Example: ADDI $r3, $r1, 12

op rs rt immed

op 3 1 12

op: opcode

rs: source register

rt: destination register

immed: constant

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Addressing Modes

Displacement mode

Example: LOAD $r1, 100($r2)

op rs rt immed

register

Memory

+

$r2 = 150; 100($r2) = 88

op 2 1 100

150

Memory

88+Address = 250

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Displacement and Immediate Displacement and Immediate ValuesValues

• Important addressing modes: Register, immediate, displacement, register indirect. Account for 88% of workload.

• Through measurements, 12-16 bits are enough to cover the majority of cases for the value of immediate

• This will allow to have instruction sets of fixed size of 32 bits – most microprocessors

opcode rs rt

6 5 5

rd

5

shamt

5

func

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R-Type Format

opcode rs rt immediate

6 5 5 16

I-Type Format

opcode immediate

6 26

J-Type Format

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Instruction Usage Instruction Usage Example:Example:

Top 10 Intel X86 Top 10 Intel X86 InstructionsInstructions

Rank Integer Average Percent total executed

1

2

3

4

5

6

7

8

9

10

instruction

load

conditional branch

compare

store

add

and

sub

move register-register

call

return

Total

Observation: Simple instructions dominate instruction usage frequency.Observation: Simple instructions dominate instruction usage frequency.

22%

20%

16%

12%

8%

6%

5%

4%

1%

1%

96%

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Instruction Set Instruction Set EncodingEncoding

Considerations affecting instruction set encoding:

– To have as many registers and address modes as possible.

– The Impact of the size of the register and addressing mode fields on the average instruction size and on the average program.

– To encode instructions into lengths that will be easy to handle in the implementation. On a minimum to be a multiple of bytes.

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Instruction FormatInstruction Format• Fixed

– Operation, address specifier 1, address specifier 2, address specifier 3.– MIPS, SPARC, Power PC.

• Variable– Operation & # of operands, address specifier1, …, specifier n.– VAX

• Hybrid– Intel x86– operation, address specifier, address field.– Operation, address specifier 1, address specifier 2, address field.– Operation, address field, address specifier 1, address specifier 2.

• Summary:– If code size is most important, use variable format.– If performance is most important, use fixed format.

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Three Examples of Instruction Set Three Examples of Instruction Set EncodingEncoding

Variable: VAX (1-53 bytes)

Operations &no of operands

Addressspecifier 1

Addressfield 1

Address specifier n

Address field n

Operation Addressfield 1

Addressfield 2

Addressfield3

Fixed: DLX, MIPS, PowerPC, SPARC

Operation Address Specifier

Addressfield

Operation AddressSpecifier 1

AddressSpecifier 2

Address field

OperationAddress Specifier Address

field 1

Address field 2

Hybrid : IBM 360/370, Intel 80x86

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Summary: ISASummary: ISA• Use general purpose registers with a load-store

architecture. • Support these addressing modes: displacement,

immediate, register indirect.• Support these simple instructions: load, store, add,

subtract, move register, shift, compare equal, compare not equal, branch, jump, call, return.

• Support these data size: 8-,16-,32-bit integer, IEEE FP standard.

• Provide at least 16 general purpose registers plus separate FP registers and aim for a minimal instruction set.

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• Design alternative:

– provide more powerful operations

– goal is to reduce number of instructions executed

– danger is a slower cycle time and/or a higher CPI

• Let’s look (briefly) at IA-32

Alternative Architectures

–“The path toward operation complexity is thus fraught with peril.

To avoid these problems, designers have moved toward simpler

instructions”

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IA - 32• 1978: The Intel 8086 is announced (16 bit architecture)• 1980: The 8087 floating point coprocessor is added• 1982: The 80286 increases address space to 24 bits, +instructions• 1985: The 80386 extends to 32 bits, new addressing modes• 1989-1995: The 80486, Pentium, Pentium Pro add a few instructions

(mostly designed for higher performance)• 1997: 57 new “MMX” instructions are added, Pentium II• 1999: The Pentium III added another 70 instructions (SSE)• 2001: Another 144 instructions (SSE2)• 2003: AMD extends the architecture to increase address space to 64 bits,

widens all registers to 64 bits and other changes (AMD64)• 2004: Intel capitulates and embraces AMD64 (calls it EM64T) and adds

more media extensions• “This history illustrates the impact of the “golden handcuffs” of compatibility:

“adding new features as someone might add clothing to a packed bag”“an architecture that is difficult to explain and impossible to love”

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IA-32 Overview• Complexity:

– Instructions from 1 to 17 bytes long

– one operand must act as both a source and destination

– one operand can come from memory

– complex addressing modese.g., “base or scaled index with 8 or 32 bit displacement”

• Saving grace:

– the most frequently used instructions are not too difficult to build

– compilers avoid the portions of the architecture that are slow

“what the 80x86 lacks in style is made up in quantity, making it beautiful from the right perspective”