1. 02/02/10 20:53
Assembly Questions
תרגול 12 1

2. 02/02/10 20:53 Q1
Consider the following assembly representation of a function foo containing a for loop

3. Q1 Fill in the blanks to provide the functionality of the loop
02/02/10 20:53 Q1
Fill in the blanks to provide the functionality of the loop
int foo(int a) {
int i;
int result = _____________;
for( ________; ________; i++)
__________________; }
return result;

4. Answer int foo(int a) { int i; int result = a + 2;
02/02/10 20:53
Answer
int foo(int a) {
int i;
int result = a + 2;
for (i=0; i < a; i++)
result += (i + 5);
result *= (i + 3); }
return result;

5. Q2 Which of the functions compiled into the assembly code shown?
02/02/10 20:53 Q2
Which of the functions compiled into the assembly code shown?

6. 02/02/10 20:53 Q3

7. 02/02/10 20:53 Q3
Is the variable val stored on the stack? If so, at what byte offset(relative to %ebp) is it stored, and why is it necessary to store it on the stack?
answer: yes, -4, Need to pass pointer to it to recursive call
Is the variable val2 stored on the stack? If so, at what byte offset(relative to %ebp) is it stored, and why is it necessary to store it on the stack?
answer: no

8. 02/02/10 20:53 Q3
What (if anything) is stored at -24(%ebp)?If something is stored there, why is it necessary to store it?
answer: the value of %ebx is saved here, because %ebx is a callee-save register
What (if anything) is stored at -8(%ebp)?If something is stored there, why is it necessary to store it?
answer: nothing is stored here.

9. Q4 Optimized the following Assembly code as much as possible:
02/02/10 20:53 Q4
movl $0, %eax
movl $0, %edx
movl 8(%ebp), %ebx
.L1:
cmpl %eax, %ebx
jle .L2
movl 12(%ebp), %ecx
addl %eax, %ecx
addl %ecx, %edx
incl %eax
jmp .L1
.L2:
movl %edx, %eax
Optimized the following Assembly code as much as possible:

10. Q4 The small and obvious things: The big improvement:
02/02/10 20:53 Q4
The small and obvious things:
Replace all movl $0 with xorl.
Instead of using both %eax and %ebx – we can initialized %eax to 8(%ebp) and reduce it till we get zero (save the need in a saved register!)
We can read the value of 12(%ebp) outside the loop and increased it by one instead of adding the index.
The big improvement:
Notice we are calculating the sum of an Arithmetic Series. Therefore, instead of using loops we can just calculate the formula – a much more efficient solution!

11. 02/02/10 20:53 Q5
We are sending an ASCII file over the net from computer A to computer B. Will it always be possible for computer B to read the file while knowing nothing on computer A?
Same as before only now we are sending a different file coded using the utf-8 coding?

12. 02/02/10 20:53 Q5
Yes. In ASCII files each character is coded using one byte. Therefore, computer B doesn’t care if computer A use big or little endians.
No. In UTF-8, some of the characters need more then one byte and then computer B must know if computer A use big or little endians.

13. 02/02/10 20:53 Q6
Computer A runs program gcc in 3.2 seconds. Computer B runs the same program in 2.9 seconds. Which computer has better performance and by how much? What about computer C, which runs the program in 3.1 seconds?

14. 02/02/10 20:53 Q6
Performance is relative so we want to measure PB/PA which is the inverse of CPUTA/CPUTB = 3.2/2.9 = Thus the performance of computer B is 10% better that computer A. Computer C is 3% (3.2/3.1 = 1.03) better than computer A. And computer B is 7% (3.1/2.9 = 1.07) faster than computer C.

15. Q7 You are given the next number in binary representation:
02/02/10 20:53 Q7
You are given the next number in binary representation:
What is its value if it is:
An unsigned int.
A float.

16. 02/02/10 20:53 Q7
float =
(-1)1*( )*2( ) = ( )*21 = -1.25*2 = -2.5
unsigned =
= ( ) =
= 3,223,322,624

17. Q8 Answer: many possible solutions
02/02/10 20:53 Q8
Transfer the next C function info Assembly code: (don’t forget comments!)
void swap(int* a, int* b) {
int temp; temp = *a; *a = *b; *b = temp; }
Answer: many possible solutions

18. Q9 What does the spatial locality principal say?
02/02/10 20:53 Q9
What does the spatial locality principal say?
What do we use MUXes for?
What is the ALU?

19. Q10 What is CPI and what is the difference between CPI and CPE
02/02/10 20:53
Q10
What is CPI and what is the difference between CPI and CPE
Latency and Throughput
Data hazards / Control hazzards
Stalling / forwarding / load-use hazard
Branch prediction