1

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

Food for thought: What to do it we need to add up 100 numbers? 1000?

2

Idea: Instead of branching forward, let’s branch backward, thus creating a loop!

3

Do you see a problem with this loop?

4

Conclusions

5

• Creating a loop is easy: just branch

backward in the program!

• The hard part is avoiding an infinite loop.

• We need a way to make decisions, so that

the program can eventually exit the loop if some condition becomes true (e.g.it has

repeated 100 times)

Decision-making instructions

6

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

7

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'

EOL3

QUIZ: What does this program do?

9

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?

10

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

11

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

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

12

main: LDA sum,d

…

BRLT negMsg

STA sum,d

DECO sum,d

finish: STOP

negMsg: CHARO 0x0045,i

BR finish

QUIZ: Change the program so it prints 'Z' (for zero) if the sum is zero, and prints the actual sum if nonzero.

13

main: LDA sum,d

…

BRLT negMsg

STA sum,d

DECO sum,d

finish: STOP

negMsg: CHARO 0x0045,i

BR finish

14

What does this program do? (p.173)

CPA must

always be

followed by a

conditional

branch!

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.)

16

Remember: Decision-making instructions

17

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?

18

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

19

Not in text

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

• They’re hard to draw in electronic documents

20

Not in text

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

• They’re hard to draw in electronic documents

21

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):

22

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!

23

… but pseudocode is more precise

24

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

25

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

26

To do for next time

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

27

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

28

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

29

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.

30

QUIZ: Desk-check the repeated division algorithm

with decimalNumber = 242, and newBase = 8

31

Answer: 362

Translating a pseudocode algorithm

32

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!

33

6.6 Testing

34

Test plan:

• Code coverage, a.k.a. clear-box testing

• Data coverage, a.k.a. black-box testing

QUIZ: Testing

35

Is the test plan on p.187:

• Code coverage, a.k.a. clear-box testing?

• Data coverage, a.k.a. black-box testing?

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?"

36

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

37

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

38

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

39

Homework Due Monday, Nov. 9

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

• The next few slides are collected references for the PEP/8, just to have everything in one place …

41

42

Pep/8 Machine Instructions

43

Pep/8 Assembly Instructions

44

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 45

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

46

What is the difference between

operations and pseudo operations?