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Revision as of 18:11, 29 June 2025

6502 Opcodes for Hackers is a reference guide for experienced programmers. It focuses on the practical application of both standard and stable non-standard NMOS 6502 opcodes, including common tricks and non-obvious behaviors. It intentionally omits highly unstable opcodes to provide a focused, practical resource.

Opcode Matrix

This table provides a complete overview of the NMOS 6502 opcode map. Each mnemonic links to its detailed description below.

Legend:

• Blue: Standard Opcodes
• Green: Stable Illegal Opcodes
• Yellow: Semi-Stable Illegal Opcodes (use with caution)
• Red: Unstable/Unreliable Opcodes (details not included below)

NMOS 6502 Opcode Matrix

HI	-0	-1	-2	-3	-4	-5	-6	-7	-8	-9	-A	-B	-C	-D	-E	-F
0-	BRK	ORA	JAM	SLO	NOP	ORA	ASL	SLO	PHP	ORA	ASL	ANC	NOP	ORA	ASL	SLO
1-	BPL	ORA	JAM	SLO	NOP	ORA	ASL	SLO	CLC	ORA	NOP	SLO	NOP	ORA	ASL	SLO
2-	JSR	AND	JAM	RLA	BIT	AND	ROL	RLA	PLP	AND	ROL	ANC	BIT	AND	ROL	RLA
3-	BMI	AND	JAM	RLA	NOP	AND	ROL	RLA	SEC	AND	NOP	RLA	NOP	AND	ROL	RLA
4-	RTI	EOR	JAM	SRE	NOP	EOR	LSR	SRE	PHA	EOR	LSR	ALR	JMP	EOR	LSR	SRE
5-	BVC	EOR	JAM	SRE	NOP	EOR	LSR	SRE	CLI	EOR	NOP	SRE	NOP	EOR	LSR	SRE
6-	RTS	ADC	JAM	RRA	NOP	ADC	ROR	RRA	PLA	ADC	ROR	ARR	JMP	ADC	ROR	RRA
7-	BVS	ADC	JAM	RRA	NOP	ADC	ROR	RRA	SEI	ADC	NOP	RRA	NOP	ADC	ROR	RRA
8-	NOP	STA	NOP	SAX	STY	STA	STX	SAX	DEY	NOP	TXA	ANE	STY	STA	STX	SAX
9-	BCC	STA	JAM	SHA	STY	STA	STX	SAX	TYA	STA	TXS	TAS	SHY	STA	SHX	SHA
A-	LDY	LDA	LDX	LAX	LDY	LDA	LDX	LAX	TAY	LDA	TAX	LAX	LDY	LDA	LDX	LAX
B-	BCS	LDA	JAM	LAX	LDY	LDA	LDX	LAX	CLV	LDA	TSX	LAS	LDY	LDA	LDX	LAX
C-	CPY	CMP	NOP	DCP	CPY	CMP	DEC	DCP	INY	CMP	DEX	SBX	CPY	CMP	DEC	DCP
D-	BNE	CMP	JAM	DCP	NOP	CMP	DEC	DCP	CLD	CMP	NOP	DCP	NOP	CMP	DEC	DCP
E-	CPX	SBC	NOP	ISC	CPX	SBC	INC	ISC	INX	SBC	NOP	SBC	CPX	SBC	INC	ISC
F-	BEQ	SBC	JAM	ISC	NOP	SBC	INC	ISC	SED	SBC	NOP	ISC	NOP	SBC	INC	ISC

Standard Opcodes

This section covers the official, documented NMOS 6502 opcodes.

ADC (ADd with Carry)

Adds memory and the Carry flag to the accumulator. To perform an 8-bit addition, a preceding `CLC` is required. The `V` flag detects signed overflow, and the `D` flag enables BCD arithmetic. Affects Flags: N, V, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	ADC #$44	$69	2	2
Zero Page	ADC $44	$65	2	3
Zero Page,X	ADC $44,X	$75	2	4
Absolute	ADC $4400	$6D	3	4
Absolute,X	ADC $4400,X	$7D	3	4+
Absolute,Y	ADC $4400,Y	$79	3	4+
Indirect,X	ADC ($44,X)	$61	2	6
Indirect,Y	ADC ($44),Y	$71	2	5+

AND (bitwise AND with accumulator)

Performs a bitwise AND between the accumulator and a memory value. Primarily used to mask (clear) bits. Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Immediate	AND #$44	$29	2	2
Zero Page	AND $44	$25	2	3
Zero Page,X	AND $44,X	$35	2	4
Absolute	AND $4400	$2D	3	4
Absolute,X	AND $4400,X	$3D	3	4+
Absolute,Y	AND $4400,Y	$39	3	4+
Indirect,X	AND ($44,X)	$21	2	6
Indirect,Y	AND ($44),Y	$31	2	5+

ASL (Arithmetic Shift Left)

Shifts an operand left by one bit. Bit 0 is cleared to 0, and the old bit 7 is shifted into the Carry flag. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Accumulator	ASL A	$0A	1	2
Zero Page	ASL $44	$06	2	5
Zero Page,X	ASL $44,X	$16	2	6
Absolute	ASL $4400	$0E	3	6
Absolute,X	ASL $4400,X	$1E	3	7

Branch Instructions

Branch instructions alter program flow based on the state of a specific flag. All use relative addressing, taking a signed 8-bit offset. A branch costs 2 cycles if not taken, 3 if taken within the same page, and 4 if the destination crosses a page boundary. To create an unconditional branch (`BRA`), branch on a flag with a known state. The `V` flag is a good candidate, as it is unaffected by most operations: `CLV` followed by `BVC` will always branch. Affects Flags: none

Mnemonic	Name	HEX
BCC	Branch on Carry Clear (C=0)	$90
BCS	Branch on Carry Set (C=1)	$B0
BEQ	Branch on EQual (Z=1)	$F0
BNE	Branch on Not Equal (Z=0)	$D0
BMI	Branch on MInus (N=1)	$30
BPL	Branch on PLus (N=0)	$10
BVC	Branch on oVerflow Clear (V=0)	$50
BVS	Branch on oVerflow Set (V=1)	$70

BIT (test BITS)

Performs a non-destructive `AND` between the accumulator and memory to set the Z flag, without altering any registers. It also copies bit 7 and bit 6 of the memory operand directly to the N and V flags, respectively. This makes it an essential tool for polling I/O registers. Affects Flags: N, V, Z

Mode	Syntax	HEX	LEN	TIM
Zero Page	BIT $44	$24	2	3
Absolute	BIT $4400	$2C	3	4

BRK (BReaK)

Forces a software interrupt. It pushes PC+2 and the processor status (with the B flag set) to the stack, then jumps via the IRQ vector ($FFFE). An `RTI` returns to the address of `BRK + 2`, allowing it to replace a 2-byte instruction for debugging purposes. Affects Flags: B

Mode	Syntax	HEX	LEN	TIM
Implied	BRK	$00	1	7

Compare Instructions

These instructions perform a non-destructive subtraction (`Register - Memory`) to set the N, Z, and C flags. They are fundamental for conditional logic and comparisons.

• C flag: Set if `Register >= Memory` (no borrow).
• Z flag: Set if `Register == Memory`.
• N flag: Set to bit 7 of the subtraction result.

CMP (CoMPare accumulator)

Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	CMP #$44	$C9	2	2
Zero Page	CMP $44	$C5	2	3
Zero Page,X	CMP $44,X	$D5	2	4
Absolute	CMP $4400	$CD	3	4
Absolute,X	CMP $4400,X	$DD	3	4+
Absolute,Y	CMP $4400,Y	$D9	3	4+
Indirect,X	CMP ($44,X)	$C1	2	6
Indirect,Y	CMP ($44),Y	$D1	2	5+

CPX (ComPare X register)

Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	CPX #$44	$E0	2	2
Zero Page	CPX $44	$E4	2	3
Absolute	CPX $4400	$EC	3	4

CPY (ComPare Y register)

Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	CPY #$44	$C0	2	2
Zero Page	CPY $44	$C4	2	3
Absolute	CPY $4400	$CC	3	4

Decrement & Increment Instructions

These modify a register or memory location by one. Register operations are 1-byte, 2-cycle instructions. Memory operations are read-modify-write and take longer.

DEC (DECrement memory)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Zero Page	DEC $44	$C6	2	5
Zero Page,X	DEC $44,X	$D6	2	6
Absolute	DEC $4400	$CE	3	6
Absolute,X	DEC $4400,X	$DE	3	7

DEX (DEcrement X register)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Implied	DEX	$CA	1	2

DEY (DEcrement Y register)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Implied	DEY	$88	1	2

INC (INCrement memory)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Zero Page	INC $44	$E6	2	5
Zero Page,X	INC $44,X	$F6	2	6
Absolute	INC $4400	$EE	3	6
Absolute,X	INC $4400,X	$FE	3	7

INX (INcrement X register)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Implied	INX	$E8	1	2

INY (INcrement Y register)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Implied	INY	$C8	1	2

EOR (bitwise Exclusive OR)

Performs a bitwise XOR between the accumulator and memory. Useful for flipping specific bits or for simple checksum calculations. Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Immediate	EOR #$44	$49	2	2
Zero Page	EOR $44	$45	2	3
Zero Page,X	EOR $44,X	$55	2	4
Absolute	EOR $4400	$4D	3	4
Absolute,X	EOR $4400,X	$5D	3	4+
Absolute,Y	EOR $4400,Y	$59	3	4+
Indirect,X	EOR ($44,X)	$41	2	6
Indirect,Y	EOR ($44),Y	$51	2	5+

Flag Instructions

These single-byte, 2-cycle instructions directly manipulate the processor status flags. It is critical to use `CLD` on startup, as the power-on state of the D flag is undefined. Affects Flags: as noted

Mnemonic	Function	HEX
CLC	C = 0	$18
SEC	C = 1	$38
CLD	D = 0	$D8
SED	D = 1	$F8
CLI	I = 0 (Enable IRQs)	$58
SEI	I = 1 (Disable IRQs)	$78
CLV	V = 0	$B8

JMP (JuMP)

Unconditionally transfers program control to a new location. The indirect mode has an infamous hardware bug: if the low byte of the indirect vector is $FF (e.g., `JMP ($30FF)`), the high byte of the jump address is fetched from `$3000`, not `$3100`. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Absolute	JMP $5597	$4C	3	3
Indirect	JMP ($5597)	$6C	3	5

JSR (Jump to SubRoutine)

Pushes the return address (the address of the last byte of the `JSR` instruction) onto the stack and jumps to a new location. Paired with `RTS` for standard subroutine calls. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Absolute	JSR $5597	$20	3	6

Load Instructions

Load instructions copy a byte from memory into a register, setting the N and Z flags based on the value loaded. Note the addressing mode asymmetries: `LDX` has a `Zero Page,Y` mode while `LDY` has an `Absolute,X` mode, but not vice-versa.

LDA (LoaD Accumulator)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Immediate	LDA #$44	$A9	2	2
Zero Page	LDA $44	$A5	2	3
Zero Page,X	LDA $44,X	$B5	2	4
Absolute	LDA $4400	$AD	3	4
Absolute,X	LDA $4400,X	$BD	3	4+
Absolute,Y	LDA $4400,Y	$B9	3	4+
Indirect,X	LDA ($44,X)	$A1	2	6
Indirect,Y	LDA ($44),Y	$B1	2	5+

LDX (LoaD X register)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Immediate	LDX #$44	$A2	2	2
Zero Page	LDX $44	$A6	2	3
Zero Page,Y	LDX $44,Y	$B6	2	4
Absolute	LDX $4400	$AE	3	4
Absolute,Y	LDX $4400,Y	$BE	3	4+

LDY (LoaD Y register)

Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Immediate	LDY #$44	$A0	2	2
Zero Page	LDY $44	$A4	2	3
Zero Page,X	LDY $44,X	$B4	2	4
Absolute	LDY $4400	$AC	3	4
Absolute,X	LDY $4400,X	$BC	3	4+

LSR (Logical Shift Right)

Shifts an operand right by one bit. Bit 7 is cleared to 0, and the old bit 0 is shifted into the Carry flag. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Accumulator	LSR A	$4A	1	2
Zero Page	LSR $44	$46	2	5
Zero Page,X	LSR $44,X	$56	2	6
Absolute	LSR $4400	$4E	3	6
Absolute,X	LSR $4400,X	$5E	3	7

NOP (No OPeration)

The official NOP wastes 2 cycles and does nothing else. Many illegal opcodes also function as NOPs with different cycle and byte counts; some still perform memory reads, which can be useful for I/O timing or acknowledging interrupts without altering registers. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Implied	NOP	$EA	1	2

ORA (bitwise OR with Accumulator)

Performs a bitwise OR between the accumulator and a memory value. Primarily used to set bits. Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Immediate	ORA #$44	$09	2	2
Zero Page	ORA $44	$05	2	3
Zero Page,X	ORA $44,X	$15	2	4
Absolute	ORA $4400	$0D	3	4
Absolute,X	ORA $4400,X	$1D	3	4+
Absolute,Y	ORA $4400,Y	$19	3	4+
Indirect,X	ORA ($44,X)	$01	2	6
Indirect,Y	ORA ($44),Y	$11	2	5+

Register Instructions

These 1-byte, 2-cycle instructions transfer data between registers. `TXS` is unique in that it does not affect any flags. Affects Flags: N, Z (except TXS)

Mnemonic	Description	HEX
TAX	Transfer A to X	$AA
TAY	Transfer A to Y	$A8
TXA	Transfer X to A	$8A
TYA	Transfer Y to A	$98
TSX	Transfer Stack Pointer to X	$BA
TXS	Transfer X to Stack Pointer	$9A

ROL (ROtate Left)

Rotates an operand left by one bit. The old bit 7 is moved to the Carry flag, and the old Carry flag is moved into bit 0. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Accumulator	ROL A	$2A	1	2
Zero Page	ROL $44	$26	2	5
Zero Page,X	ROL $44,X	$36	2	6
Absolute	ROL $4400	$2E	3	6
Absolute,X	ROL $4400,X	$3E	3	7

ROR (ROtate Right)

Rotates an operand right by one bit. The old bit 0 is moved to the Carry flag, and the old Carry flag is moved into bit 7. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Accumulator	ROR A	$6A	1	2
Zero Page	ROR $44	$66	2	5
Zero Page,X	ROR $44,X	$76	2	6
Absolute	ROR $4400	$6E	3	6
Absolute,X	ROR $4400,X	$7E	3	7

RTI (ReTurn from Interrupt)

Restores the processor state after an interrupt by pulling the processor status register and program counter from the stack. Affects Flags: all

Mode	Syntax	HEX	LEN	TIM
Implied	RTI	$40	1	6

RTS (ReTurn from Subroutine)

Returns from a subroutine by pulling the program counter from the stack and incrementing it. Can be used to implement powerful jump tables by pushing crafted return addresses onto the stack. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Implied	RTS	$60	1	6

SBC (SuBtract with Carry)

Subtracts memory and an inverted Carry flag (borrow) from the accumulator. To perform an 8-bit subtraction, a preceding `SEC` (to indicate "no borrow") is required. The `D` flag enables BCD arithmetic. Affects Flags: N, V, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	SBC #$44	$E9	2	2
Zero Page	SBC $44	$E5	2	3
Zero Page,X	SBC $44,X	$F5	2	4
Absolute	SBC $4400	$ED	3	4
Absolute,X	SBC $4400,X	$FD	3	4+
Absolute,Y	SBC $4400,Y	$F9	3	4+
Indirect,X	SBC ($44,X)	$E1	2	6
Indirect,Y	SBC ($44),Y	$F1	2	5+

Stack Instructions

These instructions interact with the hardware stack located at page one ($0100-$01FF). The 6502 stack pointer grows downwards. Affects Flags: all (for PLP) or none

Mnemonic	Description	HEX	TIM
PHA	PusH Accumulator	$48	3
PLA	PuLl Accumulator	$68	4
PHP	PusH Processor status	$08	3
PLP	PuLl Processor status	$28	4

Store Instructions

Store instructions copy the content of a register to memory. They do not affect any flags. Note the addressing mode asymmetries, particularly the absence of accumulator-relative addressing and the presence of `STX zp,Y` and `STY zp,X`.

STA (STore Accumulator)

Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Zero Page	STA $44	$85	2	3
Zero Page,X	STA $44,X	$95	2	4
Absolute	STA $4400	$8D	3	4
Absolute,X	STA $4400,X	$9D	3	5
Absolute,Y	STA $4400,Y	$99	3	5
Indirect,X	STA ($44,X)	$81	2	6
Indirect,Y	STA ($44),Y	$91	2	6

STX (STore X register)

Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Zero Page	STX $44	$86	2	3
Zero Page,Y	STX $44,Y	$96	2	4
Absolute	STX $4400	$8E	3	4

STY (STore Y register)

Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Zero Page	STY $44	$84	2	3
Zero Page,X	STY $44,X	$94	2	4
Absolute	STY $4400	$8C	3	4

Stable & Semi-Stable Illegal Opcodes

This section details non-standard opcodes that are generally stable on NMOS 6502 and its direct variants. They often combine two standard operations into one, saving bytes and/or cycles. "Semi-stable" opcodes are marked and should be used with an understanding of their specific quirks, which are detailed in their descriptions.

SLO (ASO)

Function: ` {addr} = {addr} * 2`, then `A = A OR {addr}` (ASL followed by ORA)
Performs an `ASL` on a memory location, then `ORA`s the new value with the accumulator. Useful for multi-byte shifts where the result must be combined into a register. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Zero Page	SLO $44	$07	2	5
Zero Page,X	SLO $44,X	$17	2	6
Absolute	SLO $4400	$0F	3	6
Absolute,X	SLO $4400,X	$1F	3	7
Absolute,Y	SLO $4400,Y	$1B	3	7
Indirect,X	SLO ($44,X)	$03	2	8
Indirect,Y	SLO ($44),Y	$13	2	8

Example: A 24-bit shift where the high byte is also needed in A.

; Assuming A is zero before this sequence:
  SLO data+2    ; Shifts data+2, A now holds the result
  ROL data+1
  ROL data+0
; This saves an LDA instruction compared to the standard ASL/ROL sequence.

RLA

Function: `{addr} = ROL {addr}`, then `A = A AND {addr}` (ROL followed by AND)
Performs a `ROL` on memory, then `AND`s the new value with the accumulator. Excellent for scrolling effects where a rotated tile needs to be masked by a background. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Zero Page	RLA $44	$27	2	5
Zero Page,X	RLA $44,X	$37	2	6
Absolute	RLA $4400	$2F	3	6
Absolute,X	RLA $4400,X	$3F	3	7
Absolute,Y	RLA $4400,Y	$3B	3	7
Indirect,X	RLA ($44,X)	$23	2	8
Indirect,Y	RLA ($44),Y	$33	2	8

Example: Combining a sliding sprite with a background mask.

; Standard method (16 bytes, 18 cycles):
  ROL scroll_gfx
  LDA scroll_gfx
  AND background_mask
  STA screen_ram
; Faster method (12 bytes, 14 cycles):
  LDA background_mask
  RLA scroll_gfx       ; shift left and combine in one go
  STA screen_ram

SRE (LSE)

Function: `{addr} = LSR {addr}`, then `A = A EOR {addr}` (LSR followed by EOR)
Performs an `LSR` on memory, then `EOR`s the new value with the accumulator. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Zero Page	SRE $44	$47	2	5
Zero Page,X	SRE $44,X	$57	2	6
Absolute	SRE $4400	$4F	3	6
Absolute,X	SRE $4400,X	$5F	3	7
Absolute,Y	SRE $4400,Y	$5B	3	7
Indirect,X	SRE ($44,X)	$43	2	8
Indirect,Y	SRE ($44),Y	$53	2	8

RRA

Function: `{addr} = ROR {addr}`, then `A = A ADC {addr}` (ROR followed by ADC)
Performs a `ROR` on memory, then adds the new value with carry to the accumulator. Inherits the decimal mode dependency from `ADC`. Affects Flags: N, V, Z, C

Mode	Syntax	HEX	LEN	TIM
Zero Page	RRA $44	$67	2	5
Zero Page,X	RRA $44,X	$77	2	6
Absolute	RRA $4400	$6F	3	6
Absolute,X	RRA $4400,X	$7F	3	7
Absolute,Y	RRA $4400,Y	$7B	3	7
Indirect,X	RRA ($44,X)	$63	2	8
Indirect,Y	RRA ($44),Y	$73	2	8

SAX (AXS)

Function: `{addr} = A & X`
Stores the result of a bitwise `AND` between the A and X registers into memory. The A and X registers themselves are not modified, and no flags are affected. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Zero Page	SAX $44	$87	2	3
Zero Page,Y	SAX $44,X	$97	2	4
Absolute	SAX $4400	$8F	3	4
Indirect,X	SAX ($44,X)	$83	2	6

LAX

Function: `A, X = {addr}`
Loads both the A and X registers with the same value from memory. Saves a byte and cycles over a standard `LDA`/`TAX` pair. Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Zero Page	LAX $44	$A7	2	3
Zero Page,Y	LAX $44,Y	$B7	2	4
Absolute	LAX $4400	$AF	3	4
Absolute,Y	LAX $4400,Y	$BF	3	4+
Indirect,X	LAX ($44,X)	$A3	2	6
Indirect,Y	LAX ($44),Y	$B3	2	5+

DCP (DCM)

Function: `{addr} = {addr} - 1`, then `A CMP {addr}` (DEC followed by CMP)
Decrements a memory location, then compares the new value with the accumulator. Excellent for loop counters. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Zero Page	DCP $44	$C7	2	5
Zero Page,X	DCP $44,X	$D7	2	6
Absolute	DCP $4400	$CF	3	6
Absolute,X	DCP $4400,X	$DF	3	7
Absolute,Y	DCP $4400,Y	$DB	3	7
Indirect,X	DCP ($44,X)	$C3	2	8
Indirect,Y	DCP ($44),Y	$D3	2	8

Example: A more efficient 16-bit decrement loop on `ptr`.

; Instead of DEC ptr / LDA ptr / CMP #$FF ...
  LDA #$FF
  DCP ptr       ; Decrements ptr, compares new value with A.
  BNE loop      ; Branch if ptr did not underflow from $00
  DEC ptr+1     ; Handle high byte underflow
loop:

ISC (ISB, INS)

Function: `{addr} = {addr} + 1`, then `A = A SBC {addr}` (INC followed by SBC)
Increments a memory location, then subtracts the new value from the accumulator. Inherits decimal mode dependency from `SBC`. Affects Flags: N, V, Z, C

Mode	Syntax	HEX	LEN	TIM
Zero Page	ISC $44	$E7	2	5
Zero Page,X	ISC $44,X	$F7	2	6
Absolute	ISC $4400	$EF	3	6
Absolute,X	ISC $4400,X	$FF	3	7
Absolute,Y	ISC $4400,Y	$FB	3	7
Indirect,X	ISC ($44,X)	$E3	2	8
Indirect,Y	ISC ($44),Y	$F3	2	8

Example: A loop that counts up to a value in A.

; Instead of INC counter / LDA counter / CMP #END_VALUE
  LDA #END_VALUE
  SEC
loop:
  ISC counter   ; INC counter, then SBC counter. Z will be set when they match.
  BNE loop

ANC

Function: `A = A & #{imm}`, then `C = N`
Performs a standard `AND` immediate, but also copies the resulting N flag (bit 7 of A) into the C flag. This is an excellent way to perform a mask and a bit test in a single, 2-cycle instruction. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	ANC #$44	$0B	2	2
Immediate	ANC #$44	$2B	2	2

Example: Test bit 7 and branch, without needing to preserve the Carry flag.

; Instead of PHP / AND #$80 / BEQ ... PLP
  ANC #$80      ; Sets C=1 if bit 7 is set, N=1 if bit 7 is set, Z=0 if bit 7 is set.
  BCC bit_is_clear

ALR (ASR)

Function: `A = (A & #{imm}) / 2` (AND immediate followed by LSR A)
Performs an `AND` immediate, then performs a logical shift right on the accumulator. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	ALR #$44	$4B	2	2

ARR

Function: `A = (A & #{imm})`, then a complex `ROR`-like operation.
Performs an `AND` immediate, then rotates the accumulator right. Warning: Flag calculation is unusual and decimal mode dependent. After the `AND`, V is set by `(A bit 6) EOR (A bit 7)`. During the rotate, C is set from the original A bit 6, and the new bit 7 is set from the original C flag. Bit 0 is lost. Due to its complexity, it is rarely used. Affects Flags: N, V, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	ARR #$44	$6B	2	2

SBX (AXS)

Function: `X = (A & X) - #{imm}`
Calculates `A & X`, then subtracts an immediate value from this temporary result, storing the final value in X. Flags are set as if from a `CMP` instruction (`(A & X) >= imm` sets the Carry). Does not respect decimal mode and is not affected by the initial state of the Carry flag. Affects Flags: N, Z, C

Mode	Syntax	HEX	LEN	TIM
Immediate	SBX #$44	$CB	2	2

Example: A faster decrement of X by a constant value.

; Standard method (8 cycles, 5 bytes):
  TXA
  SEC
  SBC #$10
  TAX
; Faster method (4 cycles, 3 bytes):
  TXA         ; A and X are now equal, so (A & X) = X.
  SBX #$10    ; C flag state before this instruction is irrelevant.

'Unstable High Byte' Opcodes

The following opcodes are semi-stable. They combine a load or store operation with a bitwise `AND`, but their behavior can be unreliable under specific hardware conditions. Warning:

1. Page Boundary Crossing: If the effective address calculation (`address + index`) crosses a page boundary, the result can be unpredictable. Avoid crossing page boundaries with these instructions.
2. Hardware Dependency: The `AND` operation part of the instruction may fail if the CPU's RDY line is pulled low during a specific cycle (e.g., by sprite DMA). This makes the instruction behave like a standard store (`STA`/`STX`/`STY`). To mitigate this, one can use a target address where the high byte is `$FE`, so the `AND` with `(HighByte+1)` becomes an `AND` with `$FF`, which has no effect.

SHA (AXA, AHX)

(Semi-Stable) Function: `{addr} = A & X & (HighByte({addr}) + 1)`
Stores the result of `A AND X AND (High Byte of target address + 1)` into memory. See warnings above. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Absolute,Y	SHA $4400,Y	$9F	3	5
Indirect,Y	SHA ($44),Y	$93	2	6

SHX (SXA)

(Semi-Stable) Function: `{addr} = X & (HighByte({addr}) + 1)`
Stores the result of `X AND (High Byte of target address + 1)` into memory. See warnings above. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Absolute,Y	SHX $4400,Y	$9E	3	5

SHY (SYA)

(Semi-Stable) Function: `{addr} = Y & (HighByte({addr}) + 1)`
Stores the result of `Y AND (High Byte of target address + 1)` into memory. See warnings above. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Absolute,X	SHY $4400,X	$9C	3	5

LAS

(Semi-Stable) Function: `A, X, S = {addr} & S`
Loads memory, `AND`s it with the Stack Pointer, and loads the result into A, X, and the Stack Pointer. See warnings above. Affects Flags: N, Z

Mode	Syntax	HEX	LEN	TIM
Absolute,Y	LAS $4400,Y	$BB	3	4+

TAS (SHS)

(Semi-Stable) Function: `S = A & X`, then `{addr} = S & (HighByte({addr}) + 1)`
Calculates `A & X` and puts the result in the Stack Pointer. Then it `AND`s this new S value with `(High Byte of target address + 1)` and stores the result in memory. See warnings above. Affects Flags: none

Mode	Syntax	HEX	LEN	TIM
Absolute,Y	TAS $4400,Y	$9B	3	5