CPU unofficial opcodes

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Unofficial opcodes, sometimes called illegal opcodes or undocumented opcodes, are CPU instructions that were officially left unused by the original design. The 6502 family datasheet from MOS Technology does not specify or document their function, but they actually do perform various operations.

Some of these instructions are useful; some are not predictable; some do nothing but burn cycles; some halt the CPU until reset. Most NMOS 6502 cores interpret them the same way, although there are slight differences with the less stable instructions. CMOS variants of the 6502 handle them completely differently, and later CPUs in the same family (e.g. 65C02, HuC6280, 65C816) were free to implement new instructions in the place of the unused ones.

An accurate NES emulator must implement all instructions, not just the official ones. A small number of games use them (see below).

The following documents may provide reference for these instructions:

Arrangement

The microcode of the 6502 is compressed into a 130-entry decode ROM. Instead of 256 entries telling how to process each separate opcode, it's encoded as combinational logic post-processing the output of a "sparse" ROM that acts in some ways like a programmable logic array (PLA). Each entry in the ROM means "if these bits are on, and these bits are off, do things on these six cycles." [1] Many instructions activate multiple lines of the decode ROM at once. Often this is on purpose, such as one line for the addressing mode and one for the opcode part. But many of the unofficial opcodes simultaneously trigger parts of the ROM that were intended for completely unrelated instructions.

Perhaps the pattern is easier to see by shuffling the 6502's opcode matrix. This table lists all 6502 opcodes, 32 columns per row. The columns are colored by bits 1 and 0: 00 red, 01 green, 10 blue, and 11 gray.

+00 +01 +02 +03 +04 +05 +06 +07 +08 +09 +0A +0B +0C +0D +0E +0F +10 +11 +12 +13 +14 +15 +16 +17 +18 +19 +1A +1B +1C +1D +1E +1F
00 BRK
ORA
(d,x)
STP
SLO
(d,x)
NOP
d
ORA
d
ASL
d
SLO
d
PHP
ORA
#i
ASL
ANC
#i
NOP
a
ORA
a
ASL
a
SLO
a
BPL
*+d
ORA
(d),y
STP
SLO
(d),y
NOP
d,x
ORA
d,x
ASL
d,x
SLO
d,x
CLC
ORA
a,y
NOP
SLO
a,y
NOP
a,x
ORA
a,x
ASL
a,x
SLO
a,x
20 JSR
a
AND
(d,x)
STP
RLA
(d,x)
BIT
d
AND
d
ROL
d
RLA
d
PLP
AND
#i
ROL
ANC
#i
BIT
a
AND
a
ROL
a
RLA
a
BMI
*+d
AND
(d),y
STP
RLA
(d),y
NOP
d,x
AND
d,x
ROL
d,x
RLA
d,x
SEC
AND
a,y
NOP
RLA
a,y
NOP
a,x
AND
a,x
ROL
a,x
RLA
a,x
40 RTI
EOR
(d,x)
STP
SRE
(d,x)
NOP
d
EOR
d
LSR
d
SRE
d
PHA
EOR
#i
LSR
ALR
#i
JMP
a
EOR
a
LSR
a
SRE
a
BVC
*+d
EOR
(d),y
STP
SRE
(d),y
NOP
d,x
EOR
d,x
LSR
d,x
SRE
d,x
CLI
EOR
a,y
NOP
SRE
a,y
NOP
a,x
EOR
a,x
LSR
a,x
SRE
a,x
60 RTS
ADC
(d,x)
STP
RRA
(d,x)
NOP
d
ADC
d
ROR
d
RRA
d
PLA
ADC
#i
ROR
ARR
#i
JMP
(a)
ADC
a
ROR
a
RRA
a
BVS
*+d
ADC
(d),y
STP
RRA
(d),y
NOP
d,x
ADC
d,x
ROR
d,x
RRA
d,x
SEI
ADC
a,y
NOP
RRA
a,y
NOP
a,x
ADC
a,x
ROR
a,x
RRA
a,x
80 NOP
#i
STA
(d,x)
NOP
#i
SAX
(d,x)
STY
d
STA
d
STX
d
SAX
d
DEY
NOP
#i
TXA
XAA
#i
STY
a
STA
a
STX
a
SAX
a
BCC
*+d
STA
(d),y
STP
AHX
(d),y
STY
d,x
STA
d,x
STX
d,y
SAX
d,y
TYA
STA
a,y
TXS
TAS
a,y
SHY
a,x
STA
a,x
SHX
a,y
AHX
a,y
A0 LDY
#i
LDA
(d,x)
LDX
#i
LAX
(d,x)
LDY
d
LDA
d
LDX
d
LAX
d
TAY
LDA
#i
TAX
LAX
#i
LDY
a
LDA
a
LDX
a
LAX
a
BCS
*+d
LDA
(d),y
STP
LAX
(d),y
LDY
d,x
LDA
d,x
LDX
d,y
LAX
d,y
CLV
LDA
a,y
TSX
LAS
a,y
LDY
a,x
LDA
a,x
LDX
a,y
LAX
a,y
C0 CPY
#i
CMP
(d,x)
NOP
#i
DCP
(d,x)
CPY
d
CMP
d
DEC
d
DCP
d
INY
CMP
#i
DEX
AXS
#i
CPY
a
CMP
a
DEC
a
DCP
a
BNE
*+d
CMP
(d),y
STP
DCP
(d),y
NOP
d,x
CMP
d,x
DEC
d,x
DCP
d,x
CLD
CMP
a,y
NOP
DCP
a,y
NOP
a,x
CMP
a,x
DEC
a,x
DCP
a,x
E0 CPX
#i
SBC
(d,x)
NOP
#i
ISC
(d,x)
CPX
d
SBC
d
INC
d
ISC
d
INX
SBC
#i
NOP
SBC
#i
CPX
a
SBC
a
INC
a
ISC
a
BEQ
*+d
SBC
(d),y
STP
ISC
(d),y
NOP
d,x
SBC
d,x
INC
d,x
ISC
d,x
SED
SBC
a,y
NOP
ISC
a,y
NOP
a,x
SBC
a,x
INC
a,x
ISC
a,x

Key: a is a 16-bit absolute address, and d is an 8-bit zero page address.

But if we rearrange it so that columns with the same bits 1-0 are close together, correlations become easier to see:

+00 +04 +08 +0C +10 +14 +18 +1C +01 +05 +09 +0D +11 +15 +19 +1D +02 +06 +0A +0E +12 +16 +1A +1E +03 +07 +0B +0F +13 +17 +1B +1F
00 BRK
NOP
d
PHP
NOP
a
BPL
*+d
NOP
d,x
CLC
NOP
a,x
ORA
(d,x)
ORA
d
ORA
#i
ORA
a
ORA
(d),y
ORA
d,x
ORA
a,y
ORA
a,x
STP
ASL
d
ASL
ASL
a
STP
ASL
d,x
NOP
ASL
a,x
SLO
(d,x)
SLO
d
ANC
#i
SLO
a
SLO
(d),y
SLO
d,x
SLO
a,y
SLO
a,x
20 JSR
a
BIT
d
PLP
BIT
a
BMI
*+d
NOP
d,x
SEC
NOP
a,x
AND
(d,x)
AND
d
AND
#i
AND
a
AND
(d),y
AND
d,x
AND
a,y
AND
a,x
STP
ROL
d
ROL
ROL
a
STP
ROL
d,x
NOP
ROL
a,x
RLA
(d,x)
RLA
d
ANC
#i
RLA
a
RLA
(d),y
RLA
d,x
RLA
a,y
RLA
a,x
40 RTI
NOP
d
PHA
JMP
a
BVC
*+d
NOP
d,x
CLI
NOP
a,x
EOR
(d,x)
EOR
d
EOR
#i
EOR
a
EOR
(d),y
EOR
d,x
EOR
a,y
EOR
a,x
STP
LSR
d
LSR
LSR
a
STP
LSR
d,x
NOP
LSR
a,x
SRE
(d,x)
SRE
d
ALR
#i
SRE
a
SRE
(d),y
SRE
d,x
SRE
a,y
SRE
a,x
60 RTS
NOP
d
PLA
JMP
(a)
BVS
*+d
NOP
d,x
SEI
NOP
a,x
ADC
(d,x)
ADC
d
ADC
#i
ADC
a
ADC
(d),y
ADC
d,x
ADC
a,y
ADC
a,x
STP
ROR
d
ROR
ROR
a
STP
ROR
d,x
NOP
ROR
a,x
RRA
(d,x)
RRA
d
ARR
#i
RRA
a
RRA
(d),y
RRA
d,x
RRA
a,y
RRA
a,x
80 NOP
#i
STY
d
DEY
STY
a
BCC
*+d
STY
d,x
TYA
SHY
a,x
STA
(d,x)
STA
d
NOP
#i
STA
a
STA
(d),y
STA
d,x
STA
a,y
STA
a,x
NOP
#i
STX
d
TXA
STX
a
STP
STX
d,y
TXS
SHX
a,y
SAX
(d,x)
SAX
d
XAA
#i
SAX
a
AHX
(d),y
SAX
d,y
TAS
a,y
AHX
a,y
A0 LDY
#i
LDY
d
TAY
LDY
a
BCS
*+d
LDY
d,x
CLV
LDY
a,x
LDA
(d,x)
LDA
d
LDA
#i
LDA
a
LDA
(d),y
LDA
d,x
LDA
a,y
LDA
a,x
LDX
#i
LDX
d
TAX
LDX
a
STP
LDX
d,y
TSX
LDX
a,y
LAX
(d,x)
LAX
d
LAX
#i
LAX
a
LAX
(d),y
LAX
d,y
LAS
a,y
LAX
a,y
C0 CPY
#i
CPY
d
INY
CPY
a
BNE
*+d
NOP
d,x
CLD
NOP
a,x
CMP
(d,x)
CMP
d
CMP
#i
CMP
a
CMP
(d),y
CMP
d,x
CMP
a,y
CMP
a,x
NOP
#i
DEC
d
DEX
DEC
a
STP
DEC
d,x
NOP
DEC
a,x
DCP
(d,x)
DCP
d
AXS
#i
DCP
a
DCP
(d),y
DCP
d,x
DCP
a,y
DCP
a,x
E0 CPX
#i
CPX
d
INX
CPX
a
BEQ
*+d
NOP
d,x
SED
NOP
a,x
SBC
(d,x)
SBC
d
SBC
#i
SBC
a
SBC
(d),y
SBC
d,x
SBC
a,y
SBC
a,x
NOP
#i
INC
d
NOP
INC
a
STP
INC
d,x
NOP
INC
a,x
ISC
(d,x)
ISC
d
SBC
#i
ISC
a
ISC
(d),y
ISC
d,x
ISC
a,y
ISC
a,x

Thus the 00 (red) block is mostly control instructions, 01 (green) is ALU operations, and 10 (blue) is read-modify-write (RMW) operations and data movement instructions involving X. The RMW instructions (all but row 80 and A0) in columns +06, +0E, +16, and +1E have the same addressing modes as the corresponding ALU operations.

The 11 (gray) block is unofficial opcodes combining the operations of instructions from the ALU and RMW blocks. all of them having the same addressing mode as the corresponding ALU opcode. The RMW+ALU instructions that affect memory are easiest to understand because their RMW part completes during the opcode and the ALU part completes during the next opcode's fetch. Column +0B, on the other hand, has no extra cycles; everything completes during the next opcode's fetch. This causes instructions to have strange mixing properties. Some even differ based on analog effects.

Games using unofficial opcodes

The use of unofficial opcodes is rare in NES games. It appears to occur mostly in late or unlicensed titles:

  • Puzznic (all regions) (US release November 1990) uses $89, which is a 2-byte NOP on 6502.
  • F-117A Stealth Fighter and Infiltrator also use $89.
  • Beauty and the Beast (E) (1994) uses a different 2-byte NOP ($80).[1]
  • Dynowarz uses 1-byte NOPs $DA and $FA on the first level when your dino throws his fist.
  • Gaau Hok Gwong Cheung (Ch) uses XAA ($8B), an immediate instruction. The game malfunctions after selecting Left from the main menu if that instruction is not emulated.
  • Super Cars (U) (February 1991) uses LAX ($B3)
  • Disney's Aladdin (E) (December 1994) uses SLO ($07). This is Virgin's port of the Game Boy game, itself a port of the Genesis game, not any of the pirate originals.

As hobbyists became aware of what these instructions could do, homebrew games started to use them:

  • The MUSE music engine, used in Driar and STREEMERZ: Super Strength Emergency Squad Zeta, uses the unofficial opcodes $8F (SAX), $B3 (LAX), and $CB (AXS) [2]
  • Attribute Zone uses $0B (ANC), $2F (RLA), $4B (ALR), $A7 (LAX), $B3 (LAX), $CB (AXS), $D3 (DCP), $DB (DCP).
  • The port of Zork to the Famicom uses a few unofficial opcodes.

See also

External links

References

  1. puNES 0.20 changelog indicating $80 opcode in Beauty and the Beast.