Part 3: Conditionals & Looping

Looping is a specific application of jumping on a conditional. Essentially this post is about writing conditional statements to branch one of two ways depending on some condition.

The core conditional operator is cmp for compare. The cmp operator compares two values and then sets a number of flags indicating the relation of those two values. Flags can be checked using a number of operators, namely: JE - jump if equal JZ - jump if zero JNE - jump if not equal JNZ - jump if not zero JG - jump if first operand is greater than second JGE - jump if first operand is greater or equal to second JA - JG, but performs unsigned comparison JAE - JGE, but performs unsigned comparison

To loop, a function simply calls itself based on the outcome of a conditional statement. Of course, at some point the condition must break the loop to avoid an infinite cycle.

Here is an example of a function used to print a list of arguments from the stack:

.printAllArgs:
  call .printNewline   ; fxn prints newline
  pop r11              ; pop address of the calling fxn. Remove temporarily
  mov rsi, [rsp]       ; stack pointer memory address. Holding argument to print. 
  mov rdx, 8           ; how long is the message. TO DO: calculate argument length
  push r11             ; push return address back onto the stack
  call .print
  pop r11              ; pop return address
  pop rcx              ; this is the already printed arg
  push r11             ; push return address back onto the stack
  sub rbx, 1           ; rbx is the argument count. Iterate down 1
  cmp rbx, 0           ; are there zero args left to print? 
  jne .printAllArgs    ; if more args to print, loop again
  call .printNewline   ; otherwise print Newline and return
  ret