Instructor: Erol Sahin

Instructor:

Erol Sahin

Y86 Instruction Set Architecture – SEQ processorCENG331: Introduction to Computer Systems8th Lecture

Acknowledgement: Most of the slides are adapted from the ones prepared by R.E. Bryant, D.R. O’Hallaron of Carnegie-Mellon Univ.

– 2 –

Instruction Set ArchitectureAssembly Language View

Processor state Registers, memory, …

Instructionsaddl, movl, leal, … How instructions are encoded as bytes

Layer of Abstraction Above: how to program machine

Processor executes instructions in a sequence

Below: what needs to be built Use variety of tricks to make it run fast E.g., execute multiple instructions

simultaneously

ISA

Compiler OS

CPUDesign

CircuitDesign

ChipLayout

ApplicationProgram

– 3 –

%eax%ecx%edx%ebx

%esi%edi%esp%ebp

Y86 Processor State

Program Registers Same 8 as with IA32. Each 32 bits

Condition Codes Single-bit flags set by arithmetic or logical instructions

» OF: Overflow ZF: Zero SF:Negative Program Counter

Indicates address of instruction Memory

Byte-addressable storage arrayWords stored in little-endian byte order

Program registers Condition

codes

PC

Memory

OF ZF SF

– 4 –

Y86 Instructions

Format 1--6 bytes of information read from memory

Can determine instruction length from first byteNot as many instruction types, and simpler encoding than with IA32

Each accesses and modifies some part(s) of the program state

– 5 –

Encoding RegistersEach register has 4-bit ID

Same encoding as in IA32

Register ID 8 indicates “no register” Will use this in our hardware design in multiple places

%eax%ecx%edx%ebx

%esi%edi%esp%ebp

0123

6745

– 6 –

Instruction ExampleAddition Instruction

Add value in register rA to that in register rB Store result in register rBNote that Y86 only allows addition to be applied to register data

Set condition codes based on result e.g., addl %eax,%esi Encoding: 60 06 Two-byte encoding

First indicates instruction type Second gives source and destination registers

addl rA, rB 6 0 rA rB

Encoded Representation

Generic Form

%eax%ecx%edx%ebx

%esi%edi%esp%ebp

0123

6745

– 7 –

Arithmetic and Logical Operations Refer to generically as “OPl” Encodings differ only by

“function code” Low-order 4 bytes in first

instruction word Set condition codes as side

effect

addl rA, rB 6 0 rA rB

subl rA, rB 6 1 rA rB

andl rA, rB 6 2 rA rB

xorl rA, rB 6 3 rA rB

Add

Subtract (rA from rB)

And

Exclusive-Or

Instruction Code Function Code

– 8 –

Move Operations

Like the IA32 movl instruction Simpler format for memory addresses Give different names to keep them distinct

rrmovl rA, rB 2 0 rA rB Register --> Register

Immediate --> Registerirmovl V, rB 3 0 8 rB V

Register --> Memoryrmmovl rA, D(rB) 4 0 rA rB D

Memory --> Registermrmovl D(rB), rA 5 0 rA rB D

– 9 –

Move Instruction Examples

irmovl $0xabcd, %edx movl $0xabcd, %edx 30 82 cd ab 00 00

IA32 Y86 Encoding

rrmovl %esp, %ebx movl %esp, %ebx 20 43

mrmovl -12(%ebp),%ecxmovl -12(%ebp),%ecx 50 15 f4 ff ff ff

rmmovl %esi,0x41c(%esp)movl %esi,0x41c(%esp)

—movl $0xabcd, (%eax)

—movl %eax, 12(%eax,%edx)

—movl (%ebp,%eax,4),%ecx

40 64 1c 04 00 00

%eax%ecx%edx%ebx

%esi%edi%esp%ebp

0123

6745

– 10 –

Jump Instructions Refer to generically as “jXX” Encodings differ only by

“function code” Based on values of condition

codes Same as IA32 counterparts Encode full destination address

Unlike PC-relative addressing seen in IA32

jmp Dest 7 0

Jump Unconditionally

Dest

jle Dest 7 1

Jump When Less or Equal

Dest

jl Dest 7 2

Jump When Less

Dest

je Dest 7 3

Jump When Equal

Dest

jne Dest 7 4

Jump When Not Equal

Dest

jge Dest 7 5

Jump When Greater or Equal

Dest

jg Dest 7 6

Jump When Greater

Dest

– 11 –

Y86 Program Stack Region of memory holding program

data Used in Y86 (and IA32) for supporting

procedure calls Stack top indicated by %esp

Address of top stack element Stack grows toward lower addresses

Top element is at highest address in the stack

When pushing, must first decrement stack pointer

When popping, increment stack pointer

%esp

•••

IncreasingAddresses

Stack “Top”

Stack “Bottom”

– 12 –

Stack Operations

Decrement %esp by 4 Store word from rA to memory at %esp Like IA32

Read word from memory at %esp Save in rA Increment %esp by 4 Like IA32

pushl rA a 0 rA 8

popl rA b 0 rA 8

– 13 –

Subroutine Call and Return

Push address of next instruction onto stack Start executing instructions at Dest Like IA32

Pop value from stack Use as address for next instruction Like IA32

call Dest 8 0 Dest

ret 9 0

– 14 –

Miscellaneous Instructions

Don’t do anything

Stop executing instructions IA32 has comparable instruction, but can’t execute it in user

mode We will use it to stop the simulator

nop 0 0

halt 1 0

– 15 –

Y86 Code Generation ExampleFirst Try

Write typical array code

Compile with gcc -O2 -S

Problem Hard to do array indexing on Y86

Since don’t have scaled addressing modes

/* Find number of elements in null-terminated list */int len1(int a[]){ int len; for (len = 0; a[len]; len++)

; return len;}

L18:incl %eaxcmpl $0,(%edx,%eax,4)jne L18

– 16 –

Y86 Code Generation Example #2Second Try

Write with pointer code

Compile with gcc -O2 -S

Result Don’t need to do indexed

addressing

/* Find number of elements in null-terminated list */int len2(int a[]){ int len = 0; while (*a++)

len++; return len;}

L24:movl (%edx),%eaxincl %ecx

L26:addl $4,%edxtestl %eax,%eaxjne L24

– 17 –

Y86 Code Generation Example #3IA32 Code

SetupY86 Code

Setup

len2:pushl %ebpxorl %ecx,%ecxmovl %esp,%ebpmovl 8(%ebp),%edxmovl (%edx),%eaxjmp L26

len2:pushl %ebp # Save %ebpxorl %ecx,%ecx # len = 0rrmovl %esp,%ebp # Set framemrmovl 8(%ebp),%edx# Get amrmovl (%edx),%eax # Get *ajmp L26 # Goto entry

– 18 –

Y86 Code Generation Example #4IA32 Code

Loop + FinishY86 Code

Loop + Finish

L24:movl (%edx),%eaxincl %ecx

L26:addl $4,%edx

testl %eax,%eaxjne L24movl %ebp,%espmovl %ecx,%eaxpopl %ebpret

L24:mrmovl (%edx),%eax # Get *airmovl $1,%esiaddl %esi,%ecx # len++

L26: # Entry:irmovl $4,%esiaddl %esi,%edx # a++andl %eax,%eax # *a == 0?jne L24 # No--Looprrmovl %ebp,%esp # Poprrmovl %ecx,%eax # Rtn lenpopl %ebpret

– 19 –

Y86 Program Structure

Program starts at address 0

Must set up stackMake sure don’t

overwrite code! Must initialize data Can use symbolic

names

irmovl Stack,%esp # Set up stackrrmovl %esp,%ebp # Set up frameirmovl List,%edxpushl %edx # Push argumentcall len2 # Call Functionhalt # Halt

.align 4List: # List of elements

.long 5043

.long 6125

.long 7395

.long 0

# Functionlen2:

. . .

# Allocate space for stack.pos 0x100Stack:

– 20 –

Assembling Y86 Program

Generates “object code” file eg.yoActually looks like disassembler output

unix> yas eg.ys

0x000: 308400010000 | irmovl Stack,%esp # Set up stack 0x006: 2045 | rrmovl %esp,%ebp # Set up frame 0x008: 308218000000 | irmovl List,%edx 0x00e: a028 | pushl %edx # Push argument 0x010: 8028000000 | call len2 # Call Function 0x015: 10 | halt # Halt 0x018: | .align 4 0x018: | List: # List of elements 0x018: b3130000 | .long 5043 0x01c: ed170000 | .long 6125 0x020: e31c0000 | .long 7395 0x024: 00000000 | .long 0

– 21 –

Simulating Y86 Program

Instruction set simulatorComputes effect of each instruction on processor statePrints changes in state from original

unix> yis eg.yo

Stopped in 41 steps at PC = 0x16. Exception 'HLT', CC Z=1 S=0 O=0Changes to registers:%eax: 0x00000000 0x00000003%ecx: 0x00000000 0x00000003%edx: 0x00000000 0x00000028%esp: 0x00000000 0x000000fc%ebp: 0x00000000 0x00000100%esi: 0x00000000 0x00000004

Changes to memory:0x00f4: 0x00000000 0x000001000x00f8: 0x00000000 0x000000150x00fc: 0x00000000 0x00000018

Instructor:

Erol Sahin

CISC versus RISC CENG331: Introduction to Computer Systems8th Lecture


– 23 –

CISC Instruction Sets Complex Instruction Set Computer Dominant style through mid-80’s

Stack-oriented instruction set Use stack to pass arguments, save program counter (IA32 but not int x86-

64) Explicit push and pop instructions

Arithmetic instructions can access memory addl %eax, 12(%ebx,%ecx,4)

requires memory read and write Complex address calculation

Condition codes Set as side effect of arithmetic and logical instructions

Philosophy Add instructions to perform “typical” programming tasks

– 24 –

RISC Instruction Sets Reduced Instruction Set Computer Internal project at IBM, later popularized by Hennessy (Stanford) and

Patterson (Berkeley)

Fewer, simpler instructions Might take more to get given task done Can execute them with small and fast hardware

Register-oriented instruction set Many more (typically 32) registers Use for arguments, return pointer, temporaries

Only load and store instructions can access memory Similar to Y86 mrmovl and rmmovl

No Condition codes Test instructions return 0/1 in register

– 25 –

MIPS Registers

$0$1$2$3$4$5$6$7$8$9$10$11$12$13$14$15

$0$at$v0$v1$a0$a1$a2$a3$t0$t1$t2$t3$t4$t5$t6$t7

Constant 0Reserved Temp.

Return Values

Procedure arguments

Caller SaveTemporaries:May be overwritten bycalled procedures

$16$17$18$19$20$21$22$23$24$25$26$27$28$29$30$31

$s0$s1$s2$s3$s4$s5$s6$s7$t8$t9$k0$k1$gp$sp$s8$ra

Reserved forOperating Sys

Caller Save Temp

Global Pointer

Callee SaveTemporaries:May not beoverwritten bycalled procedures

Stack PointerCallee Save TempReturn Address

– 26 –

MIPS Instruction Examples

Op Ra Rb Offset

Op Ra Rb Rd Fn00000R-R

Op Ra Rb ImmediateR-I

Load/Store

addu $3,$2,$1 # Register add: $3 = $2+$1

addu $3,$2, 3145 # Immediate add: $3 = $2+3145sll $3,$2,2 # Shift left: $3 = $2 << 2

lw $3,16($2) # Load Word: $3 = M[$2+16]sw $3,16($2) # Store Word: M[$2+16] = $3

Op Ra Rb OffsetBranch

beq $3,$2,dest # Branch when $3 = $2

– 27 –

CISC vs. RISC

Original Debate Strong opinions! CISC proponents---easy for compiler, fewer code bytes RISC proponents---better for optimizing compilers, can make run fast

with simple chip design

Current Status For desktop processors, choice of ISA not a technical issue

With enough hardware, can make anything run fastCode compatibility more important

For embedded processors, RISC makes sense Smaller, cheaper, less power

– 28 –

Summary

Y86 Instruction Set Architecture Similar state and instructions as IA32 Simpler encodings Somewhere between CISC and RISC

How Important is ISA Design? Less now than before

With enough hardware, can make almost anything go fast AMD/Intel moved away from IA32

Does not allow enough parallel execution x86-64

» 64-bit word sizes (overcome address space limitations)» Radically different style of instruction set with explicit parallelism» Requires sophisticated compilers

Instructor:

Erol Sahin

Logic Design and HCLCENG331: Introduction to Computer Systems8th Lecture


– 30 –

Computing with Logic Gates

Outputs are Boolean functions of inputs Respond continuously to changes in inputs

With some, small delay

ab out

ab out a out

out = a && b out = a || b out = !a

And Or Not

Voltage

Time

a

ba && b

Rising Delay Falling Delay

– 31 –

Combinational Circuits

Acyclic Network of Logic Gates Continously responds to changes on primary inputs Primary outputs become (after some delay) Boolean functions of

primary inputs

Acyclic Network

PrimaryInputs

PrimaryOutputs

– 32 –

Bit Equality

Generate 1 if a and b are equal

Hardware Control Language (HCL) Very simple hardware description language

Boolean operations have syntax similar to C logical operations We’ll use it to describe control logic for processors

Bit equala

b

eqbool eq = (a&&b)||(!a&&!b)

HCL Expression

– 33 –

Word Equality

32-bit word size HCL representation

Equality operationGenerates Boolean value

b31Bit equal

a31

eq31

b30Bit equal

a30

eq30

b1Bit equal

a1

eq1

b0Bit equal

a0

eq0

Eq

=B

A

Eq

Word-Level Representation

bool Eq = (A == B)

HCL Representation

– 34 –

Bit-Level Multiplexor

Control signal s Data signals a and b Output a when s=1, b when s=0

Bit MUX

b

s

a

out

bool out = (s&&a)||(!s&&b)

HCL Expression

– 35 –

Word Multiplexor

Select input word A or B depending on control signal s

HCL representationCase expression Series of test : value pairsOutput value for first successful test

Word-Level Representation

HCL Representation

b31

s

a31

out31

b30

a30

out30

b0

a0

out0

int Out = [ s : A; 1 : B;];

s

B

AOutMUX

– 36 –

HCL Word-Level Examples

Find minimum of three input words

HCL case expression Final case guarantees matchA

Min3MIN3BC

int Min3 = [ A < B && A < C : A; B < A && B < C : B; 1 : C;];

D0

D3

Out4

s0s1

MUX4D2D1

Select one of 4 inputs based on two control bits

HCL case expression Simplify tests by

assuming sequential matching

int Out4 = [ !s1&&!s0: D0; !s1 : D1; !s0 : D2; 1 : D3;];

Minimum of 3 Words

4-Way Multiplexor

– 37 –

OFZFCF

OFZFCF

OFZFCF

OFZFCF

Arithmetic Logic Unit

Combinational logicContinuously responding to inputs

Control signal selects function computedCorresponding to 4 arithmetic/logical operations in Y86

Also computes values for condition codes

ALU

Y

X

X + Y

0

ALU

Y

X

X - Y

1

ALU

Y

X

X & Y

2

ALU

Y

X

X ^ Y

3

A

B

A

B

A

B

A

B

– 38 –

Registers

Stores word of dataDifferent from program registers seen in assembly code

Collection of edge-triggered latches Loads input on rising edge of clock

I O

Clock

DC Q+

DC Q+

DC Q+

DC Q+

DC Q+

DC Q+

DC Q+

DC Q+

i7

i6

i5

i4

i3

i2

i1

i0

o7

o6

o5

o4

o3

o2

o1

o0

Clock

Structure

– 39 –

Register Operation

Stores data bits For most of time acts as barrier between input and output As clock rises, loads input

State = x

RisingclockOutput = xInput = y

x State = y

Output = y

y

– 40 –

State Machine Example

Accumulator circuit Load or accumulate

on each cycle

Comb. Logic

ALU

0

OutMUX

0

1

Clock

InLoad

x0 x1 x2 x3 x4 x5

x0 x0+x1 x0+x1+x2 x3 x3+x4 x3+x4+x5

Clock

Load

In

Out

– 41 –

Random-Access Memory

Stores multiple words of memoryAddress input specifies which word to read or write

Register fileHolds values of program registers %eax, %esp, etc.Register identifier serves as address

» ID 8 implies no read or write performed Multiple Ports

Can read and/or write multiple words in one cycle» Each has separate address and data input/output

Registerfile

A

B

W dstW

srcA

valA

srcB

valB

valW

Read ports Write port

Clock

– 42 –

Register File TimingReading

Like combinational logic Output data generated based on input

address After some delay

Writing Like register Update only as clock rises

Registerfile

A

B

srcA

valA

srcB

valB

y2

Registerfile

W dstW

valW

Clock

x2Risingclock Register

fileW dstW

valW

Clock

y2

x2

x

2

– 43 –

Hardware Control Language Very simple hardware description language Can only express limited aspects of hardware operation

Parts we want to explore and modify

Data Types bool: Boolean

a, b, c, … int: words

A, B, C, …Does not specify word size---bytes, 32-bit words, …

Statements bool a = bool-expr ; int A = int-expr ;

– 44 –

HCL Operations Classify by type of value returned

Boolean Expressions Logic Operations

a && b, a || b, !a Word Comparisons

A == B, A != B, A < B, A <= B, A >= B, A > B Set Membership

A in { B, C, D }» Same as A == B || A == C || A == D

Word Expressions Case expressions

[ a : A; b : B; c : C ]Evaluate test expressions a, b, c, … in sequenceReturn word expression A, B, C, … for first successful test

– 45 –

Summary

Computation Performed by combinational logic Computes Boolean functions Continuously reacts to input changes

Storage Registers

Hold single words Loaded as clock rises

Random-access memoriesHold multiple wordsPossible multiple read or write portsRead word when address input changesWrite word as clock rises

Instructor:

Erol Sahin

SEQuential processor implementationCENG331: Introduction to Computer Systems8th Lecture


– 47 –

Y86 Instruction SetByte 0 1 2 3 4 5

pushl rA A 0 rA 8

jXX Dest 7 fn Dest

popl rA B 0 rA 8

call Dest 8 0 Dest

rrmovl rA, rB 2 0 rA rB

irmovl V, rB 3 0 8 rB V

rmmovl rA, D(rB) 4 0 rA rB D

mrmovl D(rB), rA 5 0 rA rB D

OPl rA, rB 6 fn rA rB

ret 9 0

nop 0 0

halt 1 0

addl 6 0

subl 6 1

andl 6 2

xorl 6 3

jmp 7 0

jle 7 1

jl 7 2

je 7 3

jne 7 4

jge 7 5

jg 7 6

– 48 –

SEQ Hardware StructureState

Program counter register (PC) Condition code register (CC) Register File Memories

Access same memory space Data: for reading/writing program data Instruction: for reading instructions

Instruction Flow Read instruction at address specified by

PC Process through stages Update program counter

Instructionmemory

Instructionmemory

PCincrement

PCincrement

CCCCALUALU

Datamemory

Datamemory

Fetch

Decode

Execute

Memory

Write back

icode, ifunrA , rB

valC

Registerfile

Registerfile

A BM

E

Registerfile

Registerfile

A BM

E

PC

valP

srcA, srcBdstA, dstB

valA, valB

aluA, aluB

Bch

valE

Addr, Data

valM

PCvalE, valM

newPC

– 49 –

SEQ StagesFetch

Read instruction from instruction memory

Decode Read program registers

Execute Compute value or address

Memory Read or write data

Write Back Write program registers

PC Update program counter

Instructionmemory

Instructionmemory

PCincrement

PCincrement

CCCCALUALU

Datamemory

Datamemory

Fetch

Decode

Execute

Memory

Write back

icode, ifunrA , rB

valC

Registerfile

Registerfile

A BM

E

Registerfile

Registerfile

A BM

E

PC

valP

srcA, srcBdstA, dstB

valA, valB

aluA, aluB

Bch

valE

Addr, Data

valM

PCvalE, valM

newPC

– 50 –

Instruction Decoding

Instruction Format Instruction byte icode:ifun Optional register byte rA:rB Optional constant word valC

5 0 rA rB D

icodeifun

rArB

valC

Optional Optional

– 51 –

Executing Arith./Logical Operation

Fetch Read 2 bytes

Decode Read operand registers

Execute Perform operation Set condition codes

Memory Do nothing

Write back Update register

PC Update Increment PC by 2


– 52 –

Stage Computation: Arith/Log. Ops

Formulate instruction execution as sequence of simple steps Use same general form for all instructions

OPl rA, rBicode:ifun M1[PC]rA:rB M1[PC+1] valP PC+2

Fetch

Read instruction byteRead register byte Compute next PC

valA R[rA]valB R[rB]

DecodeRead operand ARead operand B

valE valB OP valASet CC

ExecutePerform ALU operationSet condition code register

Memory R[rB] valE

Writeback

Write back result

PC valPPC update Update PC

– 53 –

Executing rmmovl

Fetch Read 6 bytes

Decode Read operand registers

Execute Compute effective address

Memory Write to memory

Write back Do nothing



– 54 –

Stage Computation: rmmovl

Use ALU for address computation

rmmovl rA, D(rB)icode:ifun M1[PC]rA:rB M1[PC+1]valC M4[PC+2]valP PC+6

Fetch

Read instruction byteRead register byteRead displacement DCompute next PC



valE valB + valCExecute

Compute effective address

M4[valE] valAMemory Write value to memory

Writeback


– 55 –

Executing popl

Fetch Read 2 bytes

Decode Read stack pointer

Execute Increment stack pointer by 4

Memory Read from old stack pointer

Write back Update stack pointer Write result to register


popl rA b 0 rA 8

– 56 –

Stage Computation: popl

Use ALU to increment stack pointer Must update two registers

Popped valueNew stack pointer

popl rAicode:ifun M1[PC]rA:rB M1[PC+1] valP PC+2

Fetch

Read instruction byteRead register byte Compute next PC

valA R[%esp]valB R [%esp]

DecodeRead stack pointerRead stack pointer

valE valB + 4Execute

Increment stack pointer

valM M4[valA]Memory Read from stack R[%esp] valER[rA] valM

Writeback

Update stack pointerWrite back result


– 57 –

Executing Jumps

Fetch Read 5 bytes Increment PC by 5

Decode Do nothing

Execute Determine whether to take

branch based on jump condition and condition codes

Memory Do nothing

Write back Do nothing

PC Update Set PC to Dest if branch taken or

to incremented PC if not branch

jXX Dest 7 fn Dest

XX XXfall thru:

XX XXtarget:

Not taken

Taken

– 58 –

Stage Computation: Jumps

Compute both addresses Choose based on setting of condition codes and branch condition

jXX Desticode:ifun M1[PC]

valC M4[PC+1]valP PC+5

Fetch

Read instruction byte

Read destination addressFall through address

Decode

Bch Cond(CC,ifun)Execute

Take branch? Memory

Writeback

PC Bch ? valC : valPPC update Update PC

– 59 –

Executing call

Fetch Read 5 bytes Increment PC by 5


Execute Decrement stack pointer by 4

Memory Write incremented PC to new

value of stack pointer

Write back Update stack pointer

PC Update Set PC to Dest

call Dest 8 0 Dest

XX XXreturn:

XX XXtarget:

– 60 –

Stage Computation: call

Use ALU to decrement stack pointer Store incremented PC

call Desticode:ifun M1[PC]


Fetch


Read destination address Compute return point

valB R[%esp]Decode

Read stack pointervalE valB + –4

ExecuteDecrement stack pointer

M4[valE] valP Memory Write return value on stack R[%esp] valE

Writeback

Update stack pointer

PC valCPC update Set PC to destination

– 61 –

Executing ret

Fetch Read 1 byte


Execute Increment stack pointer by 4

Memory Read return address from old

stack pointer

Write back Update stack pointer

PC Update Set PC to return address

ret 9 0

XX XXreturn:

– 62 –

Stage Computation: ret

Use ALU to increment stack pointer Read return address from memory

reticode:ifun M1[PC]

Fetch


valA R[%esp]valB R[%esp]

DecodeRead operand stack pointerRead operand stack pointer

valE valB + 4Execute

Increment stack pointer

valM M4[valA] Memory Read return addressR[%esp] valE

Writeback

Update stack pointer

PC valMPC update Set PC to return address

– 63 –

Computation Steps

All instructions follow same general pattern Differ in what gets computed on each step

OPl rA, rBicode:ifun M1[PC]rA:rB M1[PC+1] valP PC+2

Fetch

Read instruction byteRead register byte[Read constant word]Compute next PC



valE valB OP valASet CC

ExecutePerform ALU operationSet condition code register

Memory [Memory read/write] R[rB] valE

Writeback

Write back ALU result[Write back memory result]


icode,ifunrA,rBvalCvalPvalA, srcAvalB, srcBvalECond codevalMdstEdstMPC

– 64 –

Irmovl: Computation Steps


irmovl V, rBicode:ifun M1[PC]rA:rB M1[PC+1] valC M4[PC+2]valP PC+6

Fetch

Read instruction byteRead register byte[Read constant word]Compute next PC

Decode

valE 0 + valCExecute

Perform ALU operation

MemoryR[rB] valE

Writeback

Write back ALU result



– 65 –

Computation Steps


call Dest

Fetch

Decode

Execute

MemoryWritebackPC update


icode:ifun M1[PC]


valB R[%esp]valE valB + –4

M4[valE] valP R[%esp] valE PC valC

Read instruction byte[Read register byte]Read constant wordCompute next PC[Read operand A]Read operand BPerform ALU operation[Set condition code reg.][Memory read/write] [Write back ALU result]Write back memory resultUpdate PC

– 66 –

Computed ValuesFetch

icode Instruction codeifun Instruction functionrA Instr. Register ArB Instr. Register BvalC Instruction constantvalP Incremented PC

DecodesrcA Register ID AsrcB Register ID BdstE Destination Register EdstM Destination Register MvalA Register value AvalB Register value B

Execute valE ALU result Bch Branch flag

Memory valM Value from

memory

– 67 –

SEQ HardwareKey

Blue boxes: predesigned hardware blocks

E.g., memories, ALU Gray boxes: control logic

Describe in HCL White ovals: labels

for signals Thick lines: 32-bit

word values Thin lines: 4-8 bit

values Dotted lines: 1-bit

valuesInstructionmemory

Instructionmemory

PCincrement

PCincrement

CCCC ALUALU

Datamemory

Datamemory

NewPC

rB

dstE dstM

ALUA

ALUB

Mem.control

Addr

srcA srcB

read

write

ALUfun.

Fetch

Decode

Execute

Memory

Write back

data out

Registerfile

Registerfile

A BM

E

Registerfile

Registerfile

A BM

E

Bch

dstE dstM srcA srcB

icode ifun rA

PC

valC valP

valBvalA

Data

valE

valM

PC

newPC

– 68 –

Fetch Logic

Predefined Blocks PC: Register containing PC Instruction memory: Read 6 bytes (PC to PC+5) Split: Divide instruction byte into icode and ifun Align: Get fields for rA, rB, and valC

Instructionmemory

Instructionmemory

PCincrement

PCincrement

rBicode ifun rA

PC

valC valP

Needregids

NeedvalC

Instrvalid

AlignAlignSplitSplit

Bytes 1-5Byte 0

– 69 –

Fetch Logic

Control Logic Instr. Valid: Is this instruction valid? Need regids: Does this instruction have a register bytes? Need valC: Does this instruction have a constant word?

Instructionmemory

Instructionmemory

PCincrement

PCincrement

rBicode ifun rA

PC

valC valP

Needregids

NeedvalC

Instrvalid

AlignAlignSplitSplit

Bytes 1-5Byte 0

– 70 –

Fetch Control Logic

pushl rA A 0 rA 8

jXX Dest 7 fn Dest

popl rA B 0 rA 8

call Dest 8 0 Dest

rrmovl rA, rB 2 0 rA rB

irmovl V, rB 3 0 8 rB V


mrmovl D(rB), rA 5 0 rA rB D


ret 9 0

nop 0 0

halt 1 0

pushl rA A 0 rA 8pushl rA A 0A 0 rA 8rA 8

jXX Dest 7 fn DestjXX Dest 7 fn7 fn Dest

popl rA B 0 rA 8popl rA B 0B 0 rA 8rA 8

call Dest 8 0 Destcall Dest 8 08 0 Dest

rrmovl rA, rB 2 0 rA rBrrmovl rA, rB 2 02 0 rA rBrA rB

irmovl V, rB 3 0 8 rB Virmovl V, rB 3 03 0 8 rB8 rB V

rmmovl rA, D(rB) 4 0 rA rB Drmmovl rA, D(rB) 4 04 0 rA rBrA rB D

mrmovl D(rB), rA 5 0 rA rB Dmrmovl D(rB), rA 5 05 0 rA rBrA rB D

OPl rA, rB 6 fn rA rBOPl rA, rB 6 fn6 fn rA rBrA rB

ret 9 0ret 9 09 0

nop 0 0nop 0 00 0

halt 1 0halt 1 01 0

bool need_regids =icode in { IRRMOVL, IOPL, IPUSHL, IPOPL,

IIRMOVL, IRMMOVL, IMRMOVL };

bool instr_valid = icode in { INOP, IHALT, IRRMOVL, IIRMOVL, IRMMOVL, IMRMOVL, IOPL, IJXX, ICALL, IRET, IPUSHL, IPOPL };

– 71 –

Decode Logic

Register File Read ports A, B Write ports E, M Addresses are register IDs or 8 (no

access)

rB

dstE dstM srcA srcB

Registerfile

Registerfile

A BM

EdstE dstM srcA srcB

icode rA

valBvalA valEvalM

Control Logic srcA, srcB: read port

addresses dstA, dstB: write port

addresses

– 72 –

A Source OPl rA, rBvalA R[rA]Decode Read operand A

rmmovl rA, D(rB)valA R[rA]Decode Read operand A

popl rAvalA R[%esp]Decode Read stack pointer

jXX DestDecode No operand

call Dest

valA R[%esp]Decode Read stack pointerret

Decode No operand

int srcA = [icode in { IRRMOVL, IRMMOVL, IOPL, IPUSHL } : rA;icode in { IPOPL, IRET } : RESP;1 : RNONE; # Don't need register

];

– 73 –

E Destination

None

R[%esp] valE Update stack pointer

None

R[rB] valEOPl rA, rB

Write-back

rmmovl rA, D(rB)

popl rA

jXX Dest

call Dest

ret

Write-back

Write-back

Write-back

Write-back

Write-back

Write back result



int dstE = [icode in { IRRMOVL, IIRMOVL, IOPL} : rB;icode in { IPUSHL, IPOPL, ICALL, IRET } : RESP;1 : RNONE; # Don't need register

];

– 74 –

Execute LogicUnits

ALU Implements 4 required functions Generates condition code values

CC Register with 3 condition code bits

bcond Computes branch flag

Control Logic Set CC: Should condition code register

be loaded? ALU A: Input A to ALU ALU B: Input B to ALU ALU fun: What function should ALU

compute?

CCCC ALUALU

ALUA

ALUB

ALUfun.

Bch

icode ifun valC valBvalA

valE

SetCC

bcondbcond

– 75 –

ALU A Input

valE valB + –4 Decrement stack pointer

No operation

valE valB + 4 Increment stack pointer

valE valB + valC Compute effective address

valE valB OP valA Perform ALU operationOPl rA, rB

Execute

rmmovl rA, D(rB)

popl rA

jXX Dest

call Dest

ret

Execute

Execute

Execute

Execute

Execute valE valB + 4 Increment stack pointer

int aluA = [icode in { IRRMOVL, IOPL } : valA;icode in { IIRMOVL, IRMMOVL, IMRMOVL } : valC;icode in { ICALL, IPUSHL } : -4;icode in { IRET, IPOPL } : 4;# Other instructions don't need ALU

];

– 76 –

ALU Operation

valE valB + –4 Decrement stack pointer

No operation

valE valB + 4 Increment stack pointer

valE valB + valC Compute effective address

valE valB OP valA Perform ALU operationOPl rA, rB

Execute

rmmovl rA, D(rB)

popl rA

jXX Dest

call Dest

ret

Execute

Execute

Execute

Execute

Execute valE valB + 4 Increment stack pointer

int alufun = [icode == IOPL : ifun;1 : ALUADD;

];

– 77 –

Memory Logic

Memory Reads or writes memory word

Control Logic Mem. read: should word be

read? Mem. write: should word be

written? Mem. addr.: Select address Mem. data.: Select data

Datamemory

Datamemory

Mem.read

Memaddr

read

write

data out

Memdata

valE

valM

valA valP

Mem.write

data in

icode

– 78 –

Memory AddressOPl rA, rB

Memory

rmmovl rA, D(rB)

popl rA

jXX Dest

call Dest

ret

No operation


valM M4[valA]Memory Read from stack

M4[valE] valP Memory Write return value on stack

valM M4[valA] Memory Read return address

Memory No operation

int mem_addr = [icode in { IRMMOVL, IPUSHL, ICALL, IMRMOVL } : valE;icode in { IPOPL, IRET } : valA;# Other instructions don't need address

];

– 79 –

Memory ReadOPl rA, rB

Memory

rmmovl rA, D(rB)

popl rA

jXX Dest

call Dest

ret

No operation


valM M4[valA]Memory Read from stack

M4[valE] valP Memory Write return value on stack

valM M4[valA] Memory Read return address

Memory No operation

bool mem_read = icode in { IMRMOVL, IPOPL, IRET };

– 80 –

PC Update Logic

New PC Select next value of PC

NewPC

Bchicode valC valPvalM

PC

– 81 –

PCUpdate

OPl rA, rB

rmmovl rA, D(rB)

popl rA

jXX Dest

call Dest

ret




PC Bch ? valC : valPPC update Update PC

PC valCPC update Set PC to destination

PC valMPC update Set PC to return address

int new_pc = [icode == ICALL : valC;icode == IJXX && Bch : valC;icode == IRET : valM;1 : valP;

];

– 82 –

SEQ Operation

State PC register Cond. Code register Data memory Register fileAll updated as clock rises

Combinational Logic ALU Control logic Memory reads

Instruction memoryRegister fileData memory

CombinationalLogic Data

memoryData

memory

Registerfile

Registerfile

PC0x00c

CCCCReadPorts

WritePorts

Read WriteRead Write

– 83 –


memoryData

memory

Registerfile

%ebx = 0x100

Registerfile

%ebx = 0x100

PC0x00c

CC100CC100

ReadPorts

WritePorts


0x00c: addl %edx,%ebx # %ebx <-- 0x300 CC <-- 000

0x00e: je dest # Not taken

Cycle 3:

Cycle 4:

0x006: irmovl $0x200,%edx # %edx <-- 0x200Cycle 2:

0x000: irmovl $0x100,%ebx # %ebx <-- 0x100Cycle 1:

ClockCycle 1 Cycle 2 Cycle 3 Cycle 4

SEQ Operation #2

state set according to second irmovl instruction

combinational logic starting to react to state changes

– 84 –



Cycle 3:

Cycle 4:




SEQ Operation #3

state set according to second irmovl instruction

combinational logic generates results for addl instruction


memoryData

memory

Registerfile

%ebx = 0x100

Registerfile

%ebx = 0x100

PC0x00c

CC100CC100

ReadPorts

WritePorts

0x00e

000


– 85 –



Cycle 3:

Cycle 4:




SEQ Operation #4

state set according to addl instruction

combinational logic starting to react to state changes


memoryData

memory

Registerfile

%ebx = 0x300

Registerfile

%ebx = 0x300

PC0x00e

CC000CC000

ReadPorts

WritePorts


– 86 –



Cycle 3:

Cycle 4:




SEQ Operation #5

state set according to addl instruction

combinational logic generates results for je instruction


memoryData

memory

Registerfile

%ebx = 0x300

Registerfile

%ebx = 0x300

PC0x00e

CC000CC000

ReadPorts

WritePorts

0x013


memoryData

memory

Registerfile

%ebx = 0x300

Registerfile

%ebx = 0x300

PC0x00e

CC000CC000

ReadPorts

WritePorts

0x013


– 87 –

SEQ Summary

Implementation Express every instruction as series of simple steps Follow same general flow for each instruction type Assemble registers, memories, predesigned combinational blocks Connect with control logic

Limitations Too slow to be practical In one cycle, must propagate through instruction memory, register file,

ALU, and data memory Would need to run clock very slowly Hardware units only active for fraction of clock cycle

Documents

Instructor: Erol Sahin