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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

• KIL

• 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 emit some of them if used in an assembly block:

• SLL – supported
• instructions using the IXH, IXL, IYH, IYL registers – supported (can only be used in Zilog syntax)
• instructions of the form RLC IX(1),B – not supported

8085

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

Intel syntax	Zilog syntax
DSUB	DSUB
ARHL, RRHL	SRA HL
RLDE, RDEL	RL DE
LDHI n	LD DE,(HL+n)
LDSI n	LD DE,(SP+n)
LHLX	LD HL,(DE)
SHLX	LD (DE),HL
JK n, JX5 n	JP K,n; JP X5,n
JNK n, JNX5 n	JP NK,n; JP NX5,n
RSTV, OVRST8	RSTV

Generation

If enabled, The compiler will only emit the following undocumented instructions from Millfork code: DSUB, LHLX, SHLX, LDSI, LDHI

The optimizer does not track the K and V flags. Use JK, JNK and RSTV with care.

8086

Undocumented instructions are not supported.