Question

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?

Answer:

[ Write answer here ]

2(b)        Convert the following code fragment (in the C language) to assembly code.

Explain 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:

[ Write answer here ]

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)

test:

        push    rbp # push S Push source onto stack

        mov     rbp, rsp   # mov S, D Move source to destination

        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 # jmp Jump to specified location/label

.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 # Pop return address from stack and jump there

Here i have also attached some meaning full sheet which helps you for better understanding purpose.

Hope this will works.