Re: How to handle undocumented opcodes

[Nick Clifton <nickc at redhat dot com>]

Hi Arnold,

> I use different machine numbers to distinguish between z80 code 
> with and without undocumented instructions. My current naming 
> scheme for processor variants is:
> z80strict - only documented Z80 instructions
> z80       - adds support for undocumented instructions that are
> 	legal on R800
> z80full   - all Z80 instructions
> r800      - documented r800 instructions

What is the difference between "z80" and "z80full" ?

> I'm not completely satisfied with this scheme as it may be too 
> finegrained, and "z80" should probably refer to the documented 
> instructions only.

If the undocumented instructions are actually well known and used then I 
think that it is fair that you include them in the default z80 target.

> Are there any standards or conventions I should adhere to? 

No :-)  But in general you should try to make your port both flexible 
and functional.  So by default I would recommend accepting all Z80 
instructions, but producing a warning when assembling undocumented 
instructions.  Provide a command line switch to disable the warnings, 
and a separate switch to enforce strict adherence to the Z80 
specification (ie effectively turning the warnings into errors).

The disassembler should be agnostic - it should disassemble anything.

The linker ought to accept mixed (Z80) format input files, but select 
the output format based on the most permissive of the input formats.

Cheers
  Nick