Writing an X86–64 Assembly Language Program

Part V: Conditionals, Jumping, and Looping

Photo by Clem Onojeghuo on Unsplash

This guide is part five of a series

• Part one: Getting Started Writing Assembly Language
• Part two: Finding an Efficient Development Cycle for writing Assembly Language
• Part three: Printing Command Line Arguments
• Part four: Sending Function Arguments and Receiving a Return Value

This guide is followed by

• Part six: How to Determine String Length
• Part seven: Quick Reference

Conditionals and Looping

Assembly language supports conditionally jumping to a specific line of the code. Looping is actually just a special implementation of jumping. It’s a jump to the beginning of the “loop” until some condition is finally met.

Essentially this post is about writing conditional statements to branch one of two ways depending on some condition.

The core conditional operator is cmp which is short for ‘compare’. The cmp operator compares two values and then sets some flags indicating the relation of the two values. Flags can be checked using some other operators, namely:

• JE means to jump if equal
• JZ means to jump if zero
• JNE means to jump if not equal
• JNZ means to jump if not zero
• JG means to jump if the first operand is greater than second
• JGE means to jump if the first operand is greater or equal to second
• JA is the same as JG, but performs an unsigned comparison
• JAE is the same as JGE, but performs an 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 demonstrating a looping function used to print a list of arguments from the stacks

I hope you enjoyed this. To learn more, find the next part of this series here: How to Determine String Length.