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CORPORATE INSTITUTE OF SCIENCE & TECHNOLOGY , BHOPALDEPARTMENT OF ELECTRONICS & COMMUNICATIONS

MICRO CODED CONTROLLER

- PROF. RAKESH K. JHA

ALTERNATIVES FOR CONTROL UNIT (CU)

Hard-wired (hardware) Random logic, programmable logic array (PLA), or ROM Fast Inflexible

Firmware Microprogrammed or microcoded CU Control implemented like a computer (microcomputer)

Microinstructions Microprogramming

Flexible Changes to instruction set possible Completely different instruction sets can be emulated

Speed limited by microcomputer memory 2

HARDWIRED CU: SINGLE-CYCLE Implemented by combinational logic.

3

Control logicopcode

Datapath

ALU control

3

2

To ALU

ALUOp

Control signals

funct. code

6

6

4

Instr. mem.PC

Add

Reg

. File

Datamem.1

mux

0

1 m

ux 0

0 m

ux 1

4

1 m

ux 0

Sign ext.

Shift left 2

ALUCont.

CO

NTR

OL

opcode

MemWriteMemRead

ALU

Branch

zero

0-15

0-5

11-15

16-20

21-25

26-31

ALU 0

mux

1

Shift left 2

0-25 Jump

Single-cycleDatapath

MemtoReg

ALUOp

ALUSrc

RegDst

RegWrite

SINGLE-CYCLE CONTROL LOGICInputsInputs OutputsOutputs

Instr.Instr.typetype

RR 00 00 00 00 00 00 11 00 00 11 00 00 00 11 00 00

lwlw 11 00 00 00 11 11 00 11 11 11 11 00 00 00 00 00

swsw 11 00 11 00 11 11 XX 11 XX 00 00 11 00 00 00 00

beqbeq 00 00 00 11 00 00 XX 00 XX 00 00 00 11 00 11 00JJ 00 00 00 00 11 00 XX XX XX 00 XX 00 XX XX XX 115

ALU

Op0

ALU

Op1

Reg

Dst

ALU

Src

Mem

toR

eg

Reg

Writ

e

Mem

Rea

d

Mem

Writ

e

Bra

nch

Op5

Op4

Op3

Op2

Op1

Op0

Jum

p

SINGLE-CYCLE CONTROL CIRCUIT

6

lw sw beq JR

RegDstALUSrcMemtoRegRegWriteMemReadMemWriteBranchALUOp1ALUOp0Jump

Op5Op4Op3Op2Op1Op0

ALU CONTROL LOGICInputsInputs Outputs to ALUOutputs to ALU

Instr.Instr.typetype

From CUFrom CU Funct. Code from IR Funct. Code from IR (bits 0-5)(bits 0-5)

3-bit 3-bit codecode

Opera-Opera-tiontion

ALUOp1ALUOp1 ALUOp0ALUOp0 F5F5 F4F4 F3F3 F2F2 F1F1 F0F0

lw, swlw, sw 00 00 XX XX XX XX XX XX 010010 AddAdd

BB 00 11 XX XX XX XX XX XX 110110 SubtractSubtract

RR

11 XX XX XX 00 00 00 00 010010 AddAdd

11 XX XX XX 00 00 11 00 110110 SubtractSubtract

11 XX XX XX 00 11 00 00 000000 ANDAND

11 XX XX XX 00 11 00 11 001001 OROR

11 XX XX XX 11 00 11 00 111111 sltslt 7

ALU CONTROL

8

ALU

3zero

result

overflow

Operation select

from control

Operation select ALU function

000 AND 001 OR 010 Add 110 Subtract 111 Set on less than

F3

F2

F1

F0

ALUOp1 ALUOp0From Control Circuit

Func

tion

code

MULTICYCLE CONTROL FSM

9

Instr. decode/reg.

fetch/branch

addr.

ALU operation

Write PC on branch condition

Write memory

data

Write jump addr.

to PC

Write register

Read memory

data

Instr. fetch/

adv. PC

Compute memory

addr.

Write register

lw or sw

lw

sw

RB

J

StartState 0 1

23

4 5

6

7

8 9

Inputs: 6 opcode bitsOutputs: 16 control signals

STATES AND OUTPUTS Suppose 10 states are encoded 0000 through 1001. State code completely determines 16 control

signals (Moore machine). States 0 (0000), 3 (0011) and 6 (0110)

Next state ← present state + 1 State 1 (0001) – opcode must decide next state

State 2 (0010) for lw or sw State 6 (0110) for R-type of instruction State 8 (1000) for branch instruction State 9 (1001) for jump instruction

State 2 (0010) – opcode must decide next state State 3 (0011) for lw State 5 (0101) for sw

States 4 (0100), 5 (0101), 7 (0111), 8 (1000) and 9 (1001) – next state is unconditionally 0 (0000) 10

A PROGRAM-LIKE IMPLEMENTATION

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Instr. decode/reg.

fetch/branch

addr.

ALU operation

Read memory

data

Instr. fetch/

adv. PC

lw or sw

lw

sw

RB

J

StartState 0000 0001

00100011

0100 0101

0110

01111000 1001

Inputs: 6 opcode bitsOutputs: 16 control signals

Compute memory

addr.

Write jump addr.

to PC

Write register

Write memory

dataWrite

register

Write PC on branch condition

IMPLEMENTING WITH ROM

12

ControlPLA or ROM

16 words

Four flip-flops

16 control signals

PLA input orROM address

6-bit opcode

State sequencer

Select one of 4 ways

16

2

4

6

ROM AND STATE SEQUENCER

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Control ROMSixteen

18-bit words4-bit address

4-bit state flip-flops

16

2

Controlsignals to datapath

4 4

0001

MUX11 10 01 00

0000

AddrCtl go to00 st. 011 st. + 101 st.

2,6,8,910 st. 3,5

Dispatch ROM 2 Dispatch ROM 1

Adder

6

6-bitOpcode from IR

Address

Advance state4

ROM Address sw, lw, R, B or J

sw or lw

DISPATCH ROM CONTENTS

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Dispatch ROM 1Dispatch ROM 1

InstructionInstruction AddressAddress(Opcode)(Opcode)

ContentContent

lwlw 100011100011 00100010

swsw 101011101011 00100010

RR 000000000000 01100110

BB 000100000100 10001000

JJ 000010000010 10011001

Dispatch ROM 2Dispatch ROM 2

InstructionInstruction AddressAddress(Opcode)(Opcode)

ContentContent

lwlw 100011100011 00110011

swsw 101011101011 01010101

Each dispatch ROM has sixty-four 4-bit wordsAddress is 6-bit opcodeContent is next state (4-bits)

CONTROL ROM CONTENTS Control ROM has

sixteen 18-bit words: bits 0-1, AddrCtl to

control mux bits 2-17, sixteen

control signals for datapath

Address is 4-bit state of control machine

Addr.Addr. bits 17-2bits 17-2 bits 1-0bits 1-000000000 10010100000010001001010000001000 1111

00010001 00000000000110000000000000011000 0101

00100010 00000000000101000000000000010100 1010

00110011 00110000000000000011000000000000 1111

01000100 00000010000000100000001000000010 0000

01010101 00101000000000000010100000000000 0000

01100110 00000000010001000000000001000100 1111

01110111 00000000000000110000000000000011 0000

10001000 01000000101001000100000010100100 0000

10011001 10000001000000001000000100000000 000015

MICROPROGRAM: BASIC IDEA The control unit in a computer generates an output

(sequence of control signals) for each instruction. Suppose we break down each instruction into a series

of smaller operations (microinstructions), such as, fetch, decode, etc.

Then, implement the control unit as a small computer (within the computer) that executes a sequence of microinstructions (microprogram) for each instruction.

M. V. Wilkes, “The Best Way to Design an Automatic Calculating Machine,” Report of Manchester University Computer Inaugural Conference, pp. 16-18, 1951.Reprinted in E. E. Swartzlander (editor), Computer Design Development: Principal Papers, pp. 266-270, Rochelle Park, NJ: Hayden, 1976.

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MICROCODED CONTROL UNIT

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Sixteen18-bit words4-bit address

4-bit state flip-flops

16

2

Controlsignals to datapath

4 4

0001

MUX11 10 01 00

0000

AddrCtl

Dispatch ROM 2 Dispatch ROM 1

Adder

6

Opcode from IR

Address

Microcodememory

μPC

Address select logic

Microcode word

Sequencing field

lw or sw sw, lw, R, B or JROM address

IMPLEMENTING THE IDEA Use a memory type implementation for

control unit. Create a software infrastructure to

automatically translate instructions into memory data (microcode):

Microinstructions – define a machine language in which instructions can be described

Microprogram – an instruction described as a sequence of microinstructions

Microassembler – converts microprogram to (binary) microcode

Is there a micro-compiler?18

MICROPROGRAMMING A microinstruction set is defined. To program the control of a computer for an

instruction set, a programmer writes a microprogram for each machine instruction.

Each micrprogram is converted into microcode, specific to the datapath hardware, by a microassembler and the entire microcode is loaded in the microcode memory of the control unit (CU).

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THE END