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62 lines
1.2 KiB
Markdown
62 lines
1.2 KiB
Markdown
# Undocumented opcodes
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Original 6502 processors accidentally supported a bunch of extra undocumented instructions.
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Millfork can emit them if so desired.
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## Mnemonics
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Since various assemblers use different mnemonics for undocumented opcodes,
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Millfork supports multiple mnemonics per opcode. The default one is given first:
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* **AHX**, AXA, SHA
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* **ALR**
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* **ANC**
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* **ARR**
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* **DCP**, DCM
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* **ISC**, INS
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* **LAS**
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* **LAX**
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* **LXA**, OAL
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* **RLA**
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* **RRA**
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* **SAX**\*
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* **SHX**, XAS
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* **SHY**, SAY\*
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* **SBX**, AXS\*\*
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* **SRE**, LSE
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* **SLO**, ASO
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* **TAS**
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* **XAA**, ANE
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\* HuC2680 has different instructions also called SAX and SAY,
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but Millfork can distinguish between them and the NMOS illegal instructions based on the addressing mode.
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\*\* AXS is also used for SAX in some assemblers. Millfork interprets AXS based on the addressing mode.
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## Generation
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In order for the compiler to emit one of those opcodes,
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an appropriate CPU architecture must be chosen (`nmos` or `ricoh`)
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and either it must appear in an assembly block or it may be a result of optimization.
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Optimization will never emit any of the following opcodes due to their instability and/or uselessness:
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AHX, LAS, LXA, SHX, SHY, TAS, XAA.
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