CPU unofficial opcodes

"This is illegal, you know."

Kravindish, Mayor of Sakado, Zelda: The Wand of Gamelon

Unofficial opcodes, sometimes misleadingly called illegal opcodes or undocumented opcodes, are opcodes in a 6502 CPU that aren't documented in MOS Technology's official 6502 family datasheet. Some of these 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.

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

Later extensions to the 6502 family (65C02, HuC6280, 65C816) replaced some of these unofficial opcodes with different opcodes. It's conjectured that Nintendo banned the use of unofficial opcodes in NES games that would make the games incompatible with these one of these CPUs in an upgraded NES. But as it became clear that the Super NES was not going to be backward compatible, Nintendo may have eased up on this for later titles.

• Puzznic (all regions) (US release November 1990) uses $89, which is a 2-byte NOP on 6502 and BIT #imm on 65C02 and later, and emulating it as a single-byte NOP will break it. That'd be expected in a port from a 65C02 game, but the arcade version uses the Z80 of Taito's L System according to System 16 and MAME. Does it share code with the PC Engine port or something?
• F-117A Stealth Fighter and Infiltrator also use $89.
• Beauty and the Beast (E) (1994) uses a different SKB ($80), which is BRA on 65C02. (source: puNES 0.20 changelog)
• Dynowarz uses 1-byte NOPs $DA and $FA on the first level when your dino throws his fist.
• Super Cars (U) (February 1991) uses LAX
• Disney's Aladdin (E) (December 1994) uses SLO. 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).

See also

• Programming with unofficial opcodes

External links

• 6502 opcode matrix including unofficial opcodes; compare
• 65C02 and 65816