x86 assembly language

Question 1

How can you preserve the values of function registers across function calls.

Answer 1

pushing to stack then poping after function

Question 2

How does using a callee saved register work?

Answer 2

The value needs to be preserved when the function returns. push at the beginning of the function and pop at the end

Question 3

What does the following command do?
movq $0x501208,%rdi

Answer 3

puts the constant 0x501208 into rdi

Question 4

What does the following command do?
movq %rax,%rdi

Answer 4

moves contents of register rax to rdi

Question 5

What does the following command do?
movq %rsi,(%rdi)

Answer 5

Stores the value in rsi in the address contained in rdi

Question 6

What does the following command do?
movq 0x501208,%rdi

Answer 6

Fetches contents in address 0x501208 and stores it in rdi

Question 7

What are the commands at the start of every function to save the frame pointer in x86?

Answer 7

pushq %rbp
movq %rsp, %rbp

Question 8

What are the instructions at the end of a function in x86?

Answer 8

leave
ret

Question 9

How do you assemble and run a program named t1.s?

Answer 9

gcc -static –g -o t1 t1.s
./t1

Question 10

What does cmpq S2, S1 do?

Answer 10

S1-S2

Question 11

What does testq S2, S1 do?

Answer 11

S1 & S2

Question 12

What are some conditional jump instructions?

Answer 12

JE (jump equal)
JNE (jump not equal)
JGE (Jump if greater or equal)
JNGE (Jump if not greater or equal)

Question 13

How do you run the below program? The function is in an assembly file?

maxarray.c:
#include <stdio.h></stdio.h>

long a[] = {4, 6, 3, 7, 9, 5 };
long maxarray(long n, long *a);
int
main()
{
printf(“maxarray(6,a)=%ld\n”, maxarray(6,a));
}

Answer 13

gcc -g -static -o maxarray maxarray.c maxarray.s

./maxarray

Question 14

How do you declare a global variable in assembly for a long named a?

Answer 14

.data
.comm a,8 # long a;

Question 15

How do you move the immediate value of a global array into a register? (double check correct)

Answer 15

Using dollar sign before $

ex: movq $a, %rsi

Question 16

What is the size of “regular” registers in x86 assembly?

Answer 16

8 bytes

Question 17

How do we do single byte manipulation in assembly?

Answer 17

Use byte registers like %r13b

Also use movb instead of movq

Question 18

What is the stack used for?

Answer 18

● store the return address
● store local variables
● Save registers when running out of them (register
spill).
● pass arguments when they don’t fit in the
registers.

Question 19

Think of an example of using the stack in an assembly program

Answer 19

.globl sum

.type sum, @function

sum:

#

pushq %rbp # Save frame pointer
movq %rsp, %rbp #
subq $24, %rsp # Create space in stack for
# tmp1, tmp2 and result

movq %rdi, -8(%rbp) # tmp1 = a
movq %rsi, -16(%rbp) # tmp2 = b
movq -8(%rbp), %rax
addq -16(%rbp), %rax
movq %rax, -24(%rbp) # result = tmp1 + tmp2 ;
movq -24(%rbp), %rax # return result ;
leave # eq. mov %rbp,%rsp
# pop %rbp

ret