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RISC Central Processing Unit

Lan-Da Van (范倫達), Ph. D.

Department of Computer Science National Chiao Tung University

Taiwan, R.O.C. Spring, 2014

Source: Prof. M. Morris Mano and Prof.

Charles R. Kime, Logic and Computer Design

Fundamentals, 3rd Edition, 2004, Prentice Hall.

Digital Systems Design

Lecture 6

Outline

Introduction

Pipelined Datapath

Pipelined Control

The Reduced Instruction Set Computer (RISC)

Summary

2


Lecture 6

Introduction

CPU: Datapath and control unit

Datapath:

— Consist of a function unit, regs, and internal buses

— May be nonpiplined or pipelined

Control unit:

— Consist of a program counter, an instr reg, and

control logic

— May be hardwired, microprogrammed, or pipelined

(if the datapath is pipelined)

3


Lecture 6

Conventional datapath v.s. Pipelined datapath

Fig 12-1 4


Lecture 6

Detailed block

diagram of the

pipelined datapath:

— the increased cost:

the pipeline platforms

Fig 12-2

5


Lecture 6

Pipeline Execution Pattern (1/2)

Execution of pipeline -ops:

Fig 12-3

6


Lecture 6

Pipeline Execution Pattern (2/2)

In the first two clock cycles, not all of the pipeline stages are active. => filling

In the next five clock cycles, all stages of the pipeline are active. => fully utilized

In the last two clock cycles, not all pipeline stages are active. => emptying

Speedup = (712)/ (95) = 1.9

More pipelines, much better?? No… — The delay contributed by the pipeline platforms

— The difference between the delay of the logic assigned to each stage

— Thus, there exists one optimized pipelined stages.

7


Lecture 6

Pipelined Control

Based on the single-cycle computer: (hardwired control)

Fig 12-2 Fig 10-15

8


Lecture 6

Block diagram

of pipelined

computer:

9


Lecture 6

Pipeline Stages

Stage 1: Instr fetch (IF)

Stage 2: Instr decoder & reg file read (DOF)

— (decode & operand fetch)

Stage 3: The function unit & data mem read

and write (EX)

Stage 4: Reg file write (WB)

Pipeleine Principle: The location of the pipeline

platforms has balanced the partitioning of the

delays.

Timing: 5 ns/stage 200 MHz, speedup = 3.4 (w.r.t single-cycle)

10


Lecture 6

Pipeline Programming and Performance (1/3)

E.g.: Load constants 1 ~ 7 into regs R1 ~ R7

1 LDI R1, 1

2 LDI R2, 2

3 LDI R3, 3

4 LDI R4, 4

5 LDI R5, 5

6 LDI R6, 6

7 LDI R7, 7

11


Lecture 6

In the first 4 clk periods: 20 ns

— 1/4 + 1/2 + 3/4 + 1 = 2.5 instrs completed

Overall time: 50 ns

— 10 clk cycles for 7 instrs

— Speedup = (7 17) / 50 = 2.38

Filling: the first 3 clks

Fully utilized: the next 4 clks

— Speedup = (4 17) / 20 = 3.4

Emptying: the last 3 clks

12



Lecture 6

For a k-stage pipelined computer:

— The speedup is not k w.r.t. the single-cycle

computer

The delays cannot be divided into k equal pieces.

The delays of the added pipeline platforms

The delay of the function unit is larger than that of ideal k

equal delays.

The filling and emptying of the pipeline

Data hazard

Control hazard

13



Lecture 6

The Reduced Instruction Set Computer

Design goal:

— A RISC with a pipelined datapath and control unit

— The instr set arch:

load/store mem access,

4 addressing modes,

A single instr format length,

Instrs that require only elementary ops.

— The ops, resembling those that can be performed by

the single-cycle computer, can be performed by a

single pass through the pipeline.

14


Lecture 6

Instruction Set Architecture

The CPU regs accessible to the programmer: Fig12-6

— Reg file: 32 regs, 32 bits/reg, R0 = 0

The size of the reg file: RISC > CISC, load/store instr set

arch

— PC: 32 bits

— No stack pointer or status reg

15


Lecture 6

Instr formats:

— 3-reg type

— 2-reg type

— Branch: target addr = PC + target offset

16

Instruction Format


Lecture 6

17

Instructions (1/2)


Lecture 6

— All of the ops are elementary and can be described

by a single reg transfer statement.

— The only ops that can access memory: Load & Store

— The immediate field: 15 bits 32 bits and using zero

fill or sign extension

— BZ, BNZ, SLT: handle the absence of stored versions

of status bits

— JML: Jump and Link

18

Instructions (2/2)


Lecture 6

Addressing Modes

4 addressing modes: specified by the opcode

i. Register: the 3-operand data manipulation instrs

ii. Register indirect: load and store instrs

iii. Immediate: the 2-reg format instrs

iv. Relative: branch and jump instrs

Implement an addressing mode not directly

provided:

— Use a sequence of RISC instrs

— E.g.: Indexed addressing, R15 M[R5 + 0 || I]

AIU R9, R5, I

LD R15, R9

19


Lecture 6

Datapath Organization

The Pipelined

computer in Fig 12-

4: 16-bit version

20


Lecture 6

Modified Datapath and Control Unit

Modifications of

datapath: Fig 12-4

Fig 12-8: 32-bit

version

— Register file

— Function unit

— Bus structure

Modifications of

control unit:

— Instruction decoder

— Control logic related

to the PC

— Pipeline platforms

21


Lecture 6

Register File and Function Unit

Reg file:

— 16 16-bit regs & all regs are identical in function

32 32-bit regs & R0 = 0

— edge triggered read-after-write reg file

Function unit:

— ALU: 16 bits 32 bits

— Shifter: Single-bit position shifter Barrel shifter

with lsr or lsl of 0 ~ 31 positions (SH: IR[4:0])

22


Lecture 6

Left/right: a control signal decoded from OPCODE SH: = IR(4:0), the shift amount field

Perform both the left and right shift by using a right rotate:

p-position right shift rotate p position to the right

p-position left shift rotate 64 p position to the right 23

32-bit Barrel Shifter


Lecture 6

Bus structure:

— zero fill constant unit:

CS = 0, zero fill ; CS = 1, sign extension

0 || IM se IM

— MUX A is added: provide a path for PC1 to the reg file

for implementing JML instr

Jump and Link: PC PC + se IM, R[DR] PC + 1

— MUX D is extended: help implement SLT instr

Set if Less Than: If R[SA] < R [SB] then R[DR] = 1

24

Bus Structure


Lecture 6

Instruction Decoder

Instruction decoder: to deal with the new instr

set

— SH is added as an IR field.

— A 1-bit CS field is added to the instr decoder.

— A 1-bit MA field is added to the instr decoder.

— MD is expanded to two bits.

— A new pipeline platform for SH & expanded 2-bit

platforms for MD

25


Lecture 6

Control Logic (1/2)

Control logic related to the PC: Permit the loading

of addrs into the PC for implementing branches

and jumps

— MUX C: in EX stage, selects from 3 different sources

for the next value of PC (BS, PS)

PC + 1

BrA: for branches and jumps PC PC + 1 + se IM

R[AA]: for reg jump PC R[AA]

— Pipeline regs PC1 & PC2

26


Lecture 6

Control Logic (2/2)

27


Lecture 6

Control Words for Instrs

28


Lecture 6

29


Lecture 6

Data Hazards

Data hazard example:

Solutions of data hazard:

— Program-based solution

— Data hazard stall

— Data forwarding

30


Lecture 6

Program-Based Solution

31

Disadv. of

program-based

sol.:

— The program is

longer (unless

some unrelated

instrs may be

placed in the NOP

positions)

— Reduce the

throughput


Lecture 6

Data Hazard Stall (1/4)

Data hazard stall: HW-based sol.

32


Lecture 6


When an operand is found at the DOF stage

that has not been written back yet, the

associated execution and write-back are

delayed by stalling the pipeline flow in IF and

DOF for one clock cycle.

— The pipeline is said to be stalled, i.e., contain a

bubble in subsequent clock cycles and stages for

that instr.

Disadv.:

— Has the same throughput penalty as the program w/

the NOPs

33


Lecture 6


The following events must all occur for HA.

— MA in the DOF stage must be 0, meaning that the A

operand is coming from the register file.

— AA in the DOF stage equals DA in the EX stage,

meaning that there is potentially a register being

read in the DOF stage that is to be written in the next

clock cycle.

— RW in the EX stage is 1, meaning that register DA in

the EX stage will definitely be written in WB during

the next clock cycle.

— The OR of all bits of DA is 1, meaning that the

register to be written is not R0 and so is a register

that must be written before being read.

34


Lecture 6


Pipelined RISC with Data hazard stall:

— Added or modified hardware:

Data hazard detection: DHS

Pipeline stalling:

DHS is inverted to initiate a bubble in the pipeline for the instr

currently in the IR and to stop the PC and IR from changing.

4

0

)()(i

iEXEXDOFEXDOF DARWAADAAMHA

4

0

)()(i

iEXEXDOFEXDOF DARWBADABMHB

HBHADHS

35


Lecture 6

36


Lecture 6

Data Forwarding (1/2)

Data forwarding: HW-based sol.

37


Lecture 6

Pipelined RISC w/ Data forwarding:


Data hazard detection: HA, HB

Data forwarding:

The information needed to form the result is available on the

inputs to the pipeline platform that provides the inputs to

MUX D. MUX D is added to produce the result on Bus

D

Add an additional input to MUX A & MUX B from Bus D

38

Data Forwarding (2/2)


Lecture 6

39


Lecture 6

Control hazard example: If R1 = 0

Solutions of control hazard:

— Program-based solution

— Branch hazard stall

— Branch prediction

1 BZ R1, 18

2 MOVA R2, R3

3 MOVA R1, R2

20 MOVA R5, R6

40

Control Hazards


Lecture 6

Program-Based Solution

41

2 NOPs are

inserted after the

branch instr.

— (These wasted

cycles can

sometimes be

avoided by

rearranging the

order of instrs.)


Lecture 6

Branch Hazard Stall

Branch hazard stall: HW-based sol.

— Just as in the case of the data hazard, a stall can be

used to deal w/ the control hazard.

Produce 2 bubbles after the branch instr

— Disadv.: The reduction in throughput will be the

same as w/ the insertion of NOPs.

42


Lecture 6

Branch Prediction (1/3)

Branch prediction

— Simplest form: predict that branches will never be

taken

Instrs will be fetched and decoded and operands fetched on

the basis of +1 to the value of the PC.

If the branch is not taken, the instrs already in the pipeline

due to the prediction will be allowed to proceed.

If the branch is taken, the instrs following the branch instr

need to be cancelled.

The cancellation may be done by inserting bubbles into EX

& WB stages.

43


Lecture 6

E.g.: Branch prediction with branch non-taken

Figure 12-16, p.555

— If the branch is taken

44



Lecture 6

Pipelined RISC w/ Branch prediction

— Figure 12-17, p.556


Branch detection: EX stage

the selection values on the inputs to MUX C are not 00

Instruction canceling: IF & DOF stages

45



Lecture 6

46


Lecture 6

Summary

Widely discussed in the following topics.

— Pipelined Datapath

— Pieplined Control

— RISC

Instruction Set Architecture

Addressing Modes

Datapath

Data Hazard

Control Hazard

47

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