I have a server socket which receives a byte (operation code) from the client.
Depending on what the value of that byte is, I need to decode further data with a specific function.
For example:
0x01, I need to call function1 to decode it.0x02, execute call function2I don't want to hardcode it all with compare and jump statements because there are more than 150 possible values and that would result to 400+ lines of code.
What would be the best option to implement this, resulting in the cleanest code? In C language, I would do this with a simple array: array[index] where array stores the function pointers and index is the operation code. This would result in O(1) speeds.
This is just a general question, but I'm working with x86_64 asm with NASM syntax if that helps.
Yes, a table of function pointers is the obvious way in asm as well.
call [dispatch_table + rax*8] in non-PIE / non-PIC code where the label address itself can fit in a 32-bit sign-extended value so you can use a label with other registers, otherwise use a RIP-relative LEA to get the table base into another register. (Look at C compiler output for a function-pointer dispatch if you need an example.)
Don't forget to validate input first, e.g. cmp eax, MAX_FUNC_NUMBER / ja error, after you do a zero-extending byte load into RAX with movzx eax, byte [mem] to zero-extend the index.
Or just a jmp instead of call to make it like a switch within a function if that's more convenient than a call that pushes a return address.