2.3 KiB
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 not emit them. The only exception is SLL, which will be emitted if it occurs in a handwritten assembly block.
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.