EECC550 - Shaaban #1 Lec # 4 Winter 2005 12-13-2005 CPU Organization (Design) Datapath Design: –Capabilities & performance characteristics of principal

EECC550 - ShaabanEECC550 - Shaaban#1 Lec # 4 Winter 2005 12-13-2005

CPU Organization (Design)CPU Organization (Design)• Datapath Design:

– Capabilities & performance characteristics of principal Functional Units (FUs) needed by ISA instructions

– (e.g., Registers, ALU, Shifters, Logic Units, ...)– Ways in which these components are interconnected (buses

connections, multiplexors, etc.).– How information flows between components.

• Control Unit Design:– Logic and means by which such information flow is controlled.– Control and coordination of FUs operation to realize the targeted

Instruction Set Architecture to be implemented (can either be implemented using a finite state machine or a microprogram).

• Hardware description with a suitable language, possibly using Register Transfer Notation (RTN).

Chapter 5.1-5.4

Components & their connections needed by ISA instructions

Control/sequencing of operations of datapath componentsto realize ISA instructions


1 Analyze instruction set to get datapath requirements:– Using independent RTN, write the micro-operations required for target ISA

instructions.• This provides the the required datapath components and how they are connected.

2 Select set of datapath components and establish clocking methodology (defines when storage or state elements can read and when they can be written, e.g clock edge-triggered)

3 Assemble datapath meeting the requirements.

4 Identify and define the function of all control points or signals needed by the datapath.– Analyze implementation of each instruction to determine setting of control points

that affects its operations.

5 Control unit design, based on micro-operation timing and control signals identified:– Combinational logic: For single cycle CPU.

– Hard-Wired: Finite-state machine implementation.

– Microprogrammed.

Major CPU Design StepsMajor CPU Design Steps

e.g Any instruction completed in one cycle


CPU Design & Implantation ProcessCPU Design & Implantation Process• Top-down Design:

– Specify component behavior from high-level requirements (ISA).

• Bottom-up Design:– Assemble components in target technology to establish critical timing

(hardware delays, critical path timing).

• Iterative refinement:– Establish a partial solution, expand and improve.

Datapath Control

ProcessorInstruction SetArchitecture (ISA):ProvidesRequirements

Reg. File Mux ALU Reg Mem Decoder Sequencer

Cells GatesTarget VLSI implementation Technology


Datapath Design StepsDatapath Design Steps• Write the micro-operation sequences required for a number of

representative target ISA instructions using independent RTN.

• Independent RTN statements specify: the required datapath components and how they are connected.

• From the above, create an initial datapath by determining possible destinations for each data source (i.e registers, ALU).– This establishes connectivity requirements (data paths, or connections)

for datapath components.– Whenever multiple sources are connected to a single input, a

multiplexor of appropriate size is added.

• Find the worst-time propagation delay in the datapath to determine the datapath clock cycle (CPU clock cycle).

• Complete the micro-operation sequences for all remaining instructions adding datapath components + connections/multiplexors as needed.

(or destination)


MIPS Instruction FormatsMIPS Instruction Formats

• op: Opcode, operation of the instruction.• rs, rt, rd: The source and destination register specifiers.• shamt: Shift amount.• funct: Selects the variant of the operation in the “op” field.• address / immediate: Address offset or immediate value.• target address: Target address of the jump instruction.

op target address

02631

6 bits 26 bits

op rs rt rd shamt funct

061116212631

6 bits 6 bits5 bits5 bits5 bits5 bits

op rs rt immediate

016212631

6 bits 16 bits5 bits5 bits

R-Type

I-Type: ALULoad/Store, Branch

J-Type: Jumps

[31:26] [25:21] [20:16] [15:11] [10:6] [5:0]

[31:26] [25:21] [20:16] [15:0]

[31:26] [25:0]


MIPS R-Type (ALU) Instruction FieldsMIPS R-Type (ALU) Instruction Fields

• op: Opcode, basic operation of the instruction. – For R-Type op = 0

• rs: The first register source operand.• rt: The second register source operand.• rd: The register destination operand.• shamt: Shift amount used in constant shift operations.• funct: Function, selects the specific variant of operation in the op field.

OP rs rt rd shamt funct

6 bits 5 bits 5 bits 5 bits 5 bits 6 bits

R-Type: All ALU instructions that use three registers

add $1,$2,$3

sub $1,$2,$3

and $1,$2,$3or $1,$2,$3

Examples:

Destination register in rd Operand register in rt

Operand register in rs

[31:26] [25:21] [20:16] [15:11] [10:6] [5:0]

1st operand 2nd operand Destination

R[rd] R[rs] funct R[rt]PC PC + 4

Rs, rt , rdare register specifier fields

R-Type = Register Type Register Addressing used (Mode 1)

Independent RTN:


MIPS ALU I-Type Instruction FieldsMIPS ALU I-Type Instruction FieldsI-Type ALU instructions that use two registers and an immediate value Loads/stores, conditional branches.

• op: Opcode, operation of the instruction.

• rs: The register source operand.

• rt: The result destination register.

• immediate: Constant second operand for ALU instruction.

OP rs rt immediate

6 bits 5 bits 5 bits 16 bits

add immediate: addi $1,$2,100

and immediate andi $1,$2,10

Examples:

Result register in rtSource operand register in rs

Constant operand in immediate

[31:26] [25:21] [20:16] [15:0]

1st operand 2nd operandDestination

R[rt] R[rs] + immediatePC PC + 4

Independent RTN for addi:

I-Type = Immediate Type Immediate Addressing used (Mode 2)


MIPS Load/Store I-Type Instruction FieldsMIPS Load/Store I-Type Instruction Fields

• op: Opcode, operation of the instruction.

– For load word op = 35, for store word op = 43.

• rs: The register containing memory base address.

• rt: For loads, the destination register. For stores, the source register of value to be stored.

• address: 16-bit memory address offset in bytes added to base register.

OP rs rt address


Store word: sw $3, 500($4)

Load word: lw $1, 32($2)

Examples: Offset base register in rssource register in rt

Destination register in rt Offsetbase register in rs

Signed addressoffset in bytes

Base Src./Dest.

[31:26] [25:21] [20:16] [15:0]

(e.g. offset)

R[rt] Mem[R[rs] + address]PC PC + 4

Mem[R[rs] + address] R[rt] PC PC + 4

Base or Displacement Addressing used (Mode 3)


MIPS Branch I-Type Instruction FieldsMIPS Branch I-Type Instruction Fields

• op: Opcode, operation of the instruction.• rs: The first register being compared• rt: The second register being compared.• address: 16-bit memory address branch target offset in words

added to PC to form branch address.

OP rs rt address


Branch on equal beq $1,$2,100

Branch on not equal bne $1,$2,100

Examples:

Register in rsRegister in rt offset in bytes equal to

instruction address field x 4

Signed addressoffset in words

Addedto PC+4 to formbranch target

[31:26] [25:21] [20:16] [15:0]

PC-Relative Addressing used (Mode 4)

(e.g. offset)

R[rs] = R[rt] : PC PC + 4 + address x 4R[rs] R[rt] : PC PC + 4

Independent RTN for beq:


MIPS J-Type Instruction FieldsMIPS J-Type Instruction Fields

• op: Opcode, operation of the instruction.– Jump j op = 2– Jump and link jal op = 3

• jump target: jump memory address in words.

J-Type: Include jump j, jump and link jal

OP jump target

6 bits 26 bits

jump target = 2500

4 bits 26 bits 2 bits

0 0PC(31-28)

Effective 32-bit jump address: PC(31-28),jump_target,00

FromPC+4

Jump j 10000

Jump and link jal 10000

Examples:

Jump memory address in bytes equal toinstruction field jump target x 4

[31:26] [25:0]

J-Type = Jump Type Pseudodirect Addressing used (Mode 5)

Jump targetin words

PC PC + 4PC PC(31-28),jump_target,00

Independent RTN for j:


A Subset of MIPS InstructionsA Subset of MIPS InstructionsADD and SUB:

add rd, rs, rt

sub rd, rs, rt

OR Immediate:

ori rt, rs, imm16

LOAD and STORE Word

lw rt, rs, imm16

sw rt, rs, imm16

BRANCH:

beq rs, rt, imm16

op rs rt rd shamt funct

061116212631

6 bits 6 bits5 bits5 bits5 bits5 bits

op rs rt immediate

016212631


op rs rt immediate

016212631


op rs rt immediate

016212631


[31:26] [25:21] [20:16] [15:0]

[31:26] [25:21] [20:16] [15:0]

[31:26] [25:21] [20:16] [15:0]

[31:26] [25:21] [20:16] [15:11] [10:6] [5:0]


Basic MIPS Instruction Processing StepsBasic MIPS Instruction Processing Steps

Obtain instruction from program storage

Determine instruction type

Obtain operands from registers

Compute result value or status

Store result in register/memory if needed

(usually called Write Back).

Update program counter to address

of next instruction } Commonsteps for all instructions

Instruction

Fetch

Instruction

Decode

Execute

Result

Store

Next

Instruction

Instruction Mem[PC]

PC PC + 4

Done by Control Unit

Instruction Memory


Overview of MIPS Instruction Micro-operationsOverview of MIPS Instruction Micro-operations• All instructions go through these common steps:

– Send program counter to instruction memory and fetch the instruction. (fetch) Instruction Mem[PC]

– Update the program counter to point to next instruction PC PC + 4– Read one or two registers, using instruction fields. (decode)

• Load reads one register only.

• Additional instruction execution actions (execution) depend on the instruction in question, but similarities exist:– All instruction classes use the ALU after reading the registers:

• Memory reference instructions use it for address calculation.• Arithmetic and logic instructions (R-Type), use it for the specified

operation.• Branches use it for comparison.

• Additional execution steps where instruction classes differ:– Memory reference instructions: Access memory for a load or store.– Arithmetic and logic instructions: Write ALU result back in register.– Branch instructions: Change next instruction address based on comparison.


A Single Cycle MIPS CPU DesignA Single Cycle MIPS CPU DesignDesign target: A single-cycle per instruction MIPS CPU design

All micro-operations of an instruction are to be carried out in a single CPU clock cycle. Cycles Per Instruction = CPI = 1

CPI = 1

Figure 5.1 page 287

Abstract view of single cycle MIPS CPU showing major functional units (components) and major connections between them

T = I x CPI x C

CPU Performance Equation:


Instruction Word Mem[PC] Fetch the instruction

PC PC + 4 Increment PC

R[rd] R[rs] + R[rt] Add register rs to register rt result in register rd

R-Type Example:R-Type Example:Micro-Operation Sequence For ADDMicro-Operation Sequence For ADD

OP rs rt rd shamt funct

6 bits 5 bits 5 bits 5 bits 5 bits 6 bits

add rd, rs, rt

Independent RTN ?

[31:26] [25:21] [20:16] [15:11] [10:6] [5:0]

CommonSteps

ProgramMemory


Initial Datapath ComponentsInitial Datapath Components

Two state elements (memory) needed to store and access instructions:1 Instruction memory: • Only read access (by user code). No read control signal needed.

2 Program counter (PC): 32-bit register.• Written at end of every clock cycle (edge-triggered) : No write control signal.

3 32-bit Adder: To compute the the next instruction address (PC + 4).

InstructionWord

32 3232

32

3232

32

Three components needed by: Instruction Fetch: Instruction Mem[PC]

Program Counter Update: PC PC + 4

Basics of logic design/logic building blocks review in Appendix B (Book CD)


More Datapath ComponentsMore Datapath Components

Register File:

• Contains all ISA registers.• Two read ports and one write port.• Register writes by asserting write control signal• Clocking Methodology: Writes are edge-triggered.

• Thus can read and write to the same register in the same clock cycle.

ISA Register File Main 32-bit ALU

32

32

32

32

32

32-bit Arithmetic and Logic Unit (ALU)

4

Basics of logic design/logic building blocks review in Appendix B (Book CD)


• Register File consists of 32 registers:– Two 32-bit output busses: busA and busB– One 32-bit input bus: busW

• Register is selected by:– RA (number) selects the register to put on busA (data):

busA = R[RA]– RB (number) selects the register to put on busB (data):

busB = R[RB]– RW (number) selects the register to be written

via busW (data) when Write Enable is 1Write Enable: R[RW] busW

• Clock input (CLK) – The CLK input is a factor ONLY during write operations.– During read operation, it behaves as a combinational logic block:

• RA or RB valid => busA or busB valid after “access time.”

Register File DetailsRegister File Details

Clk

busW

Write Enable

3232

busA

32busB

5 5 5

RW RA RB

32 32-bitRegisters


A Possible Register File Implementation

5-to-32Decoder

Register 0Write

Data In

DataOut

Register 1Write

Data In

DataOut

Register 30Write

Data In

DataOut

Register 31Write

Data In

DataOut

......

32-to-1 MUX

01

3031

32...

5

32-to-1 MUX

01

3031

32...

5

.

..

.

32

32

32

32

.

.

.

...

0

1

3031

5

.

.

.

RegisterRead Data 1(Bus A)

RegisterRead Data 2(Bus B)

Read Register 1 (RA)

Read Register 2 (RB)

Register Write Data (Bus W)

32

Register Write Enable (RegWrite)

WriteRegister RW

Clk

busW

Write Enable

3232

busA

32

busB

5 5 5RW RA RB

32 32-bitRegisters

Also see Appendix B (Book CD) - The Basics of Logic Design

Each Register contains 32 edge triggered D-Flip Flops

Clock input to registersnot shown in diagram


Idealized MemoryIdealized Memory

• Memory (idealized)

– One input bus: Data In.

– One output bus: Data Out.

• Memory word is selected by:

– Address selects the word to put on Data Out bus.

– Write Enable = 1: address selects the memoryword to be written via the Data In bus.

• Clock input (CLK):

– The CLK input is a factor ONLY during write operation,

– During read operation, this memory behaves as a combinational logic block:

• Address valid => Data Out valid after “access time.”

• Ideal Memory = Short access time.

Clk

Data In

Write Enable

32 32DataOut

Address


Clocking Methodology Used:Edge Triggered Writes

• All storage element (e.g Flip-Flops, Registers, Data Memory) writes are triggered by the same clock edge.

• Cycle Time = CLK-to-Q + Longest Delay Path + Setup + Clock Skew

Clk

Don’t Care

Setup Hold

.

.

.

.

.

.

.

.

.

.

.

.

Setup Hold

Here writes are triggered on the rising edge of the clock

Critical Path


Building The DatapathBuilding The Datapath

Portion of the datapath used for fetching instructionsand incrementing the program counter.

Instruction Fetch& PC Update:

PC PC + 4

Instruction Mem[PC]

32

32

32

32

PC write or update is edge triggered at the end of the cycle


Simplified Datapath For MIPS Simplified Datapath For MIPS R-Type InstructionsR-Type Instructions

Components and connections as specified by RTN statement

R[rd] R[rs] + R[rt]

32

32

32

32

rs

rt

rd

R[rs]

R[rt]

FromInstructionMemory

Destination register R[rd] write or update is edge triggered at the end of the cycle

4[25:21]

[20:16]

[15:11]


More Detailed Datapath More Detailed Datapath For R-Type InstructionsFor R-Type Instructions

With Control Points IdentifiedWith Control Points Identified

32

Result

ALUctr

Clk

busW

RegWr

32

32

busA

32

busB

5 5 5

Rw Ra Rb

32 32-bitRegisters

Rs RtRd

AL

U

R[rd] R[rs] + R[rt]

R[rs]

R[rt]


R-Type Register-Register TimingR-Type Register-Register Timing

32Result

ALUctr

Clk

busW

RegWr

3232

busA

32busB

5 5 5

Rw Ra Rb32 32-bitRegisters

Rs RtRdA

LU

Clk

PC

Rs, Rt, Rd,Op, Func

Clk-to-Q

ALUctr

Instruction Memory Access Time

Old Value

New Value

RegWr Old Value

New Value

Delay through Control Logic

busA, B

Register File Access TimeOld Value

New Value

busWALU Delay

Old Value

New Value

Old Value

New Value

New ValueOld Value

Register WriteOccurs Here

PC+4

All register writes occur onfalling edge of clock(clocking methodology)




R[rt] R[rs] OR ZeroExt[imm16] OR register rs with immediate field zero extended to 32 bits, result in register rt

Logical Operations with Immediate Example Example::

Micro-Operation Sequence For ORIMicro-Operation Sequence For ORI

op rs rt immediate

016212631


ori rt, rs, imm16

[31:26] [25:21] [20:16] [15:0]


Datapath For Logical Datapath For Logical Instructions With ImmediateInstructions With Immediate

R[rt] R[rs] OR ZeroExt[imm16]

32

Result

ALUctr

Clk

busW

RegWr

32

32

busA

32

busB

5 5 5

Rw Ra Rb

32 32-bitRegisters

Rs Rt

RtRdRegDst

ZeroE

xt

Mu

x

Mux

3216imm16

ALUSrc

AL

U

01

R[rs]

R[rt]

2x1 MUX (width 5 bits)

2x1 MUX (width 32 bits)




R[rt] Mem[R[rs] + SignExt[imm16]] Immediate field sign extended to 32 bits and added to register rs to form memory load address,

write word at load effective address

to register rt

Load Operations ExampleExample::

Micro-Operation Sequence For LWMicro-Operation Sequence For LW

op rs rt immediate

016212631


lw rt, rs, imm16

Data Memory

Instruction Memory

Effective Address

Address offsetin bytes

[31:26] [25:21] [20:16] [15:0]


Additional Datapath Components For Additional Datapath Components For Loads & StoresLoads & Stores

Inputs: for address and write (store) dataOutput for read (load) data

16-bit input sign-extended into a 32-bit value at the output

For SignExt[imm16]

32

32

32

Data memory write or update is edge triggered at the end of the cycle (clocking methodology)


Datapath For LoadsDatapath For Loads

R[rt] Mem[R[rs] + SignExt[imm16]]

32

ALUctr

Clk

busW

RegWr

32

32

busA

32

busB

5 5 5

Rw Ra Rb

32 32-bitRegisters

Rs

RtRdRegDst

Exten

der

Mu

x

Mux

3216

imm16

ALUSrc

ExtOp

Clk

Data InWrEn

32

Adr

DataMemory

32

AL

U

MemWr Mu

x

MemtoReg

1 0

0

1

0

1

R[rs]

R[rt]

Base Address register

EffectiveAddress

Offset




Mem[R[rs] + SignExt[imm16]] R[rt] Immediate field sign extended to 32 bits and added to register rs to form memory store effective

address, register rt written to

memory at store effective address.

Store Operations ExampleExample::

Micro-Operation Sequence For SWMicro-Operation Sequence For SW

op rs rt immediate

016212631


sw rt, rs, imm16

Effective Address

Address offsetin bytes


Datapath For StoresDatapath For Stores

Mem[R[rs] + SignExt[imm16]] R[rt]

ALUSrcExtOp

32

ALUctr

Clk

busW

RegWr

32

32

busA

32

busB

55 5

Rw Ra Rb

32 32-bitRegisters

Rs

Rt

Rt

Rd

RegDst

Exten

der

Mu

x

Mux

3216imm16

Clk

Data InWrEn

32

Adr

DataMemory

MemWr

AL

U32

Mu

xMemtoReg

1 0

0

1

0

1

R[rs]

R[rt]

Base Address register

EffectiveAddress

OffsetR[rt]




Zero R[rs] - R[rt] Calculate the branch condition R[rs] == R[rt]

(i.e R[rs] - R[rt] = 0 )

Zero : PC PC + ( SignExt(imm16) x 4 ) Calculate the next

instruction’s PC address

Conditional Branch ExampleExample::

Micro-Operation Sequence For BEQMicro-Operation Sequence For BEQ

op rs rt immediate

016212631


beq rs, rt, imm16

Branch Target

PC Offset in words

[31:26] [25:21] [20:16] [15:0]

“Zero” is zero flag of main ALU

Condition Action


Datapath For Branch Instructions

Main ALU evaluates branch conditionNew adder to compute branch target:

• Sum of incremented PC and the sign-extended lower 16-bits on the instruction.

Main ALU Evaluates Branch Condition (subtract)

R[rs]

R[rt]

Zero flag =1if R[rs] - R[rt] = 0(i.e R[rs] = R[rt])

New 32-bit Adder (Third ALU)for Branch Target

PC + 4 + ( SignExt(imm16) x 4

( SignExt(imm16) x 4

[25:21] rs

[20:16] rt

[15:0] imm16 SignExt(imm16)


More Detailed Datapath More Detailed Datapath For Branch OperationsFor Branch Operations

Clk

busW

RegWr

32

busA

32

busB

5 5 5

Rw Ra Rb

32 32-bitRegisters

Rs Rt

Eq

ual

?

Zero

32

imm16

PCClk

00

Ad

der

Mu

x

Ad

der

4 PCSrc

PC

Ext

Instruction Address

32

BranchZero

PC+4

BranchTarget

0

1Main ALU

(subtract)

Branch Target ALU New 2X1 32-bitMUX to select next PC valueSign extend

shift left 2

PC


Combining The Datapaths For Memory Combining The Datapaths For Memory Instructions and R-Type InstructionsInstructions and R-Type Instructions

Highlighted muliplexors and connections added to combine the datapaths of memory and R-Type instructions into one datapath(This is book version ORI not supported)

[25:21] rs

[20:16] rtR[rs]

R[rt]

R[rt]

0

1

1

0

rt/rdMUX not shown

4

[15:0] imm16 SignExt(imm16)


Instruction Fetch Datapath Added toInstruction Fetch Datapath Added toALU R-Type and Memory Instructions DatapathALU R-Type and Memory Instructions Datapath

R[rs]

R[rt]1

0

0

1

rs

rt

PC+ 4

PC

(This is book version ORI not supported, no zero extend of immediate needed)

rt/rdMUX not shown

4


A Simple Datapath For The MIPS ArchitectureA Simple Datapath For The MIPS ArchitectureDatapath of branches and a program counter multiplexor are added.

Resulting datapath can execute in a single cycle the basic MIPS instruction:

- load/store word - ALU operations - Branches

1

0

0

1

PC +4

Branch Target

rt/rdMUX not shown


Figure 5.11 page 300

4


Main ALU Control• The main ALU has four control lines (detailed design in Appendix B)

with the following functions:

• For our current subset of MIPS instructions only the top five functions will be used (thus only three control lines will be used)

• For R-type instruction the ALU function depends on both the opcode and the 6-bit “funct” function field

• For other instructions the ALU function depends on the opcode only.

• A local ALU control unit can be designed to accept 2-bit ALUop control lines (from main control unit) and the 6-bit function field and generate the correct 4-bit ALU control lines.

ALU Control Lines ALU Function

0000

0001

0010

0110

0111

AND

OR

add

subtract

Set-on-less-than1100 NOR


InstructionOpcode

LWSWBranch EqualR-TypeR-TypeR-TypeR-TypeR-Type

ALUOp

00 00 01 10 10 10 10 10

Funct Field

XXXXXX XXXXXX XXXXXX 100000 100010 100100 100101 101010

DesiredALU Action

addaddsubtractaddsubtractandorset on less than

ALU Control Lines

0010 0010 0110 0010 0110 0000 0001 0111

InstructionOperation

Load wordStore wordbranch equaladdsubtractANDORset on less than

MainControl

op

6

ALUControl(Local)

func

2

6ALUop

ALUctr

4

AL

U

Local ALU Decoding of “func” FieldLocal ALU Decoding of “func” Field


Local ALU Control Unit

3 ALU Control Lines

FunctionField

(2 lines From main control unit)

More details found in Appendix C (Book CD)


Single Cycle MIPS DatapathSingle Cycle MIPS DatapathNecessary multiplexors and control linesare identified here and local ALU control added:



Function Field (2 bits)


Putting It All Together: A Single Cycle DatapathPutting It All Together: A Single Cycle Datapathim

m16

32

ALUop (2-bits)

Clk

busW

RegWr

32

32

busA

32busB

55 5

Rw Ra Rb32 32-bitRegisters

Rs

Rt

Rt

RdRegDst

Exten

der

Mu

x

3216imm16

ALUSrcExtOp

Mu

x

MemtoReg

Clk

Data InWrEn32 Adr

DataMemory

MemWrA

LU

Zero

Instruction<31:0>

0

1

0

1

01

<21:25>

<16:20>

<11:15>

<0:15>

Imm16RdRtRs

=

Ad

der

Ad

der

PC

Clk

00

Mu

x

4

PCSrc

PC

Ext

Adr

InstMemory

BranchZero

0

1

PC+4

BranchTarget

R[rs]

R[rt]

MainALU

(Includes ORInot in book version)

ALUControlFunction

Field


RegDst

Instruction<31:0>

<21:25>

<16:20>

<11:15>

<0:15>

Imm16RdRsRt

Adr

InstructionMemory

DATA PATHDATA PATH

ALUSrcALOp (2-bits)

MemReadMemtoReg

Control UnitControl Unit

Op

<21:25>

Fun

BranchRegWrite

<0:25>

Jump_target

Control LinesMemWrite


The Effect of The Control Signals Signal Name

RegDst

RegWrite

ALUSrc

PCSrc

MemRead

MemWrite

MemtoReg

Effect when deasserted (=0)

The register destination number for thewrite register comes from the rt field(instruction bits 20:16).

None

The second main ALU operand comes from the second register file output (Read data 2) R[rt]

The PC is replaced by the output of the adder that computes PC + 4

None

None

The value fed to the register write data input comes from the main ALU.

Effect when asserted (=1)

The register destination number for thewrite register comes from the rd field(instruction bits 15:11).

The register on the write register inputis written with the value on the Write data input.

The second main ALU operand is the sign-extended lower 16 bits on the instruction (imm16)

The PC is replaced by the output of theadder that computes the branch target.

Data memory contents designated by the address input are put on the Read data output.

Data memory contents designated by the address input are replaced by the value on the Write data input.

The value fed to the register write data input comes from data memory.


Control Line Settings

Instruction

R-Format

lw

sw

beq

RegDst

1

0

X

X

ALUSrc

0

1

1

0

Memto-Reg

0

1

X

X

RegWrite

1

1

0

0

MemRead

0

1

0

0

MemWrite

0

0

1

0

Branch

0

0

0

1

ALUOp1

1

0

0

0

ALUOp0

0

0

0

1



The Truth Table For The Main ControlThe Truth Table For The Main Control

(Opcode)


PLA Implementation of the Main ControlPLA Implementation of the Main Control

PLA = Programmable Logic Array (Appendix B)

Figure C.2.5 (Appendix C)




PC PC(31-28),jump_target,00 Update PC with jump address

Adding Support For Jump::

Micro-Operation Sequence For Jump: JMicro-Operation Sequence For Jump: J

OP Jump_target

6 bits 26 bits

j jump_target

jump target = 2500

4 bits 26 bits 2 bits

0 0

PC(31-28)

JumpAddress

Jump addressin words

[31:26] [25:0]

4 highest bits from PC + 4


Datapath For JumpDatapath For Jump

32

PC

Clk

00

Mu

x

PCSrc

imm16

Ad

der

Ad

der

4

PC

Ext

Next Instruction Address

Mu

x

JUMP

Shift left 2jump_target

Instruction(15-0)

Instruction(25-0)

32

26

PC+4(31-28)

28 32

4

32 0

1

PC+4

BranchZero

BranchTarget

JumpAddress

PC

PC(31-28),jump_target,00


Single Cycle MIPS Datapath Extended To Handle Single Cycle MIPS Datapath Extended To Handle Jump with Control Unit AddedJump with Control Unit Added



Book figure has an error!

Function Field


Control Line Settings(with jump instruction, j added)

Instruction

R-Format

lw

sw

beq

j

RegDst

1

0

X

X

X

ALUSrc

0

1

1

0

X

Memto-Reg

0

1

X

X

X

RegWrite

1

1

0

0

0

MemRead

0

1

0

0

0

MemWrite

0

0

1

0

0

Branch

0

0

0

1

X

ALUOp1

1

0

0

0

X

ALUOp0

0

0

0

1

X

Figure 5.18 page 308 modified to include j

Jump

0

0

0

0

1


Clk

PC

Rs, Rt, Rd,Op, Func

Clk-to-Q

ALUctr

Instruction Memoey Access Time

Old Value New Value

RegWr Old Value New Value

Delay through Control Logic

busA

Register File Access Time

Old Value New Value

busB

ALU Delay

Old Value New Value

Old Value New Value

New ValueOld Value

ExtOp Old Value New Value

ALUSrc Old Value New Value

MemtoReg Old Value New Value

Address Old Value New Value

busW Old Value New

Delay through Extender & Mux

RegisterWrite Occurs

Data Memory Access Time

Worst Case Timing (Load)Worst Case Timing (Load)


Instruction Timing ComparisonInstruction Timing Comparison

PC Inst Memory mux ALU Data Mem mux

PC Reg FileInst Memory mux ALU mux

PC Inst Memory mux ALU Data Mem

PC Inst Memory cmp mux

Reg File

Reg File

Reg File

Arithmetic & Logical

Load

Store

Branch

Critical Path

setup

setup

PC Inst Memory mux

Jump


Simplified Single Cycle Datapath Timing• Assuming the following datapath/control hardware components delays:

– Memory Units: 2 ns

– ALU and adders: 2 ns

– Register File: 1 ns

– Control Unit < 1 ns

• Ignoring Mux and clk-to-Q delays, critical path analysis:

Instruction Memory

Register Read

Main ALU

Data Memory

Register Write

PC + 4 ALU

Branch Target ALU

Control Unit

Time

0 2ns 3ns 4ns 5ns 7ns 8ns

Critical Path(Load)

Obtained from low-level target VLSI implementation technology of components

ns = nanosecond = 10-9 second

}


Performance of Single-Cycle (CPI=1) CPUPerformance of Single-Cycle (CPI=1) CPU • Assuming the following datapath hardware components delays:

– Memory Units: 2 ns– ALU and adders: 2 ns– Register File: 1 ns

• The delays needed for each instruction type can be found :

• The clock cycle is determined by the instruction with longest delay: The load in this case which is 8 ns. Clock rate = 1 / 8 ns = 125 MHz• A program with I = 1,000,000 instructions executed takes:

Execution Time = T = I x CPI x C = 106 x 1 x 8x10-9 = 0.008 s = 8 msec

Instruction Instruction Register ALU Data Register Total Class Memory Read Operation Memory Write Delay

ALU 2 ns 1 ns 2 ns 1 ns 6 ns

Load 2 ns 1 ns 2 ns 2 ns 1 ns 8 ns

Store 2 ns 1 ns 2 ns 2 ns 7 ns

Branch 2 ns 1 ns 2 ns 5 ns

Jump 2 ns 2 ns

Load has longest delay of 8 nsthus determining the clock cycle of the CPU to be 8ns

Nanosecond, ns = 10-9 second


Drawbacks of Single Cycle ProcessorDrawbacks of Single Cycle Processor1. Long cycle time:

– All instructions must take as much time as the slowest• Here, cycle time for load is longer than needed for all other instructions.

– Cycle time must be long enough for the load instruction:PC’s Clock -to-Q + Instruction Memory Access Time +Register File Access Time + ALU Delay (address calculation) +Data Memory Access Time + Register File Setup Time + Clock Skew

– Real memory is not as well-behaved as idealized memory• Cannot always complete data access in one (short) cycle.

2. Impossible to implement complex, variable-length instructions and complex addressing modes in a single cycle.– e.g indirect memory addressing.

3. High and duplicate hardware resource requirements– Any hardware functional unit cannot be used more than once in a

single cycle (e.g. ALUs).

4. Does not allow overlap of instruction processing (instruction pipelining, chapter 6).

Documents

EECC550 - Shaaban #1 Lec # 4 Winter 2005 12-13-2005 CPU Organization (Design) Datapath Design: –Capabilities & performance characteristics of principal