QUIZ Pipelining

A computer pipeline has 4 processors, as shown above. Each processor takes 15 ms to execute, and each instruction must go sequentially through all 4 processors.

A program has 10 instructions. Calculate how long it takes to run it:

• without pipelining

• with pipelining

1

2

Extra-credit QUIZ

Low-level software

Components

Circuits

Gates

Transistors

Abstractions and more abstractions …

4 You are here

6.1 Computer Operations

Computer = programmable electronic device that can store, retrieve, and process data

Von Neumann architecture:

• Data and instructions to manipulate the data are represented in the same way (binary) and are stored in the same place (memory)

• For processing, both data and

instructions have to be brought

into the CPU (fetch-execute cycle)

5

6.2 Machine Language

Machine language = The language made up of binary coded instructions built into the hardware of a particular computer and used directly by the computer

Why would anyone use machine language?

(Hint: they had no choice. Why?)

6 Image source: http://www.alpcentauri.info/chapter_18.html

Remember: The Fetch-Execute Cycle

7

According to the CPU’s

machine language

Characteristics of machine language:

– Every processor type has its own set of specific machine instructions

– The relationship between the processor and the instructions it can carry out is completely integrated

– Each machine-language instruction does only one very low-level task

8

Pep/8 Virtual Computer

Virtual computer

A hypothetical machine designed to contain the important features of a real machine

Pep/8

A virtual computer designed by Stanley Warford that has 39 machine-language instructions

(We’re going to cover only a few of them!)

9

Extra-credit

10

Pep/8 Registers

– The program counter (PC) (contains the address of the next instruction to be executed)

– The instruction register (IR) (contains a copy of the instruction being executed)

– The accumulator (register A)

The memory unit is made up of 65,636 Bytes of storage

11

Can you figure out how long the Pep/8 address is?

QUIZ

The previous version of Pep, Pep/7, could use only 12 bits for memory addresses.

How many Bytes of memory could Pep/7 address?

12

13

Explain the

address format!

16 bits

24

bits

14

Explain the

address format!

Can you show what

instructions look like

in memory?

Instruction Format

15

Instruction Format

Operation code, a.k.a. opcode Specifies which instruction is to be carried out Register specifier Specifies which register is to be used (only use A in

this chapter) Addressing-mode specifier Says how to interpret the operand part of the

instruction: • Direct • Immediate

16

Say that again?!?

Instruction Format

Addressing-mode specifier Says how to interpret the operand part of the

instruction: • Direct • Immediate 2 + 3 Immediate A + B Direct

17

??

18

Immediate and direct addressing modes

Immediate and direct addressing modes

19

Example:

ADD 42 to accumulator A

What exactly am I to add?

Instruction Format

20

Addressing modes:

• immediate

• direct

Is there something we are not telling you

about Pep’s addressing modes?

PEP/8 machine instructions

21

22

What does this instruction mean, what is

its hex code, and what exactly does it do?

23

What does this instruction mean, what is

its hex code, and what exactly does it do?

24

What do these instructions mean, what are

their hex codes, and what exactly do they do?

Wait a second ...

25

Conclusions It is useless to write operands into IR.

IR should be written only in the FETCH stage of the Fetch-Execute cycle, when the 3 bytes of the instruction are brought from memory.

Operands can be written only to:

• Accumulator

• Memory

Some instructions cannot have the immediate addressing mode

26 EOL 1

QUIZ

• Name all the 4 parts of the “fetch-execute” cycle.

• What do IR, A, and PC stand for in the computer’s architecture?

• Name and explain the 2 addressing modes we’ve covered.

27

QUIZ: Immediate and direct addressing modes

28

SUB 30 from accumulator A

What exactly am I to

subtract?

29

What do these instructions mean, what are

their hex codes, and what exactly do they do?

Why is there only one on this page?

What does this instruction mean, what is

its hex code, and what exactly does it do?

31

What do these instructions mean, what are

their hex codes, and what exactly do they do?

QUIZ

• Name all the 4 parts of the “fetch-execute” cycle.

• What do IR, A, and PC stand for in the computer’s architecture?

• Name and explain the 2 addressing modes we’ve covered.

32

Immediate and direct addressing modes

33

We have this Pep

instruction:

LOAD 20 (decimal) into

accum. A

What exactly am I to load?

Problem 16 / 189

34

The PEP/8 memory has the following contents:

0001 A2

0002 11

0003 00

0004 FF

What are the contents of reg. A after this instruction

is executed:

C1 00 01

35

0001 A2

0002 11

0003 00

0004 FF

Instruction

executed:

C1 00 01

Problem 17 / 190

36

The PEP/8 memory has the following contents:

0001 A2

0002 11

0003 00

0004 FF

What are the contents of reg. A after this instruction

is executed:

C1 00 02

The first step is …

Problem 17 / 190

37

The PEP/8 memory has the following contents:

0001 A2

0002 11

0003 00

0004 FF

What are the contents of reg. A after this instruction

is executed:

C1 00 02

1100 0001 0000 0000 0000 0010

6.3 Program to write "Hello"

38

Every program ends with this!

Hand Simulation

39

What is the fetch/execute cycle?

How much is the PC incremented?

0 0 0 0

0000 50

0001 00

0002 48

0003 50

What ASCII character is this?

Hand Simulation

40

What is the fetch/execute cycle here? What ASCII character is this?

0 0 0 0

Your turn!

What does this program do?

1111

0000

1111

1 0000

42

Steps for executing a machine-language

program

Loader

Program

in

memory Execute

Machine

code

program

in a file

on disk

SKIP Pep/8 Simulator

43 EOL 2

Problem 33 / 191

44

The PEP/8 memory has the following program in

memory (in hex):

0001 49

0002 00

0003 08

0004 51

0005 00

0006 08

0007 00

Convert the program to binary and

explain in your own words what it does.

Fact: Writing programs in machine language sucks!

(i.e. it is time-consuming, boring, error-prone, unintuitive, etc.)

45

… that’s why they invented

assembly language

46

6.4 Assembly Language

Assembly language

A language that uses mnemonic codes to represent machine-language instructions

Assembler A program that reads each of the instructions in mnemonic form and translates it into the machine-language equivalent

47

48

Steps for executing an assembly-language program

Loader

Program

in

memory Execute

Pep/8 Assembly Language

49

Opcode is 31

Opcode is 38

Opcode is 39

Opcode is 04

“Hello” program in assembly

CHARO 0x0048, i ;Outputs character 'H'

CHARO 0x0065, i

CHARO 0x006C, i

CHARO 0x006C, i

CHARO 0x006F, i

STOP

.END

50

mnemonic operand addressing

mode comments

What in the

world is this?!

A New Program

51

Problem: Read and sum three

values and print the sum

How would you do it by hand?

Declaring variables in assembly: We simply reserve space in memory for them!

52

Labels – they are just

placeholders for

memory addresses

Assembler directives, a.k.a. pseudo-ops

53

What is the difference between

operations and pseudo operations?

Completed Program

54

sum: .WORD 0x0000

num1: .BLOCK 2

num2: .BLOCK 2

num3: .BLOCK 2

main: LDA sum,d

DECI num1,d

ADDA num1,d

DECI num2,d

ADDA num2,d

DECI num3,d

ADDA num3,d

STA sum,d

DECO sum,d

STOP

.END

Completed Program

55

sum: .WORD 0x0000

num1: .BLOCK 2

num2: .BLOCK 2

Num3: .BLOCK 2

main: LDA sum,d

DECI num1,d

ADDA num1,d

DECI num2,d

ADDA num2,d

DECI num3,d

ADDA num3,d

STA sum,d

DECO sum,d

STOP

.END

Houston, we have a problem! How does the CPU know that the program starts here?

Solution: “Branch” instruction

56

BR main

sum: .WORD 0x0000

num1: .BLOCK 2

num2: .BLOCK 2

num3: .BLOCK 2

main: LDA sum,d

DECI num1,d

ADDA num1,d

DECI num2,d

ADDA num2,d

DECI num3,d

ADDA num3,d

STA sum,d

DECO sum,d

STOP

.END

See comments on

next slide

57

Completed Program Unconditional

branch!

58

Your turn! Change the program so it adds only two numbers: always 7 + 42

We have covered sections 6.1, 6.2, 6.3, and part

of 6.4.

• Read the text carefully and make sure you can explain in your own words what each instruction accomplishes.

Individual work (to do in notebook for next time):

16, 17, 19, 21-26

59

EoL4

QUIZ: Write an assembly program to subtract 2010 from 4210 and put the result in memory

60

BR main

sum: .WORD 0x0000

num1: .BLOCK 2

num2: .BLOCK 2

num3: .BLOCK 2

main: LDA sum,d

DECI num1,d

ADDA num1,d

DECI num2,d

ADDA num2,d

DECI num3,d

ADDA num3,d

STA sum,d

DECO sum,d

STOP

.END

Take inspiration from the

program we studied last time:

EOL

Decision-making instructions

61

BR Set PC to operand unconditionally

BRLT i Set PC to operand if A < 0

BREQ i Set PC to operand if A = 0

Decision making problem: Same as previous “sum” program, but print the sum only if positive or zero; if negative, print an error message

62

negMsg: CHARO 0x0045,i

BR finish

main: LDA sum,d

…

BRLT negMsg

STA sum,d

DECO sum,d

finish: STOP

What ASCII code is this?

Decision making problem: Same as before, but print the sum only if positive or zero; if negative, print the letter ‘E’ (Error)

negMsg: CHARO 0x0045,i

BR finish

main: LDA sum,d

…

BRLT negMsg

STA sum,d

DECO sum,d

finish: STOP

How many ways are there to reach finish?

ASCII code for 'E'

QUIZ: What does this program do?

64

BR main

a: .BLOCK 2

main: DECI a, d

LDA a, d

SUBA 0x002A, i

STA a, d

DECO a, d

STOP

.END

QUIZ: What does this program do?

65

BR main

a: .BLOCK 2

main: DECI a, d

LDA a, d

SUBA 0x002A, i

STA a, d

BREQ yes

DECO a, d

fin: STOP

yes: CHARO 0x0046, i

BR fin

.END

ASCII code for 'F'

QUIZ: Would the decision program from the first example still work if the negMsg block were placed after the main program instead of before?

66

main: LDA sum,d

…

BRLT negMsg

STA sum,d

DECO sum,d

finish: STOP

negMsg: CHARO 0x0045,i

BR finish

67

What does this program do? (p.173)

CPA must

always be

followed by a

conditional

branch!

QUIZ: PEP Assembly

Write a program that adds the number 3 to a number entered by the user, and prints the result if equal to zero, otherwise it prints nothing.

68

QUIZ: PEP Assembly

Write a program that adds the number 3 to a number entered by the user, and prints the result if less than zero, otherwise it prints nothing.

69

QUIZ: PEP Assembly

Write a program that adds the number 3 to a number entered by the user, and prints the result if greater than zero, otherwise it prints nothing.

70

High-level software (Ch.9)

Algorithms (and data structures)

Low-level software

Components

Circuits

Gates

Transistors

6.5 Algorithms and Pseudocode

Algorithm = A sequence of steps for solving a problem

Muḥammad ibn Mūsā al-Khwārizmī (780-850A.D.)

72

Remember: Decision-making instructions

73

BR Set PC to operand unconditionally

BRLT i Set PC to operand if A < 0

BREQ i Set PC to operand if A = 0

How to describe an algorithm in an intuitive way?

74

Flowcharts!

Not in text

We use decision-making instructions to build:

• branches

• loops

Problems with flowcharts: • They’re hard to follow when they get complex

75

Not in text

Problems with flowcharts: • They’re hard to follow when they get complex

• They’re hard to draw in electronic documents

76

Not in text

Problems with flowcharts: • They’re hard to follow when they get complex

• They’re hard to draw in electronic documents

77

Not in text

Our text uses only pseudocode

6.5 Algorithms and Pseudocode

Pseudocode = A mixture of English and formatting to make the steps in an algorithm explicit

There are no syntax rules in pseudocode!

Pseudocode is not case sensitive!

Example: Repeated-division algorithm (convert base-10 number to other bases):

78

While ( the quotient is not zero )

Divide the decimal number by the new base

Make the remainder the next digit to the left in the answer

Replace the original decimal number with the quotient

Algorithms can also be described in natural language!

79

… but pseudocode is more precise

80

IF concerned about cholesterol

Put butter substitute in a pot

ELSE

Put butter in a pot

Turn on burner

Put pot on the burner

WHILE (NOT bubbling)

Leave pot on the burner

Put other ingredients in the blender

Turn on blender

WHILE (more in pot)

Pour contents into lender in slow steam

Turn off blender

Draw the flowchart for this pseudocode

81

IF concerned about cholesterol

Put butter substitute in a pot

ELSE

Put butter in a pot

Turn on burner

Put pot on the burner

WHILE (NOT bubbling)

Leave pot on the burner

Put other ingredients in the blender

Turn on blender

WHILE (more in pot)

Pour contents into lender in slow steam

Turn off blender

Pseudocode functionality

Pseudocode has all of the concepts encountered in any high-level programming language, only the syntax is informal:

– Variables

– Assignment

– I/O

– Selection / decision

– Repetition / loop

– Boolean expressions

82

To do for next time

Read pp.175-179 of the text, referring to your Python experience

83

We can test a pseudocode algorithm w/pencil & paper, a.k.a. “desk checking”

84

What is 93 in base 8?

93/8 gives 11 remainder 5

11/8 gives 1 remainder 3

1/ 8 gives 0 remainder 1

answer 1 3 5

While ( the quotient is not zero )

Divide the decimal number by the new base

Make the remainder the next digit to the left in the answer

Replace the original decimal number with

85

Organizing the solution in a computer-like way gives us

better idea of what is required for the computer to

execute it, e.g. two numbers need to be entered, etc.

86

QUIZ: Desk-check the repeated division algorithm

with decimalNumber = 242, and newBase = 8

87

Answer: 362

Translating a pseudocode algorithm

88

How we translate depends on the

language into which we are translating!

The text example (pp.184-5) is with

translation into PEP assembly, but in class

we’re translating the repeated division alg.

into Python!

Develop this code step-by-step!

89

SKIP 6.6 Testing

90

Read and take notes in notebook:

Software Piracy and Copyrighting

Have you every "borrowed" software

from a friend?

Have you ever "lent" software to a

friend?

Did you know that about 100,000 jobs are

lost in the US every year due to such

"borrowing" and "lending?"

91

Chapter Review Questions

• List the operations that a computer can perform

• Describe the important features of the Pep/8 virtual machine

• Distinguish between immediate addressing mode and direct addressing mode

• Write a simple machine-language program

• Distinguish between machine language and assembly language

92

Chapter Review Questions

• Describe the steps in creating and running an assembly-language program

• Write a simple program in assembly program

• Distinguish between instructions to the assembler (a.k.a. directives) and instructions to be translated into machine code (a.k.a. executable instructions)

• Distinguish between following an algorithm and developing one

93

Chapter Review Questions

• Describe the pseudocode constructs used in expressing an algorithm

• Use pseudocode to express and algorithm

• Distinguish between black-box and clear-box testing

94

Homework Due Friday, Apr. 8

End of chapter exercises

10, 11, 12, 13, 14, 15

18, 20, 27, 34, 36, 41

Correction in 20: use E1 instead of E0

Hint for 34: machine code 31 means decimal input

Hint for 41: How are integers represented in Pep/8?

44, 45

All PEP instructions, with machine codes 00 Stop STOP

04 Branch unconditional BR

08 Branch if Less Than BRLT

0A Branch if equal BREQ

31 Decimal Input Direct DECI

38 Decimal Output Immediate DECO

39 Decimal Output Direct DECO

49 Char. Input Direct CHARI

50 Char. Output Immediate CHARO

51 Char. Output Direct CHARO

70 Add to A Immediate ADDA

71 Add to A Direct ADDA

80 Subtract from A Immediate SUBA

81 Subtract from A Direct SUBA

C0 Load into A Immediate LDA

C1 Load into A Direct LDA

E1 Store A Direct STA 96