How x86 Jumps REALLY Work: EFLAGS Truth with GDB + pwndbg

All x86 conditional jumps (ja, jb, je, etc.) are triggered by EFLAGS — not by syntax or code order. Hands-on with GDB and pwndbg to see exactly which flag fires each branch.

July 11, 2025

Harrison Guo

Video by: HarrisonSecurityLab

Published on YouTube: 2025-07-11

x86 Assembly GDB pwndbg Reverse Engineering Malware Analysis EFLAGS

Real reverse engineers know: every x86 conditional jump is decided by EFLAGS bits — ZF, SF, CF, OF — not by what your source code looked like. This short demonstration uses GDB + pwndbg to make the flags visible while stepping through a hand-crafted instruction sequence.

If you’ve ever wondered why a jge fires when you expected a jl, the answer is always in the flag register.

Blog post: x86 jumps and EFLAGS — what GDB shows you

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