Question

Question 2

Assume that we are using the x86-64 architecture, as mentioned in lecture 4.

2(a) Explain the meaning of the following instruction:
movq %rbx, (%rcx)
        State your assumptions of the content in registers and memory, before executing this instruction.
        What will happen after executing this instruction?

(b)Convert the following code fragment (in the C language) to assembly code.
Explain your steps. [25 marks]

long test ( long a, long b, long c ) {
long t;
if (a>b) {
if (a>c)
t=a;
else
t=c;
} else {
t=b;
}
return t;
}

Answer 1

A) Here i have explained in more simplified form and also mention each and every things individually.

movq %rbx, (%rcx)

q : “quadword” refers to an eight-byte value (suffix q)

Registers %rbx, %rbp, and %r12-r15 are callee-save registers, meaning that they are saved across function calls.

Registers %rax, %rcx, %rdx, %rdi, %rsi, %rsp, and %r8-r11 are considered caller-save registers, meaning that they are not necessarily saved across function calls.

Above instruction move the eight-byte value %rbx value to the memory address of %rcx registers.

B) here i have explained all the instruction in the program i also mentioned the Registers meaning in the above answer. I have tried to be explain in more simplified way.

test:

        push    rbp # push S Push source onto stack

        mov     rbp, rsp # mov s d move the source value to destination register.

        mov     QWORD PTR [rbp-24], rdi

        mov     QWORD PTR [rbp-32], rsi

        mov     QWORD PTR [rbp-40], rdx

        mov     rax, QWORD PTR [rbp-24]

        cmp     rax, QWORD PTR [rbp-32] # cmp S2 , S1 Set condition codes according to S1 - S2

        jle     .L2 # jle / jng Label Jump if less or equal

        mov     rax, QWORD PTR [rbp-24]

        cmp     rax, QWORD PTR [rbp-40]

        jle     .L3

        mov     rax, QWORD PTR [rbp-24]

        mov     QWORD PTR [rbp-8], rax

        jmp     .L4

.L3:

        mov     rax, QWORD PTR [rbp-40]

        mov     QWORD PTR [rbp-8], rax

        jmp     .L4

.L2:

        mov     rax, QWORD PTR [rbp-32]

        mov     QWORD PTR [rbp-8], rax

.L4:

        mov     rax, QWORD PTR [rbp-8]

        pop     rbp

        ret # ret Pop return address from stack and jump there

I have cover all the command usage and also there functionality