I needed a multi-arch shellcode for both x86 and x64 in the same code. Suppose you want to attack a platform, which can either be x86 or x64 where you don’t know in advance which it is. The problem is which version you really need to use at runtime then, right?
This is a tiny trick I’ve been using for a long while now which tells whether you run on x64 or not:
INC EAX ; = DB 0x40
The idea is very simple, since x64 and x86 share most opcodes’ values, there is a small in-similarity with the range of 0x40-0x50, in x86 it used for one byte INC
Now when AMD64’s ISA (Instruction Set Architecture) was designed, they added another set of 8 GPRs, making it a total of whopping 16 GPRs. In a world where x86 ruled, you only needed 3 bits in the ModRM byte (some byte in the instruction that tells the processor how to read its operands) to access a specific register from 0 to 8. With the new ISA, an extra bit was required in order to be able to address all 16 registers. Therefore, a new prefix (called the REX prefix) was added to solve this problem with an extra bit (and there’s more to it, not relevant for now). The new prefix used the range of 0x40-0x50, thus eliminating old one byte INC/DEC (no worries however, now compilers use the 2 bytes existent variation for these instructions).
Back to our assembly code, it depends on the fact that in x86 the INC EAX, really increments EAX by one, and so it will become 1 if the code runs on x86. And when it’s run on x64, it becomes a prefix to the NOP instruction, which doesn’t do anything anyway. And hence, EAX stays zero. Just a final note for the inexperienced that in x64, operations on 32 bit registers are automatically promoted to 64 bit registers, so RAX is also 0.