SCI 199 Y Tutorial Sept. 14, 2009

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

2. Von Neumann Architecture Data OS Programs 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 • 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

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 1001 %put N into accumulator • Subtract-memory 1002 % compute N-I • Conditional-jump-on-negative 16 %if N-I was negative • Get-memory 2000 % get value of A(I) • Subtract-memory 1000 % 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 1002 % 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 1002 % load I • Add-constant 1 % increment I • Jump 6 % go back to start of loop • Get-memory 1002 % get value of I • Put-memory 1003 %put this value into 1003 (answer) • Output 1003 % output answer

9. Issues • Code can modify itself! • 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