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spjewkes / jrnz

A work-in-progress ZX Spectrum emulator
MIT License
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NEG instruction is broken. #32

Closed spjewkes closed 3 months ago

spjewkes commented 3 months ago

I debugged this issue by investigating a problem with Trashman. Here are my notes:

Breaking on 0xB097 on the ‘enter your name screen’. Repeatedly breaking at this point, the value of DE starts to drift. Some instruction is setting this incorrectly.

I think this might be a debugger issue.

I can see if I break beyond this subroutine to 0xA84A that both emulators agree with the DE field.

Seems to be the NEG instruction.

On Fuse the value in A goes from 0x28 to 0xD8

On JRNZ it stays the same for some reason.

0x28 == 00101000 0xD8 == 11011000

So my 2’s complement is failing!

I see the problem. I’ve tried to replicate the NEG instruction by subtracting A from 0. However, I’ve got it the wrong way around.

So it has the operands the wrong way around I believe. I tried to be clever by using a SUB instruction. It might be simpler to just write a neg instruction to make it slightly clearer.

spjewkes commented 3 months ago

Fixed. There is an issue with half carry which I'll raise as a separate bug. I suspect none of my instructions are handling this properly at the moment.