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millfork/docs/abi/undocumented.md
Karol Stasiak 7ea2fe6a4e Almost full LR35902 opcode space coverage
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Undocumented opcodes

6502

Original 6502 processors accidentally supported a bunch of extra undocumented instructions. Millfork can emit them if so desired.

Mnemonics

Since various assemblers use different mnemonics for undocumented opcodes, Millfork supports multiple mnemonics per opcode. The default one is given first:

  • AHX, AXA, SHA

  • ALR

  • ANC

  • ARR

  • DCP, DCM

  • ISC, INS

  • LAS

  • LAX

  • LXA, OAL

  • RLA

  • RRA

  • SAX*

  • SHX, XAS

  • SHY, SAY*

  • SBX, AXS**

  • SRE, LSE

  • SLO, ASO

  • TAS

  • XAA, ANE

* HuC2680 has different instructions also called SAX and SAY, but Millfork can distinguish between them and the NMOS illegal instructions based on the addressing mode.

** AXS is also used for SAX in some assemblers. Millfork interprets AXS based on the addressing mode.

Generation

In order for the compiler to emit one of those opcodes, an appropriate CPU architecture must be chosen (nmos or ricoh) and either it must appear in an assembly block or it may be a result of optimization.

Optimization will never emit any of the following opcodes due to their instability and/or uselessness: AHX, LAS, LXA, SHX, SHY, TAS, XAA.

Z80

Original Z80 processors accidentally supported a bunch of extra undocumented instructions. Millfork will not emit them. The only exception is SLL, which will be emitted if it occurs in a handwritten assembly block.