Disassembling/executing instructions with unused bits

[hop]

How does the 68000 decode instructions with unused bits? Is this well-defined or undefined behaviour?

For example

ORI.B $#ff,D1

assembles to two words:

0001 00FF

. The upper byte in the second word is not required to define the byte-sized instruction.

What should happen if the CPU/disassembler/emulator encounters non-zero bits in there? For example:

0001 FFFF

. Will the $ff00 bits in the second word simply be ignored (by the spec), or will the decoder class this as an illegal instruction.

Sorry if this is in the manual - I couldn't find any information on it.

[paraj]

The unused bits are just ignored (by all M68K versions), same with 000/010 and the previously unused bits in the extension word (scale/full extension word bit). The latter is at least documented (68000PRM 2.4).

An emulator should obviously behave the same way or it will fail on actual code.

A good disassembler will reproduce the original instruction, i.e. preserve the unused bits. Otherwise you can't reassemble the same binary. Maybe via a dc.w sequence if the original instruction can't be accurately re-used in context (e.g. for the extension word thing)

[jimshaw]

You'd have to tell a disassembler which cpu the code was intended for, else it won't know the right thing to do with bits that are undefined on 68000 and used for something new on the 68020 or later.