Working on Single-Step Breakpoints in a Debugger

Introduction

Single-step breakpoints are fundamental debugging mechanism that allows execution to pause after every single instruction. Unlike hardware breakpoints which trigger at a specific address, single-step mode traces every instruction sequentially.

In this post, we will see these breakpoints in detail with NASM + QEMU.

Single-Step Breakpoints and Trap Flag

A debugger can determine single-step breakpoints with DR6 register. DR6 register holds BS (Single-Step Bit) and a debugger can use this bit. BS is 14 bit of DR6 register:

I assigned the value 0x5 to the step_count variable, so each step is performed based on this value. Also my breakpoint is based on DebugTarget function:

DebugTarget:

    mov  rax, 0x1

    mov  rbx, 0x2

    add  rax, rbx

    mov  rcx, rax

    mov  rdx, 0x0

    ret