How does the CPU execute programs? - Radboud Universiteit · 2017-12-06 · 2 Overview So far… • Circuits • Memory • ALU Today… • How are instructions executed? • Fetch-decode-execute

How does the CPU execute programs?Nils Jansen December 5, 2017 (Based on slides by Jeroen Keiren and Niek Janssen)

2

Overview

2

Overview

So far…• Circuits• Memory• ALU

2

Overview

So far…• Circuits• Memory• ALU

Today…• How are instructions executed?

• Fetch-decode-execute cycle• Data Path

• RUN1718 CPU - your own processor in the practicum

3

Recall: A Six-level Computer

3


recall the levels and their meaning

3


gates get signals 0 or 1, compute output functions (AND, OR),

may form memory or even computing engine

ALU (Arithmetic Logic Unit), registers

controls data path between ALU and registers

human-understandable

machine language

recall the levels and their meaning

4

“Von Neumann”-architecture

4

“Von Neumann”-architecture recall the structure

Tanenbaum, Structured Computer Organization, Fifth Edition, © 2006 Pearson Education, Inc. All rights reserved. 0-13-148521-0

4


Screen Keyboard

Program

Memory

recall the structure


4


Screen Keyboard

Program

Memory

recall the structure

How to execute programs?

5

Program Inside the Memory

• Consists of machine code

• RUN1718 CPU: Each machine code instruction consists of 32 bits

• Other processors: 8–120 bits

Details next week!

6

Microarchitecture — Program Execution Cycle

1.Read instruction (PC) (Fetch)2.Increase PC3.Decode instruction (Decode)4.Execute instruction (Execute)

6



Decode, ExecuteFetch

6



Decode, ExecuteFetch

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7

Fetch-Phase: Read Instruction

Screen Keyboard

Memory

Program


7

Fetch-Phase: Read Instruction

Screen Keyboard

Memory

Program

Instruction


8

Fetch-phase: Increase PC

Screen Keyboard

Memory

Program

Instruction


8

Fetch-phase: Increase PC

Screen KeyboardPC

Memory

Program

Instruction


9

Decode-phase

Screen Keyboard

Memory

Program

Instruction


9

Decode-phase

Screen Keyboard

Memory

Program

Instruction

what is done in the decode phase?


9

Decode-phase

Screen Keyboard

Memory

Program

Instruction

Next week: • READ: 110000

• WRITE: 111000

what is done in the decode phase?


10

Execute-phase: Computation

Screen Keyboard

Memory

Program


10


Screen Keyboard

Memory

Program

what is done during a computation?


10


Screen Keyboard

Memory

Program

operands



10


Screen Keyboard

Memory

Program

flags

resultoperands



11

Execute-phase: Write

Screen Keyboard

WRITE

Memory

Program


11


Screen Keyboard

WRITE

Memory

Program

what is done during writing?


11


Screen Keyboard

WRITE

Memory

Program

WRITE

what is done during writing?

12

Data Path (MIPS processor)

Read address Instruction

Instruction Register

Readreg. 1 Read data1

Registers

Readreg. 2

Writereg.Write data Read data2

Address Read data

Data Memory

Write data

12

Data Path (MIPS processor)

ADD R1, R2, R2 (R2 := R1 + R2)




Registers

Readreg. 2


Address Read data

Data Memory

Write data

13

Data Path (MIPS processor): Fetch

ADD R1, R2, R2 (R2 := R1 + R2)




Registers

Readreg. 2


Address Read data

Data Memory

Write data

14

Data Path (MIPS processor): Decode

ADD R1, R2, R2 (R2 := R1 + R2)




Registers

Readreg. 2


Address Read data

Data Memory

Write data

15

Data Path (MIPS processor): Execute

ADD R1, R2, R2 (R2 := R1 + R2)




Registers

Readreg. 2


Address Read data

Data Memory

Write data

15

Data Path (MIPS processor): Execute

ADD R1, R2, R2 (R2 := R1 + R2)




Registers

Readreg. 2


Address Read data

Data Memory

Write data

The Practicum: RUN1718 CPUNils Jansen December 5, 2017 (Based on slides by Jeroen Keiren)

17

Finite automaton Fetch-D+Execute Cycle

Decode+ ExecuteFetch

17

Finite automaton Fetch-D+Execute Cycle


What is a finite automaton?

18

How to construct the RUN1718 CPU

3 phases:

18


3 phases:1. Fetch read instructions

18


3 phases:1. Fetch read instructions2. Test preconditions satisfied?

18


3 phases:1. Fetch read instructions2. Test preconditions satisfied?3. D(ecode) + Execute decoding only combinatorial

19


Test

Pause

CLK↘ [TEST_SUCCEEDS=1]CLK↗

CLK↘ CLK↗

Finite automaton for RUN1718 CPU

19


Test

Pause

CLK↘ [TEST_SUCCEEDS=1]CLK↗

CLK↘ CLK↗

CLK↘ [TEST_SUCCEEDS=0]


20

Run of the finite automaton

clock pulse

20


clock pulse

20


Fetch

clock pulse

20


Fetch

Test

clock pulse

20



Test

clock pulse

20



Test Pause

clock pulse

20



Test Pause

Fetch

clock pulse

20



Test Pause

Fetch

Test

clock pulse

20



Test Pause

FetchFetch

Test

clock pulse

20



Test Pause

FetchFetch

Test Test

clock pulse

21

Reset

Halted


Test

Pause


21

Reset

Halted


Test

Pause


21

Reset

Halted


Test

Pause


22

Structure of the RUN1718 CPU

22


• ALU

22


• ALU • registerbank

22


• ALU • registerbank • control unit ▪ flag register bank ▪ tester ▪ timer ▪ instruction decoder with instruction register

23

Structure of the RUN1718 CPU (Overview)

23

Structure of the RUN1718 CPU (Overview)

24

How do I start?

24

How do I start?

1. Data Path: How do the components interact?

24

How do I start?

1. Data Path: How do the components interact?▪ during the fetch phase?

24

How do I start?

1. Data Path: How do the components interact?▪ during the fetch phase?▪ during the test phase?

24

How do I start?

1. Data Path: How do the components interact?▪ during the fetch phase?▪ during the test phase?▪ during the execution phase?

24

How do I start?


2. Interfaces for each component

24

How do I start?


2. Interfaces for each component▪ based on given information

24

How do I start?



3. Combined data path

24

How do I start?



3. Combined data path▪ necessary connections (and multiplexers)

24

How do I start?




24

How do I start?




Afterwards: design your own parts.

24

How do I start?




Afterwards: design your own parts. (HADES is no design tool)

25

Summary

• Finite automatonReset ➔ Fetch ➔ Test ➔ (Decode+Execute) ➔ Pause ➔ Halted

• Data path: which data is required when for execution of instruction?

• Start working on the processor, plan carefully!

How does the CPU execute programs? - Radboud Universiteit · 2017-12-06 · 2 Overview So far… • Circuits • Memory • ALU Today… • How are instructions executed? • Fetch-decode-execute

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