Difference between revisions of "General Coding Tricks"

Revision as of 16:34, 6 August 2016

Data is code, code is data

Code is nothing more than data that the CPU interprets. For example, consider this multi-byte instruction:

        mov ah,37h

This assembles to B4 37. B4 by itself isn't interesting, but 37 is the opcode for AAS. Let's say you had this code before a loop, and you needed to perform AAS at the top of a loop. Rather than put AAS at the top of the loop, you can reuse the opcode that will already be there as part of the mov ah,37 that comes before it. Just jump directly into the middle of the mov ah,37h, which will get interpreted and executed as AAS:

label:
        mov ah,37h
        ;misc. stuff
        loop label+1

The +1 specifies the jump should go to 1 byte past the actual location.