Multiple cycle implementation

Each instruction takes more than one clock cycles to execution

Q: How to break an instruction? Break each instruction into a series of

steps corresponding to the functional unit operations Execution steps

Abstract of multicycle datapath

3 main func. units: memory, register, ALU

PC

Memory

Address

Instructionor data

Data

Instructionregister

Registers

Register #

Data

Register #

Register #

ALU

Memorydata

register

A

B

ALUOut

buffers

Change from single-cycle to multi-cycle

Usage of functional units A functional unit can be used more than on

ce per instruction on different clock cycles Ex. Instruction memory, data memory -> one m

emory Ex. ALU, adder -> single ALU

Temporary registers as buffers after operational units IR, MDR, A, B, ALUout 除了 IR 外，不需 write control signal 。 IR

需保存指令至指令結束

Ref: next page

同一功能單元可在同一指令的不同時脈中使用

不同時脈間的執行結果需存在暫存器中

Recall: single-cycle implementation

PC

Instructionmemory

Readaddress

Instruction[31– 0]

Instruction [15– 11]

Instruction [20– 16]

Instruction [25– 21]

Add

Instruction [5– 0]

MemtoReg

ALUOp

MemWrite

RegWrite

MemRead

BranchRegDst

ALUSrc

Instruction [31– 26]

4

16 32Instruction [15– 0]

0

0Mux

0

1

ALUcontrol

Control

Shiftleft 2

Add ALUresult

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Signextend

Mux

1

ALUresult

Zero

Datamemory

Writedata

Readdata

Mux

1ALU

Address

Change from single-cycle to multi-cycle (cont.) How to select input to shared functio

nal units? Use Multiplexor Ex. ALU input

Input1: register file 1 PC Input2: register file 2 constant 4 sign-extended offset shifted offset

Ref: prev. page

Shiftleft 2

PC

Memory

MemData

Writedata

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Mux

0

1

Mux

0

1

4

Instruction[15– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

1 Mux

0

3

2

Mux

ALUresult

ALUZero

Memorydata

register

Instruction[15– 11]

A

B

ALUOut

0

1

Address

Multicycle datapath for MIPS: add MUXs

Shiftleft 2

MemtoReg

IorD MemRead MemWrite

PC

Memory

MemData

Writedata

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

ALUOpALUSrcB

RegDst RegWrite

Instruction[15– 0]

Instruction [5– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

1 Mux

0

3

2

ALUcontrol

Mux

0

1ALU

resultALU

ALUSrcA

ZeroA

B

ALUOut

IRWrite

Address

Memorydata

register

Multicycle datapath for MIPS: add control lines

Do notneed writecontrol

Possible sources for PC1. Normal: PC+42. Branch: calculate branch target

from ALU3. Jump:

6 bits 26 bits

PC

31… 28

26 bits 00

J-format

Jump target address

Shiftleft 2

PCMux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

Instruction[15– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

ALUcontrol

ALUresult

ALUZero

Memorydata

register

A

B

IorD

MemRead

MemWrite

MemtoReg

PCWriteCond

PCWrite

IRWrite

ALUOp

ALUSrcB

ALUSrcA

RegDst

PCSource

RegWrite

Control

Outputs

Op[5– 0]

Instruction[31-26]

Instruction [5– 0]

Mux

0

2

Jumpaddress [31-0]Instruction [25– 0] 26 28

Shiftleft 2

PC [31-28]

1

1 Mux

0

3

2

Mux

0

1ALUOut

Memory

MemData

Writedata

Address

Multicycle datapath for MIPS: add PC control lines

PCsource

PC control lines

0 1

Break instruction execution into clock cycles

PC

Memory

Address

Instructionor data

Data

Instructionregister

Registers

Register #

Data

Register #

Register #

ALU

Memorydata

register

A

B

ALUOut

Clock cycle length 時脈長度 : longest of these 3 operations

Maximally 5 steps to execute 3 to 5 steps to execute an

instructionInstruction

fetchData/register

readInstructionexecution

Memory/registerread/write

Registerwrite

IR=Memory[PC];PC=PC+4

A=Reg[IR[25-21]];B=Reg[IR[20-16]];calc. branch address

Memory referenceR-type inst.BranchJump

Shiftleft 2

PCMux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

Instruction[15– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

ALUcontrol

ALUresult

ALUZero

Memorydata

register

A

B

IorD

MemRead

MemWrite

MemtoReg

PCWriteCond

PCWrite

IRWrite

ALUOp

ALUSrcB

ALUSrcA

RegDst

PCSource

RegWrite

Control

Outputs

Op[5– 0]

Instruction[31-26]

Instruction [5– 0]

Mux

0

2

Jumpaddress [31-0]Instruction [25– 0] 26 28

Shiftleft 2

PC [31-28]

1

1 Mux

0

3

2

Mux

0

1ALUOut

Memory

MemData

Writedata

AddressPCWrite

PCSource = 10

ALUSrcA = 1ALUSrcB = 00ALUOp = 01PCWriteCond

PCSource = 01

ALUSrcA =1ALUSrcB = 00ALUOp= 10

RegDst = 1RegWrite

MemtoReg = 0

MemWriteIorD = 1

MemReadIorD = 1

ALUSrcA = 1ALUSrcB = 10ALUOp = 00

RegDst = 0RegWrite

MemtoReg =1

ALUSrcA = 0ALUSrcB = 11ALUOp = 00

MemReadALUSrcA = 0

IorD = 0IRWrite

ALUSrcB = 01ALUOp = 00

PCWritePCSource = 00

Instruction fetchInstruction decode/

register fetch

Jumpcompletion

BranchcompletionExecution

Memory addresscomputation

Memoryaccess

Memoryaccess R-type completion

Write-back step

(Op

= 'J

')

(Op =

'LW

')

4

01

9862

753

Start

Shiftleft 2

PCMux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

Instruction[15– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

ALUcontrol

ALUresult

ALUZero

Memorydata

register

A

B

IorD

MemRead

MemWrite

MemtoReg

PCWriteCond

PCWrite

IRWrite

ALUOp

ALUSrcB

ALUSrcA

RegDst

PCSource

RegWrite

Control

Outputs

Op[5– 0]

Instruction[31-26]

Instruction [5– 0]

Mux

0

2

Jumpaddress [31-0]Instruction [25– 0] 26 28

Shiftleft 2

PC [31-28]

1

1 Mux

0

3

2

Mux

0

1ALUOut

Memory

MemData

Writedata

Address

PCWritePCSource = 10

ALUSrcA = 1ALUSrcB = 00ALUOp = 01PCWriteCond

PCSource = 01

ALUSrcA =1ALUSrcB = 00ALUOp= 10

RegDst = 1RegWrite

MemtoReg = 0

MemWriteIorD = 1

MemReadIorD = 1

ALUSrcA = 1ALUSrcB = 10ALUOp = 00

RegDst = 0RegWrite

MemtoReg =1

ALUSrcA = 0ALUSrcB = 11ALUOp = 00

MemReadALUSrcA = 0

IorD = 0IRWrite

ALUSrcB = 01ALUOp = 00

PCWritePCSource = 00

Instruction fetchInstruction decode/

register fetch

Jumpcompletion

BranchcompletionExecution

Memory addresscomputation

Memoryaccess

Memoryaccess R-type completion

Write-back step

(Op

= 'J

')

(Op =

'LW

')

4

01

9862

753

Start

Example: Memory reference (load)

Shiftleft 2

MemtoReg

IorD MemRead MemWrite

PC

Memory

MemData

Writedata

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

ALUOpALUSrcB

RegDst RegWrite

Instruction[15– 0]

Instruction [5– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

1 Mux

0

3

2

ALUcontrol

Mux

0

1ALU

resultALU

ALUSrcA

ZeroA

B

ALUOut

IRWrite

Address

Memorydata

register

lw $t1, offset($t2)

3. ALUout=A+sign(IR[15-0]); ALUSrcA=1,ALUSrcB=10

4. MDR = Memory[ALUout]; IorD=1, MemRead=1,

5. Reg[IR[20-16]]=MDR; RegWrite=1, RegDst=0, MemtoReg=1

Quiz: R-type

Shiftleft 2

MemtoReg

IorD MemRead MemWrite

PC

Memory

MemData

Writedata

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

ALUOpALUSrcB

RegDst RegWrite

Instruction[15– 0]

Instruction [5– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

1 Mux

0

3

2

ALUcontrol

Mux

0

1ALU

resultALU

ALUSrcA

ZeroA

B

ALUOut

IRWrite

Address

Memorydata

register

Add $t1, $t2, $t3

Instructionfetch

Data/registerread

Instructionexecution

Memory/registerread/write

Registerwrite

Control for multicycle datapath

Control for multi-cycle datapath

Recall: we build truth table in single cycle datapath

Multicycle: different control signals at the series of steps in a instruction

2 methods Finite state machine microprogramming

指令中每個 cycle 都需要不同的控制訊號

Idea for multi-cycle control 在 datapath 中每個階段都需要不同的

控制訊號Instruction

fetchData/register

readInstructionexecution

Memory/registerread/write

Registerwrite

ControlSignal 1

ControlSignal 2

Memory referenceR-type inst.BranchJump

state statetransition function

Finite state machine

PCWritePCSource = 10

ALUSrcA = 1ALUSrcB = 00ALUOp = 01PCWriteCond

PCSource = 01

ALUSrcA =1ALUSrcB = 00ALUOp= 10

RegDst = 1RegWrite

MemtoReg = 0

MemWriteIorD = 1

MemReadIorD = 1

ALUSrcA = 1ALUSrcB = 10ALUOp = 00

RegDst = 0RegWrite

MemtoReg =1

ALUSrcA = 0ALUSrcB = 11ALUOp = 00

MemReadALUSrcA = 0

IorD = 0IRWrite

ALUSrcB = 01ALUOp = 00

PCWritePCSource = 00

Instruction fetchInstruction decode/

register fetch

Jumpcompletion

BranchcompletionExecution

Memory addresscomputation

Memoryaccess

Memoryaccess R-type completion

Write-back step

(Op

= 'J

')

(Op =

'LW

')

4

01

9862

753

Start

Preview of FSM for control

Inst. Fetch+Reg. read

execution

MemoryRead/write

Write backregister

MemWriteIorD = 1

MemReadIorD = 1

ALUSrcA = 1ALUSrcB = 10ALUOp = 00

RegWriteMemtoReg = 1

RegDst = 0

Memory address computation

(Op = 'LW') or (Op = 'SW')

Memoryaccess

Write-back step

(Op = 'SW

')

(Op

= 'L

W')

4

2

53

From state 1

To state 0(Figure 5.37)

Memoryaccess

Shiftleft 2

MemtoReg

IorD MemRead MemWrite

PC

Memory

MemData

Writedata

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

ALUOpALUSrcB

RegDst RegWrite

Instruction[15– 0]

Instruction [5– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

1 Mux

0

3

2

ALUcontrol

Mux

0

1ALU

resultALU

ALUSrcA

ZeroA

B

ALUOut

IRWrite

Address

Memorydata

register

Implement FSM

Datapath control outputs

State registerInputs from instructionregister opcode field

Outputs

Combinationalcontrol logic

Inputs

Next state

State transition

ROM,PLA

state

Control for multicycle datapath

Microprogramming

Motivation If there are over 100 instructions,

the graphical representation of FSM is impossible…大指令集，要畫出 FSM 的狀態轉移圖很難

Ideas from programminginstructions

datapathControlsignal

Instructions for datapah

microinstruction: define the datapath control signals in a given state

microprogramming: design the control as a program composed of micro-instructions

Microinstruction format 依所控制的功能單元分欄位

ALU

register

memoryPC

Shiftleft 2

MemtoReg

IorD MemRead MemWrite

PC

Memory

MemData

Writedata

Mux

0

1

RegistersWriteregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Instruction[15– 11]

Mux

0

1

Mux

0

1

4

ALUOpALUSrcB

RegDst RegWrite

Instruction[15– 0]

Instruction [5– 0]

Signextend

3216

Instruction[25– 21]

Instruction[20– 16]

Instruction[15– 0]

Instructionregister

1 Mux

0

3

2

ALUcontrol

Mux

0

1ALU

resultALU

ALUSrcA

ZeroA

B

ALUOut

IRWrite

Address

Memorydata

register

MIPS microprogram

Implement microprogramming

Microprogram counter

Address select logic

Adder

1

Input

Datapathcontroloutputs

Microcodestorage

Inputs from instructionregister opcode field

Outputs

Sequencingcontrol

下一個指令：循序執行或有 branch

微指令對應到的控制訊號