x86 assembly language just like most other programming languages provides us with the ability to control the flow of the program using various instructions.
This article provides an overview of those instructions that can be used to control the flow of a program.
See the last article in this series, How to diagnose and locate segmentation faults in x86 assembly.
Using comparison instructions to control applications at the x86 level
x86 instruction set comes with two popular instructions for comparison. They are CMP and TEST. Let us explore the following program to understand how these two instructions work.
mov eax, 101
mov ebx, 100
mov ecx, 100
cmp eax, ebx
cmp ebx, ecx
xor eax, eax
test eax, eax
First, let us assemble and link this program using the following commands.
$ nasm comparison.nasm -o comparison.o -f elf32
$ ld comparison.o -o comparison -m elf_i386
Now, let us load the program in GDB as shown below.
$ gdb ./comparison
Set up a breakpoint at the entry point of the program and run the program as shown