1. Phillipa Gill phillipa@cs.toronto.edu
SCI 199 Y Tutorial Sept. 14, 2009
Phillipa Gill

2. Von Neumann ArchitectureOS
Programs
Data
Memory
Control
Unit
Arithmetic
Logic Unit
Accumulator
Input
Output

3. 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

4. Example Program: 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

5. Example Program: Code ANSWER=0 I=1 WHILE ANSWER = 0 and I <= N
IF X=A(I) then ANSWER = I
I=I+1
ENDWHILE
OUTPUT ANSWER

6. Example Program: Memory
1000 X
1001 N
1002 I
1003
ANSWER
2000 A
2001 …
N-1

7. Example Program: Memory (2)
Write-constant-into-memory 0,1003 % set ANSWER=0
Write-constant-into-memory 1,1002 % set I=1
Get-memory %put N into accumulator
Subtract-memory % compute N-I
Conditional-jump-on-negative 16 %if N-I was negative
Get-memory % get value of A(I)
Subtract-memory % compute A(I) – X
Conditional-jump-on-zero 14 % if A(I)-X=0
Get-memory 6 % load word 6 into memory
Add-constant 1 % increment address in word 6
Get-memory % load I

8. Example Program: Memory (3)
Get-memory 6 % load word 6 into memory
Add-constant 1 % increment address in word 6
Get-memory % load I
Add-constant 1 % increment I
Jump % go back to start of loop
Get-memory % get value of I
Put-memory %put this value into 1003 (answer)
Output % output answer

9. Issues Code can modify itself! The above code changes the statement:
Get-memory 6 % load word 6 into memory
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