x86 - Assembly - How to multiply/divide a constant by another constant in assembly?

Question

So, I have an assembly function, which is called in C. It compiles and gives me no warnings, but when I try to run it, it gives me a segmentation fault. I think it's because I can't move a constant into a register, but to use the mul/div command it requires a value to be in EAX register. How can I multiply or divide two constants in Assembly?

Here's the code so far...

.section .data
.global n
.equ A, 50
.equ B, 5

.section .text
.global loop_function

loop_function:
    # prologue
    pushl %ebp      # save previous stack frame pointer
    movl %esp, %ebp  # the stack frame pointer for sum function
    # beginning 
    movl i, %ebx # place i (declared in c) in ebx
    movl A, %eax # place A in eax
    movl B, %ecx # place B in ecx
    jmp loop
loop:
    movl $0, %edx # clean edx register
    cdq
    idivl %ecx # A / B, result in eax
    imull %ebx # i * A / B, result in eax

    incl %ebx 
    cmpl %ebx, n # if i <= n
    jle loop # then jumps to loop
    jmp end # else jumps to end

end:
    # epilogue
    movl %ebp, %esp  #  restore the previous stack pointer ("clear" the stack)
    popl %ebp     #  restore the previous stack frame pointer
    ret

Answer 1

GAS supports the * operators for assemble-time multiplication of constants. For example, mov $(5 * 50), %eax assembles to exactly the same machine code as mov $250, %eax. Other operators like + - / %, and bitwise stuff are also available. I'll just use * for the examples, but you can construct arbitrary expressions out of compile-time constants as long as they evaluate to a single number (or offset from a symbol that the linker can resolve).

This works with assembler constants like .equ A, 50 or A = 50 as well.

.equ A, 50
.equ B, 5

aa = 3
bb = 7

.globl _start
_start:                    # machine code         .intel_syntax disassembly
    mov $(5 * 50), %eax    # b8 fa 00 00 00    mov  eax,0xfa  # 250

    mov $(aa * B), %ecx    # b9 0f 00 00 00    mov  ecx,0xf   # 3*5 = 15
    mov $A * B,    %edx    # ba fa 00 00 00    mov  edx,0xfa  # 250

Note that the whole immediate constant only uses one $, rather than a $ on every symbol name. For example, mov $(5 + $A), %eax tries to put the address of a symbol called $A (plus 5) into %eax, so you get a link-time error for an undefined symbol.

mov $( $A * $B ), %eax doesn't even assemble:
Error: invalid operands (*UND* and *UND* sections) for '*'
This is because you're trying to multiply the address of two unknown symbols ($A and $B), rather than your assembler constants A and B.

In GAS, all symbols have an associated section. When you define a symbol with .equ or =, it's an "absolute" symbol (rather than a .data section or .text section symbol like you'd get from a label like A:).

Assembler constants aren't really different from symbols defined with labels. However, other than + and -, all assemble-time math operators require both args to be absolute, and the result is absolute.

---

Your code appears to be trying to put the constants into registers to multiply them at runtime. If you insist on doing that as an exercise,

mov   $A, %ecx           # put symbol's value in ECX
imul  $B, %ecx, %eax     # EAX = A * B

mov A, %eax is a load from the symbol's value. i.e. a load from absolute address 50, which obviously segfaults. Single-step with a debugger and look at the disassembly to understand what happened.

AT&T syntax uses $ for immediate constants, so use that to get the value. (Remember, .equ symbols behave the same as labels, like how you'd use $my_string to get the address as an immediate.)