Arithmetic Instructions

The add and sub Instructions

• Description:
  • Adds and subtracts arbitrary values. The destination (where the result is stored) is the first value provided (i.e. the left value).
• Basic Use:
  • We can use a combination of registers and immediates as operands:

mov rax, 1
add rax, 2      ; rax now contains 3
sub rax, 1      ; rax now contains 2
mov rcx, 2
add rax, rcx    ; as above, rax now contains 4
sub rax, rcx    ; rax is now back to 2

The mul Instruction

• Description:
  • Allow multiplication of arbitrary values. Takes a single argument, multiples rax/eax/ax (depending on operand size) by src (whatever follows mul instruction). Result is stored in rax/eax/ax.
• Basic Use:

mov eax, 10
mov ecx, 10
mul ecx             ; rax now contains 100

mov rax, 5
mov rcx, 7
mul rcx             ; rax now contains 35

Mul: Storing Results

• Results are stored in the source (possible), or in a combination of registers in the configuration below:

The div Instruction

• Description:
  • As with mul, div takes a single argument, and divides the value stored in the dividend register(s) by it. This is typically AX/EAX/RAX (and the *dx equivalents), but may vary a bit depending on the size.
  • RDX is also needed. RDX is where the remainder will be stored. This register will need to be set to 0 before division can take place. Otherwise you'll get a SIGFPE.
  • TL;DR: RAX/src (src = rcx in this case). Results stored in RAX, remainder stored in RDX.
• Basic Use:

; clearing the register where the
; high bits would be stored, we're only using what's in rax!
mov rdx, 0
mov rax, 10
mov rcx, 2
div rcx         ; rax now contains 5

Div: Storing Results

• Where to retrieve the results of a div from depends on the size of the arguments. The table below illustrates this relationship:

inc and dec

• Description:
  • Adds or subtracts one from the provided register, storing the result in place.
• Basic Use:

mov rax, 1      ; rax now contains 1
inc rax         ; rax now contains 2
inc rax         ; rax now contains 3
dec rax         ; rax now contains 2