Description
It can be a pain to identify where we are in the decoding process when implementing an emulator, and this would avoid having to store fetched bytes elsewhere temporarily. For example DJNZ has an extended M1 cycle followed by an operand read, so at the stage where the extension has to be inserted, the decoder result is still an error, and we can't blindly fetch the operand; cycle timing is critical, because some input lines are sampled regularly at a certain clock (edge) of every machine cycle.
When fetching the opcode byte, it's not possible to identify DJNZ definitely unless we know both the opcode and the decoder state from a try_decode() call either before or after it. It can be too restrictive depending on how low-level the emulator gets.
Another example is LD (IX/IY+d), n. This is the only indexed instruction that is not in the bitwise instruction table and does not end with the displacement fetch. When this occurs, the displacement computation overlaps with the final fetch cycle. But since the decoder state (when invoking try_decode() with each pushed byte) goes RootOpcode, IndexedOpcode, IndexedOperand, IndexedOperand, we can't distinguish between other indexed 4-byte instructions unless we know the full context of the decoding process. So we either need to save some more intermediary information or add IndexedDisplacement to replace IndexedOperand when the next value that the decoder needs is the displacement.
Activity