The Zilog Z80 and Ascii R800 version of as have a few machine dependent options.
-z80Produce code for the Z80 processor. There are additional options to request warnings and error messages for undocumented instructions. -ignore-undocumented-instructions-WnudSilently assemble undocumented Z80-instructions that have been adopted as documented R800-instructions. -ignore-unportable-instructions-WnupSilently assemble all undocumented Z80-instructions. -warn-undocumented-instructions-WudIssue warnings for undocumented Z80-instructions that work on R800, do not assemble other undocumented instructions without warning. -warn-unportable-instructions-WupIssue warnings for other undocumented Z80-instructions, do not treat any undocumented instructions as errors. -forbid-undocumented-instructions-FudTreat all undocumented z80-instructions as errors. -forbid-unportable-instructions-FupTreat undocumented z80-instructions that do not work on R800 as errors.
-r800Produce code for the R800 processor. The assembler does not support undocumented instructions for the R800. In line with common practice, as uses Z80 instruction names for the R800 processor, as far as they exist.