Programming with unofficial opcodes

Using the CPU's unofficial opcodes could get your game rejected from Nintendo lot check, as it would cause games to fail on a hypothetical future revision of the 2A03 CPU. It could also screw up emulators that have not implemented unofficial opcodes. Luckily, Nintendo hasn't performed lot check on NES games since 1994, it hasn't revised the CPU in a way that affects opcode behavior, and testing anything that doesn't depend on open bus behavior on an NES is easy with the PowerPak. Some of the unofficial opcodes listed here appear useful, and many are already supported in ca65's 6502X mode. Others do things that appear extremely obscure with such limited applicability, and still others even vary from one chip to another due to analog effects. Here are the ones that appear most useful:

Combined operations

Because of how the 6502's microcode is compressed, some opcodes that share bits with two other opcodes will end up performing operations from both opcodes. A lot of these involve a bitwise AND operation, which is a side effect of the open-drain behavior of NMOS logic. When two instructions put a value into a temporary register inside the 6502 core called "special bus", this creates a bus conflict, and the lower voltage wins because transistors can pull down stronger than resistors can pull up.

ALR #i ($4B ii; 2 cycles)

Equivalent to AND #i then LSR A. Some sources call this "ASR"; we do not follow this out of confusion with the mnemonic for a pseudoinstruction that combines CMP #$80 (or ANC #$FF) then ROR.

ANC #i ($0B ii, $2B ii; 2 cycles)

Does AND #i, setting N and Z flags based on the result. Then it copies N (bit 7) to C. ANC #$FF could be useful for sign-extending, much like CMP #$80. ANC #$00 acts like LDA #$00 followed by CLC.

ARR #i ($6B ii; 2 cycles)

Similar to AND #i then ROR A, except sets the flags differently. N and Z are normal, but C is bit 6 and V is bit 6 xor bit 5.

AXS #i ($CB ii, 2 cycles)

Sets X to {(A AND X) - #value without borrow}, and updates NZC. One might use TXA AXS #-element_size to iterate through an array of structures or other elements larger than a byte, where the 6502 architecture usually prefers a structure of arrays. For example, TXA AXS #$FC could step to the next OAM entry or to the next APU channel, saving one byte and four cycles over four INXs. Also called SBX.

LAX (d,X) ($A3 dd; 6 cycles)

LAX d ($A7 dd; 3 cycles)

LAX a ($AF aa aa; 4 cycles)

LAX (d),Y ($B3 dd; 5 cycles)

LAX d,Y ($B7 dd; 4 cycles)

LAX a,Y ($BF aa aa; 4 cycles)

Equivalent to LDX value then TXA, or LDA value then TAX. Saves a byte and two cycles and allows use of the X register with the (d),Y addressing mode.

SAX (d,X) ($83 dd; 6 cycles)

SAX d ($87 dd; 3 cycles)

SAX a ($8F aa aa; 4 cycles)

SAX d,Y ($97 aa aa; 4 cycles)

Stores the bitwise AND of A and X. As with STA and STX, no flags are affected.

RMW instructions

The read-modify-write instructions (INC, DEC, ASL, LSR, ROR) have few valid addressing modes, but these instructions have three more: (d,X), (d),Y, and a,Y. In some cases, it could be worth it to use these and ignore the side effect on the accumulator.

DCP (d,X) ($C3 dd; 8 cycles)

DCP d ($C7 dd; 5 cycles)

DCP a ($CF aa aa; 6 cycles)

DCP (d),Y ($D3 dd; 8 cycles)

DCP d,X ($D7 dd; 6 cycles)

DCP a,Y ($DB aa aa; 7 cycles)

DCP a,X ($DF aa aa; 7 cycles)

Equivalent to DEC value then CMP value, except supporting more addressing modes.

ISC (d,X) ($E3 dd; 8 cycles)

ISC d ($E7 dd; 5 cycles)

ISC a ($EF aa aa; 6 cycles)

ISC (d),Y ($F3 dd; 8 cycles)

ISC d,X ($F7 dd; 6 cycles)

ISC a,Y ($FB aa aa; 7 cycles)

ISC a,X ($FF aa aa; 7 cycles)

Equivalent to INC value then SBC value, except supporting more addressing modes.

RLA (d,X) ($23 dd; 8 cycles)

RLA d ($27 dd; 5 cycles)

RLA a ($2F aa aa; 6 cycles)

RLA (d),Y ($33 dd; 8 cycles)

RLA d,X ($37 dd; 6 cycles)

RLA a,Y ($3B aa aa; 7 cycles)

RLA a,X ($3F aa aa; 7 cycles)

Equivalent to ROL value then AND value, except supporting more addressing modes.

RRA (d,X) ($63 dd; 8 cycles)

RRA d ($67 dd; 5 cycles)

RRA a ($6F aa aa; 6 cycles)

RRA (d),Y ($73 dd; 8 cycles)

RRA d,X ($77 dd; 6 cycles)

RRA a,Y ($7B aa aa; 7 cycles)

RRA a,X ($7F aa aa; 7 cycles)

Equivalent to ROR value then ADC value, except supporting more addressing modes. Essentially this computes A + value / 2, where value is 9-bit and the division is rounded up.

SLO (d,X) ($03 dd; 8 cycles)

SLO d ($07 dd; 5 cycles)

SLO a ($0F aa aa; 6 cycles)

SLO (d),Y ($13 dd; 8 cycles)

SLO d,X ($17 dd; 6 cycles)

SLO a,Y ($1B aa aa; 7 cycles)

SLO a,X ($1F aa aa; 7 cycles)

Equivalent to ASL value then ORA value, except supporting more addressing modes.

SRE (d,X) ($43 dd; 8 cycles)

SRE d ($47 dd; 5 cycles)

SRE a ($4F aa aa; 6 cycles)

SRE (d),Y ($53 dd; 8 cycles)

SRE d,X ($57 dd; 6 cycles)

SRE a,Y ($5B aa aa; 7 cycles)

SRE a,X ($5F aa aa; 7 cycles)

Equivalent to LSR value then EOR value, except supporting more addressing modes.

Watermarking instructions

Some instructions are equivalent to other instructions; other instructions do nothing at all or next to nothing. These are useful for watermarking your binary if you want to make leaked executables traceable, such as copies of the ROM sent to testers or even individual cartridges sent to end users.

ADC #i ($69 ii, $E9 ii^$FF, $EB ii^$FF; 2 cycles)

SBC #i ($69 ii^$FF, $E9 ii, $EB ii; 2 cycles)

$69 and $E9 are official; $EB is not. These three opcodes are nearly equivalent, except that $E9 and $EB add 255-i instead of i. Have your preprocessor randomly choose one of these opcodes.

NOP ($1A, $3A, $5A, $7A, $DA, $EA, $FA; 2 cycles)

The official NOP ($EA) and six unofficial NOPs do nothing, but they are useful for watermarking your binary. You can have a preprocessor insert a randomly chosen NOP at random points in a non-time-sensitive subroutine.

SKB #i ($80 ii, $82 ii, $89 ii, $C2 ii, $E2 ii; 2 cycles)

These unofficial opcodes just read an immediate byte and skip it. Because the second byte can be absolutely anything, your preprocessor can insert even more watermark data here. One of these even works almost the same way on 65C02, Hu6280, and 65C816: BIT #i ($89 ii), which affects the NVZ flags like the other BIT instruction. Use this SKB if you want your code to be portable to Lynx, TG16, or Super NES. Puzznic uses $89.

CLD ($D8; 2 cycles)

CLV ($B8; 2 cycles)

SED ($F8; 2 cycles)

These are official. CLD and SED control decimal mode, but on second-source 6502 CPUs without decimal mode such as the 2A03, they do almost nothing; their effect is visible only after a PHP. Use them like NOP. And the V flag that CLV clears is rarely used; only ADC, BIT, SBC, the stack ops PLP and RTI, and the unofficial instructions ARR, ISC, and RRA affect it.

AND #i ($29 ii; 2 cycles)

DEX ($CA; 2 cycles)

DEY ($88; 2 cycles)

EOR #i ($49 ii; 2 cycles)

INX ($E8; 2 cycles)

INY ($C8; 2 cycles)

LDA #i ($A9 ii; 2 cycles)

LDX #i ($A2 ii; 2 cycles)

LDY #i ($A0 ii; 2 cycles)

ORA #i ($09 ii; 2 cycles)

These are official and can be used much like NOP or SKB immediately before LDA, LDX, LDY, PLA, TAX, TAY, TSX, TXA, or TYA as appropriate.