tl;dr: I would like to add a long x86 instruction which doesn’t conform to any existing format that I know; I’m not sure where to start.
I am attempting to add an instruction into X86, to be simulated in gem5. I’ve already added a simple, opcode-only instruction which I can successfully decode and run in gem5, so I am roughly familiar with .td files and how backends are built out of them.
My goal now is to make a more complex instruction – specifically, I need to add large operands. The format would look something like this:
- 1 byte opcode (0x06, which I hijacked from PUSHES, which isn’t implemented in gem5)
- n byte destination (memory location)
- n byte source (memory location)
- n byte source (memory location or immediate)
If n=4, then the total opcode length is 13 bytes, which is under the 15 byte x86 limit.
As far as I know, this doesn’t conform to any existing x86 format. Because that’s the case, I’m not sure how to go about encoding an instruction like this; presumably, I can’t use the existing I<…> class, which is what I’d used previously.
Can anyone point me in the general direction of what I will need to do to encode this rather arbitrary instruction format? Should I look into implementing a new Instruction class? Is there an easier way?
Gus Smith, PSU