Assembly Primer for Hackers (Part 9) Conditional Branching

This is Part 9 of the "Assembly Language Primer for Hackers" video series. Please start here by watching Part 1 of this series, if you have not already done so. In this video we will look at Conditional Branching in Assembly Language using the JXX family of instructions and the LOOP instruction.

The conditional jump instructions such as JA, JAE, JZ, JNZ etc. use various flags in the EFLAGS register such as the Zero Flag (ZF), the Parity Flag (PF), Overflow Flag (OF), Sign Flag (SF) etc.  to determine which instruction path to take next. In this video we will look at the JZ condition jump instruction in great detail. JZ using the Zero Flag (ZF) to determine if the last instruction resulted in the Zero operation or not and then chooses to jump to a specified location if it was set. We will also look at the LOOP instruction which used the ECX register to loop over a set of instructions over and over again.

Please download the ConditionalBranching.s sample code file to try the example shown in this video.

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Vivek Ramachandran is a security evangelist and has been working in computer security related fields for the past 7 years. In 2007, Vivek spoke at world renowned conferences Defcon (WEP Cloaking Exposed) and Toorcon (The Caffe Latte Attack). The discovery of the Caffe Latte Attack was covered by CBS5 news, BBC online, Network World etc news agencies.In 2006, Vivek was announced as one of winners of the Microsoft Security Shootout contest held in India among 65,000 participants. He has also been a recipient of a Team Achievement at Cisco Systems for his work on 802.1x and Port Security modules on the Catalyst 6500 switches. Currently he spends all of his time maintaining Security- Freak.Net , SecurityTube.Net and is the co-founder of Axonize. Vivek, is a Bachelor in Electronics and Communications Engineering from the prestigious Indian Institute of Technology, Guwahati.You can contact him at vivek[at]securitytube.net