CPU DESIGNThe Single-Cycle Implementation

Computer OrganizationCSE

2021

Shakil M. Khan

(adapted from Profs. H. Roumani, A. Asif)

Dept of CS & Eng, York University

CSE-2021 June-30-2011 2

Sequential vs. Combinational Circuits

Digital circuits can be classified into two categories:

1. Combinational Circuits: mux, ALU

2. Sequential Circuits: flip-flops, registers, memory

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Clocks

Clock cycle

S ta teelement

1Combinational logic

S ta tee leme nt

2

• Periodic signal oscillating between low and high states with fixed cycle time

• Clock frequency = inverse of clock cycle time

• Clock controls when the state of a memory element changes

Clock period Rising e dge

Falling edge

CPU DESIGN

• The Datapath

• Single-Cycle Control

• Performance

Focus on the Subset:

addi, add/sub/and/or/slt, lw/sw, beq, j

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Building the

Datapath

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The Basic Datapath Components (1)

6

PC

1. Program counter

contains address of next instruction

16 32Sign

extend

2. Sign-extension unit

extends a 16-bit integer to a 32-bit integer

Add Sum

3. Adder

adds two 32-bit integers

4. AL U

ALU control

ALUre sult

ALU

Zero

3

add/subtract/and/or/compare two 32-bit integers

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The Basic Datapath Components (2)

7

InstructionMemory

Instructionaddress

Instruction

5.Instruction memory

Register

numbers

7. Register Files

RegWrite

RegistersWriteregister

Readdata 1

Readdata 2

Readregister 1

Readregister 2

WritedataData

5

5

5 6. Data memory unit

MemRead

MemWrite

Datamemory

Writedata

Readdata

Address

The Basic Datapath Components

A

L

U

RFIM

BUS

P

C

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The Basic Datapath [computational R-Type]

A

L

U

RFIM

P

C

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Recall the ML Formats:

6 5 5 5 5 6

R opCode rs rt rd sa funCode

Register rs = source, rt = target, rd = destination.

6 5 5 16

I opCode rs rt immediate

6 26

J opCode immediate

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The Basic Datapath [computational R-Type]

A

L

U

RFIM

25 – 21

20 – 16

15 – 11

P

C

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The PC Circuitry

A

L

U

RF

P

C

IM

4

25 – 21

20 – 16

15 – 11

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Add support for computational I-Types

0

1

A

L

U

P

C

IM

4

25 – 21

20 – 16

15-11

RF

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15 – 11

Add support for computational I-Types

0

1

A

L

U

RF

0

1

SE

P

C

IM

4

25 – 21

20 – 16

15-11

CSE-2021 June-30-2011 14

0

1

A

L

U

RF

0

1

SE

P

C

IM

4

25 – 21

20 – 16

15-11

Add support for lw

DM

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Add support for lw

DM

0

1

A

L

U

RF

0

1

SE

1

0

P

C

IM

4

25 – 21

20 – 16

15-11

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Add support for sw

DM

0

1

A

L

U

RF

0

1

SE

1

0

P

C

IM

4

25 – 21

20 – 16

15-11

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Add Support for branch

DM

0

1

A

L

U

RF

0

1

SE

1

0

P

C

IM

4 0

1

sll

25 – 21

20 – 16

15-11

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19

Combined Datapath (w/o Jump)

PC

Instructionmemory

Readaddress

Instruction

16 32

Add ALUresult

Mux

Registers

Writeregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Shift

left 2

4

Mux

ALU operation3

RegWrite

MemRead

MemWrite

PCSrc

ALUSrc

MemtoReg

ALUresult

ZeroALU

Datamemory

Address

Writedata

Readdata M

ux

Sign

extend

Add

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20

add/sub/or/and/slt $s1,$s2,$s3

PC

Instructionmemory

Readaddress

Instruction

16 32

Add ALUresult

Mux

Registers

Writeregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Shift

left 2

4

Mux

ALU operation3

RegWrite

MemRead

MemWrite

PCSrc

ALUSrc

MemtoReg

ALUresult

ZeroALU

Datamemory

Address

Writedata

Readdata M

ux

Sign

extend

Add

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21

lw $s1, offset($s2)

PC

Instructionmemory

Readaddress

Instruction

16 32

Add ALUresult

Mux

Registers

Writeregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Shift

left 2

4

Mux

ALU operation3

RegWrite

MemRead

MemWrite

PCSrc

ALUSrc

MemtoReg

ALUresult

ZeroALU

Datamemory

Address

Writedata

Readdata M

ux

Sign

extend

Add

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22

sw $s1, offset($s2)

PC

Instructionmemory

Readaddress

Instruction

16 32

Add ALUresult

Mux

Registers

Writeregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Shift

left 2

4

Mux

ALU operation3

RegWrite

MemRead

MemWrite

PCSrc

ALUSrc

MemtoReg

ALUresult

ZeroALU

Datamemory

Address

Writedata

Readdata M

ux

Sign

extend

Add

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23

beq $s1, $s2, w_offset

PC

Instructionmemory

Readaddress

Instruction

16 32

Add ALUresult

Mux

Registers

Writeregister

Writedata

Readdata 1

Readdata 2

Readregister 1

Readregister 2

Shift

left 2

4

Mux

ALU operation3

RegWrite

MemRead

MemWrite

PCSrc

ALUSrc

MemtoReg

ALUresult

ZeroALU

Datamemory

Address

Writedata

Readdata M

ux

Sign

extend

Add

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Shift left 2

PC

Instruction memory

Read address

Instruction [31– 0]

Data memory

Read data

Write data

RegistersWrite register

Write data

Read data 1

Read data 2

Read register 1

Read register 2

Instruction [15– 11]

Instruction [20– 16]

Instruction [25– 21]

Add

ALU result

Zero

Instruction [5– 0]

MemtoReg

ALUOp

MemWrite

RegWrite

MemRead

Branch

JumpRegDst

ALUSrc

Instruction [31– 26]

4

M u x

Instruction [25–0] Jump address [31– 0]

PC+4 [31– 28]

Sign extend

16 32Instruction [15– 0]

1

M u x

1

0

M u x

0

1

M u x

0

1

ALU control

Control

AddALU

result

M u x

0

1 0

ALU

Shift left 2

26 28

Address

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Building the

Control

Control

DM

0

1

A

L

U

RF

0

1

SE

1

0

P

C

IM

4 0

1

sll

clk

clk

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Exercise

SIGNAL VALUE

ALUSrc

MemToReg

RegDst

RegWrite

MemRead

MemWrite

Branch

Jump

Operation (3-bit)

add $t0, $s0, $a0

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Exercise

SIGNAL VALUE

ALUSrc

MemToReg

RegDst

RegWrite

MemRead

MemWrite

Branch

Jump

Operation (3-bit)

sw $t0, 500($s0)

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Exercise

SIGNAL VALUE

ALUSrc

MemToReg

RegDst

RegWrite

MemRead

MemWrite

Branch

Jump

Operation (3-bit)

beq $t0, $s0, 401

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Generating the Control Signals

All signals depend on the instruction, i.e. on a total of

12 bits complex.

Note that non-ALU signals depend only on the 6-bit

op_code simpler.

Hence, split the control into a main control unit that

sees only the opcode, and an auxiliary one that sees

the funtion code.

The two communicate via a new signal, ALUop

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Splitting the Control

Main Control Unit31 – 26

ALU Control Unit5 – 0

Operation

8 control

signals8

3

2

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The Operation Signal

A 3-bit signal through which the auxiliary

control unit tells the ALU to:

000 = and

001 = or

010 = add

110 = sub

111 = slt

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The ALUop Signal

A 2-bit signal through which the main control

unit tells the auxiliary to:

00 = add (no matter what the fun_code is)

01 = subtract (no matter what the fun_code is)

10 = R-Type (follow the fun_code)

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The Main Control Unit

Combinational

Logic

31-26

RegDst

ALUsrc

MemToReg

RegWrite

MemRead

MemWrite

Branch

Jump

ALUop-1 ALUop-0CSE-2021 June-30-2011 34

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Instruction RegDst ALUSrc MemtoReg RegWrite MemRd MemWrt Branch ALUOp1 ALUOp0

R-format 1 0 0 1 0 0 0 1 0

lw 0 1 1 1 1 0 0 0 0

sw X 1 X 0 0 1 0 0 0

beq X 0 X 0 0 0 1 0 1

Instruction Opcode in

Decimal

Opcode in Binary

Op5 Op4 Op3 Op2 Op1 Op0

R-format 0ten 0 0 0 0 0 0

lw 35ten 1 0 0 0 1 1

sw 43ten 1 0 1 0 1 1

beq 4ten 0 0 0 1 0 0

Inputs of Control Unit:

Outputs of Control Unit:

The Main Control Unit (1)

35

R-format Iw sw beq

Op0

Op1

Op2

Op3

Op4

Op5

Inputs

Outputs

RegDst

ALUSrc

MemtoReg

RegWrite

MemRead

MemWrite

Branch

ALUOp1

ALUOpO

The Main Control Unit (2)

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ALU Control

AUXILURY

Control Unit

ALUop-1

Operation-2

Operation-1

Operation-0

ALUop-0

5-0

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Aux Controller Implementation (1)

Instruction

(opcode)

Inputs

Desired ALU action

Outputs Operation

(Op3 – Op0)ALUOp

(ALUOp1 – ALUOp0)

Function Field

(F5 – F0)

lw (I) 0 0 (0 0) X X X X X X add 0 0 1 0

sw (I) 0 0 (0 0) X X X X X X add 0 0 1 0

beq (I) 0 1 (0 1) X X X X X X sub 0 1 1 0

add (32) 1 0 (1 0) X X 0 0 0 0 add 0 0 1 0

sub (34) 1 X (1 0) X X 0 0 1 0 sub 0 1 1 0

and (36) 1 0 (1 0) X X 0 1 0 0 and 0 0 0 0

or (37) 1 0 (1 0) X X 0 1 0 1 or 0 0 0 1

slt (42) 1 X (1 1) X X 1 0 1 0 slt 0 1 1 1

38

Aux Controller Implementation (2)

Operation2

Operation1

Operation0

Operation

ALUOp1

F3

F2

F1

F0

F (5– 0)

ALUOp0

ALUOp

ALU control block

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Op0 ALUOp1 (F0 F3)

Op1 ALUOp1 F2

Op2 ALUOp0 ALUOP1F1

The Single-Cycle

Performance

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CSE-2021 June-30-2011

• Load = 5 functional units:

inst. fetch, register access, ALU, data memory access, register access

• Store = 4 functional units:

instruction fetch, register access, ALU, data memory access

• R-type = 4 functional units:

instruction fetch, register access, ALU, register access

• Branch = 3 functional units:

instruction fetch, register access, ALU

• Jump = 1 functional unit:

instruction fetch

Performance Analysis

41

Component DelaysRF=50, ALU=100, and MEM (both IM and DM)=200 ps.

Compute CPU Time to execute various instructionsj, beq, add, sw, lw

Compute Max GHz for the CPU ClockAnswer: 1.66 GHz

Critique of S/Cycle+very simple

-caters to the slowest

-h/w redundancy

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