SCI 199 YTutorial Sept. 14, 2009

Phillipa Gill

phillipa@cs.toronto.edu

Von Neumann Architecture

Memory

ControlUnit

Arithmetic Logic Unit

Accumulator

Input Output

OS Programs Data

General Purpose Machines

• As opposed to fixed purpose– E.g., your calculator is not a word processor

or gaming console

• Machine can be used to solve problems using programs stored in memory

Example Program: Problem

• Problem:– Input: a value X, a list A with length N– Output:

• the position in the list where X resides if X is in the list.

• 0 otherwise.

• Example: A = 2,4,6,5,10 if X is 6 this program will return 3

Example Program: Code

ANSWER=0

I=1

WHILE ANSWER = 0 and I <= NIF X=A(I) then ANSWER = I

I=I+1

ENDWHILE

OUTPUT ANSWER

Example Program: Memory

1000 X

1001 N

1002 I

1003 ANSWER

2000 A

2001

…

2000 + N-1

Example Program: Memory (2)1. Write-constant-into-memory 0,1003 % set

ANSWER=0

2. Write-constant-into-memory 1,1002 % set I=1

3. Get-memory 1001 %put N into accumulator

4. Subtract-memory 1002 % compute N-I

5. Conditional-jump-on-negative 16 %if N-I was negative

6. Get-memory 2000 % get value of A(I)

7. Subtract-memory 1000 % compute A(I) – X

8. Conditional-jump-on-zero 14 % if A(I)-X=0

9. Get-memory 6 % load word 6 into memory

10. Add-constant 1 % increment address in word 6

11. Get-memory 1002 % load I

Example Program: Memory (3)

9. Get-memory 6 % load word 6 into memory

10. Add-constant 1 % increment address in word 6

11. Get-memory 1002 % load I

12. Add-constant 1 % increment I

13. Jump 6 % go back to start of loop

14. Get-memory 1002 % get value of I

15. Put-memory 1003 %put this value into 1003 (answer)

16. Output 1003 % output answer

Issues

• Code can modify itself!9. Get-memory 6 % load word 6 into memory

10. Add-constant 1 % increment address in word 6

• The above code changes the statement:– Get-memory 2000 into Get-memory 2001

• Why is this a bad idea?

• Recent developments have tried to fix this