CS61CL Machine Structures

Lec 8 – State and Register Transfers

David CullerElectrical Engineering and Computer Sciences

University of California, Berkeley

CS61CL Road Map

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Hardware

Software

Machine Lang. pgm

Instruction Set Architecture

Machine Organization

HLL Program Asm Lang. Pgm

Co

mp

iler

Ass

emb

ler

foo.cfoo.s

foo.o

I/O systemInstr. Set Proc.

Digital Design

Circuit Design

Datapath & Control

Layout & fab

Semiconductor Materials

foo.exe

Lin

ker

Review: Combinational Logic• Any boolean function can be expressed as an acyclic connection of gates

• Often specified by a truth table

• Outputs are purely a function of the inputs– no history, no state

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Combinational

Logic

inp

uts

ou

tpu

ts

inputs outputs

k

2k

Examples: Logical Operations

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C = A & B

A B

C

A31 B31

C31

A30 B30

C30

A0 B0

C0

°°°

A31:0 B31:0

C31:0

A31:0 B31:0

C31:0

A31:0 B31:0

C31:0

A31:0

C31:0

Example: Multiplexor

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C = S ? A : B

A B

C

S

C = (S & A) | (~S & B)

S

A31:0 B31:0

C31:0

Example: Adder

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A B Ci

Co S

A B Ci

Co S

A B Ci

Co S

A B Ci

Co S

A B Ci

Co S

A31:0 B31:0

C31:0

Example: Arithmetic Logic Unit

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A31:0 B31:0

C31:0

S1:0

ALU

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Element of Time

• Logical change is not instantaneous• Broader digital design methodology has to make it appears

as such– Clocking, delay estimation, glitch avoidance

Vout

+3

0T

Propagation delay

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What makes Digital Systems tick?

Combinational

Logic

time

clk

Administrative Issues

• HW 6 due tonight

• Project 2 dues Monday 10/26– bimodal check-off

– testing tools available tomorrow

– they are really picky

• Project 1 grading almost done– Friday

• HW 7 – discuss

• Midterm 2 on 11/9 as in original schedule– 11/11 is holiday

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8/30/2007 12

A Bit of state: D-type edge-triggered flip-flop

• The edge of the clock is used to sample the "D" input & send it to "Q” (positive edge triggering).

– At all other times the output Q is independent of the input D (just stores previously sampled value).

– The input must be stable for a short time before the clock edge.

01 01 010

9/18/07

13

R S R S R S

D Q D Q D Q D Q

OUT1 OUT2 OUT3 OUT4

CLK

IN1 IN2 IN3 IN4

R S

"0"

Registers

• Collections of flip-flops with similar controls and logic– Stored values somehow related (e.g., form binary value)

– Share clock, reset, and set lines

– Similar logic at each stage

What “registers” do we need?

• “read” vs use the output

• “write” on the clock edge => Load

• Load Control

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0r0r1°°°r31PClohi

Programmable storage

2^32 x bytes

31 x 32-bit GPRs (R0=0)

32 x 32-bit FP regs (paired DP)

HI, LO, PC

Register with Load Control

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R0R31clock

load1 0

Register File

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R0

°°°

R1

R2

R31

AselBsel

BoutAout

Dsel

1

ld

Dinde

code

r

Towards a Data Path

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°°°

AselBselDsel

ld

aluOP

Exercise a Data Path

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°°°

AselBselDsel

ld

aluOP

7

3

24

2

1

7 3

10

10

10 3

13

13

13 3

16

What about RAM - Randomly Accessed Memory?

• Like a HUGE register file– dense, slower, low-cost storage cell (6T)– fewer ports– wider address lines– accessed over a “bus”

• Bus: means of composition in hardware system– logically related collection of wires– interfacing one or more sources to one or more destinations

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RAM

data

address

Recall: Instruction Cycle

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°°°

000..0:

FFF..F:

n:

0B20

0B20:

Instruction Fetch

Execute

PC

32 2 3 1

“add $1,$2,$3”40

61101Operand

Result

Next

Decode

+

0B24

main:

Register Transfers

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Synchronous Circuit Design

• clock– distributed to all flip-flops

• ALL CYCLES GO THROUGH A REG!

• Combinational Logic Blocks (CL)– Acyclic

– no internal state (no feedback)

– output only a function of inputs

• Registers (reg)– collections of flip-flops

reg regCL CL

clock input

output

option feedback

input output