Chapter 4

The Processor

Chapter 4 — The Processor — 4

Introduction n  CPU performance factors

n  Instruction count n  Determined by ISA and compiler

n  CPI and Cycle time n  Determined by CPU hardware

n  We will examine two MIPS implementations n  A simplified version n  A more realistic pipelined version

n  Simple subset, shows most aspects n  Memory reference: lw, sw n  Arithmetic/logical: add, sub, and, or, slt n  Control transfer: beq, j

§4.1 Introduction

Chapter 4 — The Processor — 5

Instruction Execution n  PC → instruction memory, fetch instruction

n  Register numbers → register file, read registers

n  Depending on instruction class n  Use ALU to calculate

n  Arithmetic result n  Memory address for load/store n  Branch target address

n  Access data memory for load/store n  PC ← target address or PC + 4

Chapter 4 — The Processor — 6

CPU Overview

Chapter 4 — The Processor — 7

Multiplexers n  Can’t just join

wires together n  Use multiplexers

Chapter 4 — The Processor — 8

Control

Chapter 4 — The Processor — 9

Logic Design Basics §4.2 Logic D

esign Conventions

n  Information encoded in binary n  Low voltage = 0, High voltage = 1 n  One wire per bit n  Multi-bit data encoded on multi-wire buses

n  Combinational element n  Operate on data n  Output is a function of input

n  State (sequential) elements n  Store information

Chapter 4 — The Processor — 10

Combinational Elements

n  AND-gate n  Y = A & B A B Y

I0 I1 Y

M u x

S

n  Multiplexer n  Y = S ? I1 : I0

A

B Y +

A

B

Y ALU

F

n  Adder n  Y = A + B

n  Arithmetic/Logic Unit n  Y = F(A, B)

Chapter 4 — The Processor — 11

Sequential Elements n  Register: stores data in a circuit

n  Uses a clock signal to determine when to update the stored value

n  Edge-triggered: update when Clk changes from 0 to 1

D

Clk

Q Clk

D

Q

Chapter 4 — The Processor — 12

Sequential Elements n  Register with write control

n  Only updates on clock edge when write control input is 1

n  Used when stored value is required later

D

Clk

Q Write

Write

D

Q

Clk

Chapter 4 — The Processor — 13

Clocking Methodology n  Combinational logic transforms data during

clock cycles n  Between clock edges n  Input from state elements, output to state

element n  Longest delay determines clock period

Chapter 4 — The Processor — 14

Building a Datapath n  Datapath

n  Elements that process data and addresses in the CPU

n  Registers, ALUs, mux’s, memories, …

n  We will build a MIPS datapath incrementally n  Refining the overview design

§4.3 Building a D

atapath

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Instruction Fetch

32-bit register

Increment by 4 for next instruction

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R-Format Instructions n  Read two register operands n  Perform arithmetic/logical operation n  Write register result

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Load/Store Instructions n  Read register operands n  Calculate address using 16-bit offset

n  Use ALU, but sign-extend offset n  Load: Read memory and update register n  Store: Write register value to memory

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Branch Instructions n  Read register operands n  Compare operands

n  Use ALU, subtract and check Zero output n  Calculate target address

n  Sign-extend displacement n  Shift left 2 places (word displacement) n  Add to PC + 4

n  Already calculated by instruction fetch

Chapter 4 — The Processor — 19

Branch Instructions

Just re-routes

wires

Sign-bit wire replicated

Chapter 4 — The Processor — 20

Composing the Elements n  Data path does an instruction in one clock cycle

n  Each datapath element can only do one function at a time

n  Hence, we need separate instruction and data memories

n  Use multiplexers where alternate data sources are used for different instructions

Chapter 4 — The Processor — 21

R-Type/Load/Store Datapath

Chapter 4 — The Processor — 22

Full Datapath

Chapter 4 — The Processor — 23

ALU Control n  ALU used for

n  Load/Store: F = add n  Branch: F = subtract n  R-type: F depends on funct field

§4.4 A Sim

ple Implem

entation Schem

e

ALU control Function 0000 AND 0001 OR 0010 add 0110 subtract 0111 set-on-less-than 1100 NOR

Chapter 4 — The Processor — 24

ALU Control n  Assume 2-bit ALUOp derived from opcode

n  Combinational logic derives ALU control

opcode ALUOp Operation funct ALU function ALU control lw 00 load word XXXXXX add 0010

sw 00 store word XXXXXX add 0010 beq 01 branch equal XXXXXX subtract 0110 R-type 10 add 100000 add 0010

subtract 100010 subtract 0110 AND 100100 AND 0000 OR 100101 OR 0001

set-on-less-than 101010 set-on-less-than 0111

Chapter 4 — The Processor — 25

The Main Control Unit n  Control signals derived from instruction

0 rs rt rd shamt funct 31:26 5:0 25:21 20:16 15:11 10:6

35 or 43 rs rt address 31:26 25:21 20:16 15:0

4 rs rt address 31:26 25:21 20:16 15:0

R-type

Load/ Store

Branch

opcode always read

read, except for load

write for R-type

and load

sign-extend and add

Chapter 4 — The Processor — 26

Datapath With Control

Chapter 4 — The Processor — 27

R-Type Instruction

Chapter 4 — The Processor — 28

Load Instruction

Chapter 4 — The Processor — 29

Branch-on-Equal Instruction

Chapter 4 — The Processor — 30

Implementing Jumps

n  Jump uses word address n  Update PC with concatenation of

n  Top 4 bits of old PC n  26-bit jump address n  00

n  Need an extra control signal decoded from opcode

2 address 31:26 25:0

Jump

Chapter 4 — The Processor — 31

Datapath With Jumps Added

Chapter 4 — The Processor — 32

Performance Issues n  Longest delay determines clock period

n  Critical path: load instruction n  Instruction memory → register file → ALU →

data memory → register file n  Not feasible to vary period for different

instructions n  Violates design principle

n  Making the common case fast n  We will improve performance by pipelining

Chapter 4 — The Processor — 33

Pipelining Analogy n  Pipelined laundry: overlapping execution

n  Parallelism improves performance

§4.5 An O

verview of P

ipelining n  Four loads: n  Speedup

= 8/3.5 = 2.3 n  Non-stop:

n  Speedup = 2n/0.5n + 1.5 ≈ 4 = number of stages

Chapter 4 — The Processor — 34

MIPS Pipeline n  Five stages, one step per stage

1.  IF: Instruction fetch from memory 2.  ID: Instruction decode & register read 3.  EX: Execute operation or calculate address 4.  MEM: Access memory operand 5.  WB: Write result back to register