6502 Instruction Set

show illegal opcodes

Description

Address Modes

*

16-bit address words are little endian, lo(w)-byte first, followed by the hi(gh)-byte.
(An assembler will use a human readable, big-endian notation as in $HHLL.)

**

The available 16-bit address space is conceived as consisting of pages of 256 bytes each, with
address hi-bytes represententing the page index. An increment with carry may affect the hi-byte
and may thus result in a crossing of page boundaries, adding an extra cycle to the execution.
Increments without carry do not affect the hi-byte of an address and no page transitions do occur.
Generally, increments of 16-bit addresses include a carry, increments of zeropage addresses don't.
Notably this is not related in any way to the state of the carry bit of the accumulator.

***

Branch offsets are signed 8-bit values, -128 ... +127, negative offsets in two's complement.
Page transitions may occur and add an extra cycle to the exucution.

Instructions by Name

ADC

add with carry

AND

and (with accumulator)

ASL

arithmetic shift left

BCC

branch on carry clear

BCS

branch on carry set

BEQ

branch on equal (zero set)

BIT

bit test

BMI

branch on minus (negative set)

BNE

branch on not equal (zero clear)

BPL

branch on plus (negative clear)

BRK

break / interrupt

BVC

branch on overflow clear

BVS

branch on overflow set

CLC

clear carry

CLD

clear decimal

CLI

clear interrupt disable

CLV

clear overflow

CMP

compare (with accumulator)

CPX

compare with X

CPY

compare with Y

DEC

decrement

DEX

decrement X

DEY

decrement Y

EOR

exclusive or (with accumulator)

INC

increment

INX

increment X

INY

increment Y

JMP

jump

JSR

jump subroutine

LDA

load accumulator

LDX

load X

LDY

load Y

LSR

logical shift right

NOP

no operation

ORA

or with accumulator

PHA

push accumulator

PHP

push processor status (SR)

PLA

pull accumulator

PLP

pull processor status (SR)

ROL

rotate left

ROR

rotate right

RTI

return from interrupt

RTS

return from subroutine

SBC

subtract with carry

SEC

set carry

SED

set decimal

SEI

set interrupt disable

STA

store accumulator

STX

store X

STY

store Y

TAX

transfer accumulator to X

TAY

transfer accumulator to Y

TSX

transfer stack pointer to X

TXA

transfer X to accumulator

TXS

transfer X to stack pointer

TYA

transfer Y to accumulator

Registers

Note: The status register (SR) is also known as the P register.

SR Flags (bit 7 to bit 0)

• The zero flag (Z) indicates a value of all zero bits and the negative flag (N) indicates the presence of a set sign bit in bit-position 7. These flags are always updated, whenever a value is transferred to a CPU register (A,X,Y) and as a result of any logical ALU operations. The Z and N flags are also updated by increment and decrement operations acting on a memory location.
• The carry flag (C) flag is used as a buffer and as a borrow in arithmetic operations. Any comparisons will update this additionally to the Z and N flags, as do shift and rotate operations.
• All arithmetic operations update the Z, N, C and V flags.
• The overflow flag (V) indicates overflow with signed binary arithmetics. As a signed byte represents a range of -128 to +127, an overflow can never occur when the operands are of opposite sign, since the result will never exceed this range. Thus, overflow may only occur, if both operands are of the same sign. Then, the result must be also of the same sign. Otherwise, overflow is detected and the overflow flag is set. (I.e., both operands have a zero in the sign position at bit 7, but bit 7 of the result is 1, or, both operands have the sign-bit set, but the result is positive.)
• The decimal flag (D) sets the ALU to binary coded decimal (BCD) mode for additions and subtractions (ADC, SBC).
• The interrupt inhibit flag (I) blocks any maskable interrupt requests (IRQ).
• The break flag (B) is not an actual flag implemented in a register, and rather appears only, when the status register is pushed onto or pulled from the stack. When pushed, it will be 1 when transfered by a BRK or PHP instruction, and zero otherwise (i.e., when pushed by a hardware interrupt). When pulled into the status register (by PLP or on RTI), it will be ignored.
  In other words, the break flag will be inserted, whenever the status register is transferred to the stack by software (BRK or PHP), and will be zero, when transferred by hardware. Since there is no actual slot for the break flag, it will be always ignored, when retrieved (PLP or RTI). The break flag is not accessed by the CPU at anytime and there is no internal representation. Its purpose is more for patching, to discern an interrupt caused by a BRK instruction from a normal interrupt initiated by hardware.
• Any of these flags (but the break flag) may be set or cleared by dedicated instructions. Moreover, there are branch instructions to conditionally divert the control flow depending on the respective state of the Z, N, C or V flag.

Processor Stack

LIFO, top-down, 8 bit range, 0x0100 - 0x01FF

Bytes, Words, Addressing

8 bit bytes, 16 bit words in lobyte-hibyte representation (Little-Endian).
16 bit address range, operands follow instruction codes.

Signed values are two's complement, sign in bit 7 (most significant bit).
(%11111111 = $FF = -1, %10000000 = $80 = -128, %01111111 = $7F = +127)
Signed binary and binary coded decimal (BCD) arithmetic modes.

System Vectors

$FFFA, $FFFB ... NMI (Non-Maskable Interrupt) vector, 16-bit (LB, HB)
$FFFC, $FFFD ... RES (Reset) vector, 16-bit (LB, HB)
$FFFE, $FFFF ... IRQ (Interrupt Request) vector, 16-bit (LB, HB)

Start/Reset Operations

An active-low reset line allows to hold the processor in a known disabled
state, while the system is initialized. As the reset line goes high, the
processor performs a start sequence of 7 cycles, at the end of which the
program counter (PC) is read from the address provided in the 16-bit reset
vector at $FFFC (LB-HB). Then, at the eighth cycle, the processor transfers
control by performing a JMP to the provided address.
Any other initializations are left to the thus executed program. (Notably,
instructions exist for the initialization and loading of all registers, but
for the program counter, which is provided by the reset vector at $FFFC.)

Instructions by Type

• Transfer Instructions

  Load, store, interregister transfer

  LDA

  load accumulator

  LDX

  load X

  LDY

  load Y

  STA

  store accumulator

  STX

  store X

  STY

  store Y

  TAX

  transfer accumulator to X

  TAY

  transfer accumulator to Y

  TSX

  transfer stack pointer to X

  TXA

  transfer X to accumulator

  TXS

  transfer X to stack pointer

  TYA

  transfer Y to accumulator

• Stack Instructions

  These instructions transfer the accumulator or status register (flags) to and from the stack. The processor stack is a last-in-first-out (LIFO) stack of 256 bytes length, implemented at addresses $0100 - $01FF. The stack grows down as new values are pushed onto it with the current insertion point maintained in the stack pointer register.
  (When a byte is pushed onto the stack, it will be stored in the address indicated by the value currently in the stack pointer, which will be then decremented by 1. Conversely, when a value is pulled from the stack, the stack pointer is incremented. The stack pointer is accessible by the TSX and TXS instructions.)

  PHA

  push accumulator

  PHP

  push processor status register (with break flag set)

  PLA

  pull accumulator

  PLP

  pull processor status register

• Decrements & Increments

  DEC

  decrement (memory)

  DEX

  decrement X

  DEY

  decrement Y

  INC

  increment (memory)

  INX

  increment X

  INY

  increment Y

• Arithmetic Operations

  ADC

  add with carry (prepare by CLC)

  SBC

  subtract with carry (prepare by SEC)

  See the Primer of 6502 Arithmetic Instructions below for details.

• Logical Operations

  AND

  and (with accumulator)

  EOR

  exclusive or (with accumulator)

  ORA

  (inclusive) or with accumulator

• Shift & Rotate Instructions

  All shift and rotate instructions preserve the bit shifted out in the carry flag.

  ASL

  arithmetic shift left (shifts in a zero bit on the right)

  LSR

  logical shift right (shifts in a zero bit on the left)

  ROL

  rotate left (shifts in carry bit on the right)

  ROR

  rotate right (shifts in zero bit on the left)

• Flag Instructions

  CLC

  clear carry

  CLD

  clear decimal (BCD arithmetics disabled)

  CLI

  clear interrupt disable

  CLV

  clear overflow

  SEC

  set carry

  SED

  set decimal (BCD arithmetics enabled)

  SEI

  set interrupt disable

• Comparisons

  Generally, comparison instructions subtract the operand from the given register without affecting this register. Flags are still set as with a normal subtraction and thus the relation of the two values becomes accessible by the Zero, Carry and Negative flags.
  (See the branch instructions below for how to evaluate flags.)

  Relation R − Op	Z	C	N
  Register < Operand	0	0	sign bit of result
  Register = Operand	1	1	0
  Register > Operand	0	1	sign bit of result

  CMP

  compare (with accumulator)

  CPX

  compare with X

  CPY

  compare with Y

• Conditional Branch Instructions

  Branch targets are relative, signed 8-bit address offsets. (An offset of #0 corresponds to the immedately following address — or a rather odd and expensive NOP.)

  BCC

  branch on carry clear

  BCS

  branch on carry set

  BEQ

  branch on equal (zero set)

  BMI

  branch on minus (negative set)

  BNE

  branch on not equal (zero clear)

  BPL

  branch on plus (negative clear)

  BVC

  branch on overflow clear

  BVS

  branch on overflow set

• Jumps & Subroutines

  JSR and RTS affect the stack as the return address is pushed onto or pulled from the stack, respectively.
  (JSR will first push the high-byte of the return address [PC+2] onto the stack, then the low-byte. The stack will then contain, seen from the bottom or from the most recently added byte, [PC+2]-L [PC+2]-H.)

  JMP

  jump

  JSR

  jump subroutine

  RTS

  return from subroutine

• Interrupts

  A hardware interrupt (maskable IRQ and non-maskable NMI), will cause the processor to put first the address currently in the program counter onto the stack (in HB-LB order), followed by the value of the status register. (The stack will now contain, seen from the bottom or from the most recently added byte, SR PC-L PC-H with the stack pointer pointing to the address below the stored contents of status register.) Then, the processor will divert its control flow to the address provided in the two word-size interrupt vectors at $FFFA (IRQ) and $FFFE (NMI).
  A set interrupt disable flag will inhibit the execution of an IRQ, but not of a NMI, which will be executed anyways.
  The break instruction (BRK) behaves like a NMI, but will push the value of PC+2 onto the stack to be used as the return address. Also, as with any software initiated transfer of the status register to the stack, the break flag will be found set on the respective value pushed onto the stack. Then, control is transferred to the address in the NMI-vector at $FFFE.
  In any way, the interrupt disable flag is set to inhibit any further IRQ as control is transferred to the interrupt handler specified by the respective interrupt vector.

  The RTI instruction restores the status register from the stack and behaves otherwise like the JSR instruction. (The break flag is always ignored as the status is read from the stack, as it isn't a real processor flag anyway.)

  BRK

  break / software interrupt

  RTI

  return from interrupt

• Other

  BIT

  bit test (accumulator & memory)

  NOP

  no operation

6502 Address Modes in Detail

(This section, especially the diagrams included, is heavily inspired by the Acorn Atom manual "Atomic Theory and Practice" by David Johnson Davies, Acorn Computers Limited, 2^nd ed. 1980, p 118–121.)

• Implied Addressing

  These instructions act directly on one or more registers or flags internal to the CPU. Therefor, these instructions are principally single-byte instructions, lacking an explicit operand. The operand is implied, as it is already provided by the very instruction.

  Instructions targeting exclusively the contents of the accumulator may or may not be denoted by using an explicit "A" as the operand, depending on the flavor of syntax. (This may be regarded as a special address mode of its own, but it is really a special case of an implied instruction. It is still a single-byte instruction and no operand is provided in machine language.)

  Mnemonic Examples:

  CLC

  clear the carry flag

  ROL A

  rotate contents of accumulator left by one position

  ROL

  same as above, implicit notation (A implied)

  TXA

  transfer contents of X-register to the accumulator

  PHA

  push the contents of the accumulator to the stack

  RTS

  return from subroutine (by pulling PC from stack)

  Mind that some of these instructions, while simple in appearance, may be quite complex operations, like "PHA", which involves the accumulator, the stack pointer and memory access.

• Immediate Addressing

  Here, a literal operand is given immediately after the instruction. The operand is always an 8-bit value and the total instruction length is always 2 bytes. In memory, the operand is a single byte following immediately after the instruction code. In assembler, the mode is usually indicated by a "#" prefix adjacent to the operand.

  Mnemonic Examples:

  LDA #$07

  load the literal hexidecimal value "$7" into the accumulator

  ADC #$A0

  add the literal hexidecimal value "$A0" to the accumulator

  CPX #$32

  compare the X-register to the literal hexidecimal value "$32"

• Absolute Addressing

  Absolute addressing modes provides the 16-bit address of a memory location, the contents of which used as the operand to the instruction. In machine language, the address is provided in two bytes immediately after the instruction (making these 3-byte instructions) in low-byte, high-byte order (LLHH) or little-endian. In assembler, conventional numbers (HHLL order or big-endian words) are used to provide the address.

  Absolute addresses are also used for the jump instructions JMP and JSR to provide the address for the next instruction to continue with in the control flow.

  Mnemonic Examples:

  LDA $3010

  load the contents of address "$3010" into the accumulator

  ROL $08A0

  rotate the contents of address "$08A0" left by one position

  JMP $4000

  jump to (continue with) location "$4000"

• Zero-Page Addressing

  The 16-bit address space available to the 6502 is thought to consist of 256 "pages" of 256 memory locations each ($00…$FF). In this model the high-byte of an address gives the page number and the low-byte a location inside this page. The very first of these pages, where the high-byte is zero (addresses $0000…$00FF), is somewhat special.

  The zero-page address mode is similar to absolute address mode, but these instructions use only a single byte for the operand, the low-byte, while the high-byte is assumed to be zero by definition. Therefore, these instructions have a total length of just two bytes (one less than absolute mode) and take one CPU cycle less to execute, as there is one byte less to fetch.

  Mnemonic Examples:

  LDA $80

  load the contents of address "$0080" into the accumulator

  BIT $A2

  perform bit-test with the contents of address "$00A2"

  ASL $9A

  arithmetic shift left of the contents of location "$009A"

  (One way to think of the zero-page is as a page of 256 additional registers, somewhat slower than the internal registers, but with zero-page instructions also faster executing than "normal" instructions. The zero-page has a few more tricks up its sleeve, making these addresses perform more like real registers, see below.)

• Indexed Addressing: Absolute,X and Absolute,Y

  Indexed addressing adds the contents of either the X-register or the Y-register to the provided address to give the effective address, which provides the operand.

  These instructions are usefull to e.g., load values from tables or to write to a continuous segment of memory in a loop. The most basic forms are "absolute,X" and "absolute,X", where either the X- or the Y-register, respectively, is added to a given base address. As the base address is a 16-bit value, these are generally 3-byte instructions. Since there is an additional operation to perform to determine the effective address, these instructions are one cycle slower than those using absolute addressing mode.*

  Mnemonic Examples:

  LDA $3120,X

  load the contents of address "$3120 + X" into A

  LDX $8240,Y

  load the contents of address "$8240 + Y" into X

  INC $1400,X

  increment the contents of address "$1400 + X"

  *) If the addition of the contents of the index register effects in a change of the high-byte given by the base address so that the effective address is on the next memory page, the additional operation to increment the high-byte takes another CPU cycle. This is also known as a crossing of page boundaries.

• Indexed Addressing: Zero-Page,X (and Zero-Page,Y)

  As with absolute addressing, there is also a zero-page mode for indexed addressing. However, this is generally only available with the X-register. (The only exception to this is LDX, which has an indexed zero-page mode utilizing the Y-register.)
  As we have already seen with normal zero-page mode, these instructions are one byte less in total length (two bytes) and take one CPU cycle less than instructions in absolute indexed mode.

  Unlike absolute indexed instructions with 16-bit base addresses, zero-page indexed instructions never affect the high-byte of the effective address, which will simply wrap around in the zero-page, and there is no penalty for crossing any page boundaries.

  Mnemonic Examples:

  LDA $80,X

  load the contents of address "$0080 + X" into A

  LSR $82,X

  shift the contents of address "$0082 + X" left

  LDX $60,Y

  load the contents of address "$0060 + Y" into X

• Indirect Addressing

  This mode looks up a given address and uses the contents of this address and the next one (in LLHH little-endian order) as the effective address. In its basic form, this mode is available for the JMP instruction only. (Its generally use is jump vectors and jump tables.)
  Like the absolute JMP instruction it uses a 16-bit address (3 bytes in total), but takes two additional CPU cycles to execute, since there are two additional bytes to fetch for the lookup of the effective jump target.

  Generally, indirect addressing is denoted by putting the lookup address in parenthesis.

  Mnemonic Example:

  JMP ($FF82)

  jump to address given in addresses "$FF82" and "$FF83"

• Pre-Indexed Indirect, "(Zero-Page,X)"

  Indexed indirect address modes are generally available only for instructions supplying an operand to the accumulator (LDA, STA, ADC, SBC, AND, ORA, EOR, etc). The placement of the index register inside or outside of the parenthesis indicating the address lookup will give you clue what these instructions are doing.

  Pre-indexed indirect address mode is only available in combination with the X-register. It works much like the "zero-page,X" mode, but, after the X-register has been added to the base address, instead of directly accessing this, an additional lookup is performed, reading the contents of resulting address and the next one (in LLHH little-endian order), in order to determine the effective address.

  Like with "zero-page,X" mode, the total instruction length is 2 bytes, but there are two additional CPU cycles in order to fetch the effective 16-bit address. As "zero-page,X" mode, a lookup address will never overflow into the next page, but will simply wrap around in the zero-page.

  These instructions are useful, whenever we want to loop over a table of pointers to disperse addresses, or where we want to apply the same operation to various addresses, which we have stored as a table in the zero-page.

  Mnemonic Examples:

  LDA ($70,X)

  load the contents of the location given in addresses
  "$0070+X" and "$0070+1+X" into A

  STA ($A2,X)

  store the contents of A in the location given in
  addresses "$00A2+X" and "$00A3+X"

  EOR ($BA,X)

  perform an exlusive OR of the contents of A and the contents
  of the location given in addresses "$00BA+X" and "$00BB+X"

• Post-Indexed Indirect, "(Zero-Page),Y"

  Post-indexed indirect addressing is only available in combination with the Y-register. As indicated by the indexing term ",Y" being appended to the outside of the parenthesis indicating the indirect lookup, here, a pointer is first read (from the given zero-page address) and resolved and only then the contents of the Y-register is added to this to give the effective address.

  Like with "zero-page,Y" mode, the total instruction length is 2 bytes, but there it takes an additional CPU cycles to resolve and index the 16-bit pointer. As with "absolute,X" mode, the effective address may overflow into the next page, in the case of which the execution uses an extra CPU cycle.

  These instructions are useful, wherever we want to perform lookups on varying bases addresses or whenever we want to loop over tables, the base address of which we have stored in the zero-page.

  Mnemonic Examples:

  LDA ($70),Y

  add the contents of the Y-register to the pointer provided in
  "$0070" and "$0071" and load the contents of this address into A

  STA ($A2),Y

  store the contents of A in the location given by the pointer
  in "$00A2" and "$00A3" plus the contents of the Y-register

  EOR ($BA),Y

  perform an exlusive OR of the contents of A and the address
  given by the addition of Y to the pointer in "$00BA" and "$00BB"

• Relative Addressing (Conditional Branching)

  This final address mode is exlusive to conditional branch instructions, which branch in the execution path depending on the state of a given CPU flag. Here, the instruction provides only a relative offset, which is added to the contents of the program counter (PC) as it points to the immediate next instruction. The relative offset is a signed single byte value in two's complement encoding (giving a range of −128…+127), which allows for branching up to half a page forwards and backwards.
  On the one hand, this makes these instructions compact, fast and relocatable at the same time. On the other hand, we have to mind that our branch target is no farther away than half a memory page.

  Generally, an assembler will take care of this and we only have to provide the target address, not having to worry about relative addressing.

  These instructions are always of 2 bytes length and perform in 2 CPU cycles, if the branch is not taken (the condition resolving to 'false'), and 3 cycles, if the branch is taken (when the condition is true). If a branch is taken and the target is on a different page, this adds another CPU cycle (4 in total).

  Mnemonic Examples:

  BEQ $1005

  branch to location "$1005", if the zero flag is set.
  if the current address is $1000, this will give an offset of $03.

  BCS $08C4

  branch to location "$08C4", if the carry flag is set.
  if the current address is $08D4, this will give an offset of $EE (−$12).

  BCC $084A

  branch to location "$084A", if the carry flag is clear.

Vendor

MOS Technology, 1975

Image: Wikimedia Commons.

6502 Instructions in Detail

ADC

Add Memory to Accumulator with Carry

A + M + C -> A, C

N	Z	C	I	D	V
+	+	+	-	-	+

addressing	assembler	opc	bytes	cycles
immediate	ADC #oper	69	2	2
zeropage	ADC oper	65	2	3
zeropage,X	ADC oper,X	75	2	4
absolute	ADC oper	6D	3	4
absolute,X	ADC oper,X	7D	3	4*
absolute,Y	ADC oper,Y	79	3	4*
(indirect,X)	ADC (oper,X)	61	2	6
(indirect),Y	ADC (oper),Y	71	2	5*

AND

AND Memory with Accumulator

A AND M -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	AND #oper	29	2	2
zeropage	AND oper	25	2	3
zeropage,X	AND oper,X	35	2	4
absolute	AND oper	2D	3	4
absolute,X	AND oper,X	3D	3	4*
absolute,Y	AND oper,Y	39	3	4*
(indirect,X)	AND (oper,X)	21	2	6
(indirect),Y	AND (oper),Y	31	2	5*

ASL

Shift Left One Bit (Memory or Accumulator)

C <- [76543210] <- 0

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
accumulator	ASL A	0A	1	2
zeropage	ASL oper	06	2	5
zeropage,X	ASL oper,X	16	2	6
absolute	ASL oper	0E	3	6
absolute,X	ASL oper,X	1E	3	7

BCC

Branch on Carry Clear

branch on C = 0

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BCC oper	90	2	2**

BCS

Branch on Carry Set

branch on C = 1

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BCS oper	B0	2	2**

BEQ

Branch on Result Zero

branch on Z = 1

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BEQ oper	F0	2	2**

BIT

Test Bits in Memory with Accumulator

bits 7 and 6 of operand are transfered to bit 7 and 6 of SR (N,V);
the zero-flag is set according to the result of the operand AND
the accumulator (set, if the result is zero, unset otherwise).
This allows a quick check of a few bits at once without affecting
any of the registers, other than the status register (SR).

A AND M -> Z, M7 -> N, M6 -> V

N	Z	C	I	D	V
M7	+	-	-	-	M6

addressing	assembler	opc	bytes	cycles
zeropage	BIT oper	24	2	3
absolute	BIT oper	2C	3	4

BMI

Branch on Result Minus

branch on N = 1

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BMI oper	30	2	2**

BNE

Branch on Result not Zero

branch on Z = 0

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BNE oper	D0	2	2**

BPL

Branch on Result Plus

branch on N = 0

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BPL oper	10	2	2**

BRK

Force Break

BRK initiates a software interrupt similar to a hardware
interrupt (IRQ). The return address pushed to the stack is
PC+2, providing an extra byte of spacing for a break mark
(identifying a reason for the break.)
The status register will be pushed to the stack with the break
flag set to 1. However, when retrieved during RTI or by a PLP
instruction, the break flag will be ignored.
The interrupt disable flag is not set automatically.

interrupt,
push PC+2, push SR

N	Z	C	I	D	V
-	-	-	1	-	-

addressing	assembler	opc	bytes	cycles
implied	BRK	00	1	7

BVC

Branch on Overflow Clear

branch on V = 0

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BVC oper	50	2	2**

BVS

Branch on Overflow Set

branch on V = 1

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
relative	BVS oper	70	2	2**

CLC

Clear Carry Flag

0 -> C

N	Z	C	I	D	V
-	-	0	-	-	-

addressing	assembler	opc	bytes	cycles
implied	CLC	18	1	2

CLD

Clear Decimal Mode

0 -> D

N	Z	C	I	D	V
-	-	-	-	0	-

addressing	assembler	opc	bytes	cycles
implied	CLD	D8	1	2

CLI

Clear Interrupt Disable Bit

0 -> I

N	Z	C	I	D	V
-	-	-	0	-	-

addressing	assembler	opc	bytes	cycles
implied	CLI	58	1	2

CLV

Clear Overflow Flag

0 -> V

N	Z	C	I	D	V
-	-	-	-	-	0

addressing	assembler	opc	bytes	cycles
implied	CLV	B8	1	2

CMP

Compare Memory with Accumulator

A - M

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	CMP #oper	C9	2	2
zeropage	CMP oper	C5	2	3
zeropage,X	CMP oper,X	D5	2	4
absolute	CMP oper	CD	3	4
absolute,X	CMP oper,X	DD	3	4*
absolute,Y	CMP oper,Y	D9	3	4*
(indirect,X)	CMP (oper,X)	C1	2	6
(indirect),Y	CMP (oper),Y	D1	2	5*

CPX

Compare Memory and Index X

X - M

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	CPX #oper	E0	2	2
zeropage	CPX oper	E4	2	3
absolute	CPX oper	EC	3	4

CPY

Compare Memory and Index Y

Y - M

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	CPY #oper	C0	2	2
zeropage	CPY oper	C4	2	3
absolute	CPY oper	CC	3	4

DEC

Decrement Memory by One

M - 1 -> M

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	DEC oper	C6	2	5
zeropage,X	DEC oper,X	D6	2	6
absolute	DEC oper	CE	3	6
absolute,X	DEC oper,X	DE	3	7

DEX

Decrement Index X by One

X - 1 -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	DEX	CA	1	2

DEY

Decrement Index Y by One

Y - 1 -> Y

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	DEY	88	1	2

EOR

Exclusive-OR Memory with Accumulator

A EOR M -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	EOR #oper	49	2	2
zeropage	EOR oper	45	2	3
zeropage,X	EOR oper,X	55	2	4
absolute	EOR oper	4D	3	4
absolute,X	EOR oper,X	5D	3	4*
absolute,Y	EOR oper,Y	59	3	4*
(indirect,X)	EOR (oper,X)	41	2	6
(indirect),Y	EOR (oper),Y	51	2	5*

INC

Increment Memory by One

M + 1 -> M

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	INC oper	E6	2	5
zeropage,X	INC oper,X	F6	2	6
absolute	INC oper	EE	3	6
absolute,X	INC oper,X	FE	3	7

INX

Increment Index X by One

X + 1 -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	INX	E8	1	2

INY

Increment Index Y by One

Y + 1 -> Y

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	INY	C8	1	2

JMP

Jump to New Location

operand 1st byte -> PCL
operand 2nd byte -> PCH

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolute	JMP oper	4C	3	3
indirect	JMP (oper)	6C	3	5

JSR

Jump to New Location Saving Return Address

push (PC+2),
operand 1st byte -> PCL
operand 2nd byte -> PCH

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolute	JSR oper	20	3	6

LDA

Load Accumulator with Memory

M -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	LDA #oper	A9	2	2
zeropage	LDA oper	A5	2	3
zeropage,X	LDA oper,X	B5	2	4
absolute	LDA oper	AD	3	4
absolute,X	LDA oper,X	BD	3	4*
absolute,Y	LDA oper,Y	B9	3	4*
(indirect,X)	LDA (oper,X)	A1	2	6
(indirect),Y	LDA (oper),Y	B1	2	5*

LDX

Load Index X with Memory

M -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	LDX #oper	A2	2	2
zeropage	LDX oper	A6	2	3
zeropage,Y	LDX oper,Y	B6	2	4
absolute	LDX oper	AE	3	4
absolute,Y	LDX oper,Y	BE	3	4*

LDY

Load Index Y with Memory

M -> Y

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	LDY #oper	A0	2	2
zeropage	LDY oper	A4	2	3
zeropage,X	LDY oper,X	B4	2	4
absolute	LDY oper	AC	3	4
absolute,X	LDY oper,X	BC	3	4*

LSR

Shift One Bit Right (Memory or Accumulator)

0 -> [76543210] -> C

N	Z	C	I	D	V
0	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
accumulator	LSR A	4A	1	2
zeropage	LSR oper	46	2	5
zeropage,X	LSR oper,X	56	2	6
absolute	LSR oper	4E	3	6
absolute,X	LSR oper,X	5E	3	7

NOP

No Operation

---

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	NOP	EA	1	2

ORA

OR Memory with Accumulator

A OR M -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	ORA #oper	09	2	2
zeropage	ORA oper	05	2	3
zeropage,X	ORA oper,X	15	2	4
absolute	ORA oper	0D	3	4
absolute,X	ORA oper,X	1D	3	4*
absolute,Y	ORA oper,Y	19	3	4*
(indirect,X)	ORA (oper,X)	01	2	6
(indirect),Y	ORA (oper),Y	11	2	5*

PHA

Push Accumulator on Stack

push A

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	PHA	48	1	3

PHP

Push Processor Status on Stack

The status register will be pushed with the break
flag and bit 5 set to 1.

push SR

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	PHP	08	1	3

PLA

Pull Accumulator from Stack

pull A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	PLA	68	1	4

PLP

Pull Processor Status from Stack

The status register will be pulled with the break
flag and bit 5 ignored.

pull SR

N	Z	C	I	D	V
from stack

addressing	assembler	opc	bytes	cycles
implied	PLP	28	1	4

ROL

Rotate One Bit Left (Memory or Accumulator)

C <- [76543210] <- C

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
accumulator	ROL A	2A	1	2
zeropage	ROL oper	26	2	5
zeropage,X	ROL oper,X	36	2	6
absolute	ROL oper	2E	3	6
absolute,X	ROL oper,X	3E	3	7

ROR

Rotate One Bit Right (Memory or Accumulator)

C -> [76543210] -> C

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
accumulator	ROR A	6A	1	2
zeropage	ROR oper	66	2	5
zeropage,X	ROR oper,X	76	2	6
absolute	ROR oper	6E	3	6
absolute,X	ROR oper,X	7E	3	7

RTI

Return from Interrupt

The status register is pulled with the break flag
and bit 5 ignored. Then PC is pulled from the stack.

pull SR, pull PC

N	Z	C	I	D	V
from stack

addressing	assembler	opc	bytes	cycles
implied	RTI	40	1	6

RTS

Return from Subroutine

pull PC, PC+1 -> PC

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	RTS	60	1	6

SBC

Subtract Memory from Accumulator with Borrow

A - M - C̅ -> A

N	Z	C	I	D	V
+	+	+	-	-	+

addressing	assembler	opc	bytes	cycles
immediate	SBC #oper	E9	2	2
zeropage	SBC oper	E5	2	3
zeropage,X	SBC oper,X	F5	2	4
absolute	SBC oper	ED	3	4
absolute,X	SBC oper,X	FD	3	4*
absolute,Y	SBC oper,Y	F9	3	4*
(indirect,X)	SBC (oper,X)	E1	2	6
(indirect),Y	SBC (oper),Y	F1	2	5*

SEC

Set Carry Flag

1 -> C

N	Z	C	I	D	V
-	-	1	-	-	-

addressing	assembler	opc	bytes	cycles
implied	SEC	38	1	2

SED

Set Decimal Flag

1 -> D

N	Z	C	I	D	V
-	-	-	-	1	-

addressing	assembler	opc	bytes	cycles
implied	SED	F8	1	2

SEI

Set Interrupt Disable Status

1 -> I

N	Z	C	I	D	V
-	-	-	1	-	-

addressing	assembler	opc	bytes	cycles
implied	SEI	78	1	2

STA

Store Accumulator in Memory

A -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	STA oper	85	2	3
zeropage,X	STA oper,X	95	2	4
absolute	STA oper	8D	3	4
absolute,X	STA oper,X	9D	3	5
absolute,Y	STA oper,Y	99	3	5
(indirect,X)	STA (oper,X)	81	2	6
(indirect),Y	STA (oper),Y	91	2	6

STX

Store Index X in Memory

X -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	STX oper	86	2	3
zeropage,Y	STX oper,Y	96	2	4
absolute	STX oper	8E	3	4

STY

Sore Index Y in Memory

Y -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	STY oper	84	2	3
zeropage,X	STY oper,X	94	2	4
absolute	STY oper	8C	3	4

TAX

Transfer Accumulator to Index X

A -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	TAX	AA	1	2

TAY

Transfer Accumulator to Index Y

A -> Y

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	TAY	A8	1	2

TSX

Transfer Stack Pointer to Index X

SP -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	TSX	BA	1	2

TXA

Transfer Index X to Accumulator

X -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	TXA	8A	1	2

TXS

Transfer Index X to Stack Register

X -> SP

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	TXS	9A	1	2

TYA

Transfer Index Y to Accumulator

Y -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
implied	TYA	98	1	2

*

add 1 to cycles if page boundary is crossed

**

add 1 to cycles if branch occurs on same page
add 2 to cycles if branch occurs to different page

Legend to Flags:

+

modified

-

not modified

1

set

0

cleared

M6

memory bit 6

M7

memory bit 7

Note on assembler syntax:
Some assemblers employ "OPC *oper" or a ".b" extension
to the mneomonic for forced zeropage addressing.

"Illegal" Opcodes and Undocumented Instructions

ALR (ASR)

AND oper + LSR

A AND oper, 0 -> [76543210] -> C

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	ALR #oper	4B	2	2

ANC

AND oper + set C as ASL

A AND oper, bit(7) -> C

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	ANC #oper	0B	2	2

ANC (ANC2)

AND oper + set C as ROL

effectively the same as instr. 0B

A AND oper, bit(7) -> C

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	ANC #oper	2B	2	2

ANE (XAA)

* OR X + AND oper

Highly unstable, do not use.

A base value in A is determined based on the contets of A and a constant, which may be typically $00, $ff, $ee, etc. The value of this constant depends on temerature, the chip series, and maybe other factors, as well.
In order to eliminate these uncertaincies from the equation, use either 0 as the operand or a value of $FF in the accumulator.

(A OR CONST) AND X AND oper -> A

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	ANE #oper	8B	2	2	††

ARR

AND oper + ROR

This operation involves the adder:
V-flag is set according to (A AND oper) + oper
The carry is not set, but bit 7 (sign) is exchanged with the carry

A AND oper, C -> [76543210] -> C

N	Z	C	I	D	V
+	+	+	-	-	+

addressing	assembler	opc	bytes	cycles
immediate	ARR #oper	6B	2	2

DCP (DCM)

DEC oper + CMP oper

M - 1 -> M, A - M

Decrements the operand and then compares the result to the accumulator.

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	DCP oper	C7	2	5
zeropage,X	DCP oper,X	D7	2	6
absolute	DCP oper	CF	3	6
absolut,X	DCP oper,X	DF	3	7
absolut,Y	DCP oper,Y	DB	3	7
(indirect,X)	DCP (oper,X)	C3	2	8
(indirect),Y	DCP (oper),Y	D3	2	8

ISC (ISB, INS)

INC oper + SBC oper

M + 1 -> M, A - M - C̅ -> A

N	Z	C	I	D	V
+	+	+	-	-	+

addressing	assembler	opc	bytes	cycles
zeropage	ISC oper	E7	2	5
zeropage,X	ISC oper,X	F7	2	6
absolute	ISC oper	EF	3	6
absolut,X	ISC oper,X	FF	3	7
absolut,Y	ISC oper,Y	FB	3	7
(indirect,X)	ISC (oper,X)	E3	2	8
(indirect),Y	ISC (oper),Y	F3	2	8

LAS (LAR)

LDA/TSX oper

M AND SP -> A, X, SP

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolut,Y	LAS oper,Y	BB	3	4*

LAX

LDA oper + LDX oper

M -> A -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	LAX oper	A7	2	3
zeropage,Y	LAX oper,Y	B7	2	4
absolute	LAX oper	AF	3	4
absolut,Y	LAX oper,Y	BF	3	4*
(indirect,X)	LAX (oper,X)	A3	2	6
(indirect),Y	LAX (oper),Y	B3	2	5*

LXA (LAX immediate)

Store * AND oper in A and X

Highly unstable, involves a 'magic' constant, see ANE

(A OR CONST) AND oper -> A -> X

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	LXA #oper	AB	2	2	††

RLA

ROL oper + AND oper

M = C <- [76543210] <- C, A AND M -> A

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	RLA oper	27	2	5
zeropage,X	RLA oper,X	37	2	6
absolute	RLA oper	2F	3	6
absolut,X	RLA oper,X	3F	3	7
absolut,Y	RLA oper,Y	3B	3	7
(indirect,X)	RLA (oper,X)	23	2	8
(indirect),Y	RLA (oper),Y	33	2	8

RRA

ROR oper + ADC oper

M = C -> [76543210] -> C, A + M + C -> A, C

N	Z	C	I	D	V
+	+	+	-	-	+

addressing	assembler	opc	bytes	cycles
zeropage	RRA oper	67	2	5
zeropage,X	RRA oper,X	77	2	6
absolute	RRA oper	6F	3	6
absolut,X	RRA oper,X	7F	3	7
absolut,Y	RRA oper,Y	7B	3	7
(indirect,X)	RRA (oper,X)	63	2	8
(indirect),Y	RRA (oper),Y	73	2	8

SAX (AXS, AAX)

A and X are put on the bus at the same time (resulting effectively in an AND operation) and stored in M

A AND X -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	SAX oper	87	2	3
zeropage,Y	SAX oper,Y	97	2	4
absolute	SAX oper	8F	3	4
(indirect,X)	SAX (oper,X)	83	2	6

SBX (AXS, SAX)

CMP and DEX at once, sets flags like CMP

(A AND X) - oper -> X

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
immediate	SBX #oper	CB	2	2

SHA (AHX, AXA)

Stores A AND X AND (high-byte of addr. + 1) at addr.

unstable: sometimes 'AND (H+1)' is dropped, page boundary crossings may not work (with the high-byte of the value used as the high-byte of the address)

A AND X AND (H+1) -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolut,Y	SHA oper,Y	9F	3	5	†
(indirect),Y	SHA (oper),Y	93	2	6	†

SHX (A11, SXA, XAS)

Stores X AND (high-byte of addr. + 1) at addr.

unstable: sometimes 'AND (H+1)' is dropped, page boundary crossings may not work (with the high-byte of the value used as the high-byte of the address)

X AND (H+1) -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolut,Y	SHX oper,Y	9E	3	5	†

SHY (A11, SYA, SAY)

Stores Y AND (high-byte of addr. + 1) at addr.

unstable: sometimes 'AND (H+1)' is dropped, page boundary crossings may not work (with the high-byte of the value used as the high-byte of the address)

Y AND (H+1) -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolut,X	SHY oper,X	9C	3	5	†

SLO (ASO)

ASL oper + ORA oper

M = C <- [76543210] <- 0, A OR M -> A

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	SLO oper	07	2	5
zeropage,X	SLO oper,X	17	2	6
absolute	SLO oper	0F	3	6
absolut,X	SLO oper,X	1F	3	7
absolut,Y	SLO oper,Y	1B	3	7
(indirect,X)	SLO (oper,X)	03	2	8
(indirect),Y	SLO (oper),Y	13	2	8

SRE (LSE)

LSR oper + EOR oper

M = 0 -> [76543210] -> C, A EOR M -> A

N	Z	C	I	D	V
+	+	+	-	-	-

addressing	assembler	opc	bytes	cycles
zeropage	SRE oper	47	2	5
zeropage,X	SRE oper,X	57	2	6
absolute	SRE oper	4F	3	6
absolut,X	SRE oper,X	5F	3	7
absolut,Y	SRE oper,Y	5B	3	7
(indirect,X)	SRE (oper,X)	43	2	8
(indirect),Y	SRE (oper),Y	53	2	8

TAS (XAS, SHS)

Puts A AND X in SP and stores A AND X AND (high-byte of addr. + 1) at addr.

unstable: sometimes 'AND (H+1)' is dropped, page boundary crossings may not work (with the high-byte of the value used as the high-byte of the address)

A AND X -> SP, A AND X AND (H+1) -> M

N	Z	C	I	D	V
-	-	-	-	-	-

addressing	assembler	opc	bytes	cycles
absolut,Y	TAS oper,Y	9B	3	5	†

USBC (SBC)

SBC oper + NOP

effectively same as normal SBC immediate, instr. E9.

A - M - C̅ -> A

N	Z	C	I	D	V
+	+	+	-	-	+

addressing	assembler	opc	bytes	cycles
immediate	USBC #oper	EB	2	2

NOPs (including DOP, TOP)

Instructions effecting in 'no operations' in various address modes. Operands are ignored.

N	Z	C	I	D	V
-	-	-	-	-	-

opc	addressing	bytes	cycles
1A	implied	1	2
3A	implied	1	2
5A	implied	1	2
7A	implied	1	2
DA	implied	1	2
FA	implied	1	2
80	immediate	2	2
82	immediate	2	2
89	immediate	2	2
C2	immediate	2	2
E2	immediate	2	2
04	zeropage	2	3
44	zeropage	2	3
64	zeropage	2	3
14	zeropage,X	2	4
34	zeropage,X	2	4
54	zeropage,X	2	4
74	zeropage,X	2	4
D4	zeropage,X	2	4
F4	zeropage,X	2	4
0C	absolute	3	4
1C	absolut,X	3	4*
3C	absolut,X	3	4*
5C	absolut,X	3	4*
7C	absolut,X	3	4*
DC	absolut,X	3	4*
FC	absolut,X	3	4*

JAM (KIL, HLT)

These instructions freeze the CPU.

The processor will be trapped infinitely in T1 phase with $FF on the data bus. — Reset required.

Instruction codes: 02, 12, 22, 32, 42, 52, 62, 72, 92, B2, D2, F2

Have a look at this table of the instruction layout in order to see how most of these
"illegal" instructions are a result of executing both instructions at c=1 and c=2 in
a given slot (same column, rows immediately above) at once.
Where c is the lowest two bits of the instruction code. E.g., "SAX abs", instruction
code $8F, binary 10001111, is "STA abs", 10001101 ($8D) and "STX abs", 10001110 ($8E).

Honorable Mention: Rev. A 6502 ROR, Pre-June 1976

Famously, the Rev. A 6502 as delivered from September 1975 to June 1976 had a
"ROR bug". However, the "ROR" instruction isn't only missing from the original
documentation, as it turns out, the chip is actually missing crucial control lines,
which would have been required to make this instruction work. The instruction is
simply not implemented and it wasn't even part of the design. (This was actually
added on popular demand in Rev. B, as rumor has it, demand by Steve Wozniak. Even,
if not true, this makes for a good story. And how could there be a page on the 6502
without mentioning "Woz" once?) So, for all means, "ROR" is an undocumented or
"illegal" instruction on the Rev. A 6502.

And this is how ROR behaves on these Rev. A chips, much like ASL: it shifts all bits
to the left, shifting in a zero bit at the LSB side, but, unlike ASL, it does not
shift the high-bit into the carry. (So there are no connections to the carry at all.)

ROR  Rev. A (pre-June 1976)

As ASL, but does not update the carry.

N and Z flags are set correctly for the operation performed.

[76543210] <- 0

N	Z	C	I	D	V
+	+	-	-	-	-

addressing	assembler	opc	bytes	cycles
accumulator	ROR A	6A	1	2
zeropage	ROR oper	66	2	5
zeropage,X	ROR oper,X	76	2	6
absolute	ROR oper	6E	3	6
absolute,X	ROR oper,X	7E	3	7

Compare Instructions

The 6502 MPU features three basic compare instructions in various address modes:

The various compare instructions subtract the operand from the respective register
(as if the carry was set) without setting the result and adjust the N, Z, and C
flags accordingly to this operation.
Flags will be set as follows:

And for the derivative relations "less/greater than or equal":

Mind that the negative flag is not significant and all conditions may be evaluated
by checking the carry and/or zero flag(s).

The BIT Instruction

A Primer of 6502 Arithmetic Operations

6502 Jump Vectors and Stack Operations

Similarly, as a JSR instruction is encountered, PC is dumped onto the stack
and recovered by the JSR instruction. (Here, the value stored is actually the
address before the location, the program will eventually return to. Thus, the
effective return address is PC+1.)

Curious Interrupt Behavior

• If the instruction was a taken branch instruction with 3 cycles execution time (without crossing page boundaries), the interrupt will trigger only after an extra CPU cycle.
• On the NMOS6502, an NMI hardware interrupt occuring at the start of a BRK intruction will hijack the BRK instruction, meanining, the BRK instruction will be executed as normal, but the NMI vector will be used instead of the IRQ vector.
• The 65C02 will clear the decimal flag on any interrupts (and BRK).

The Break Flag and the Stack

E.g.,

1)
         SR: N V - B D I Z C
             0 0 - - 0 0 1 1

    PHP  ->  0 0 1 1 0 0 1 1  =  $33

    PLP  <-  0 0 - - 0 0 1 1  =  $03

  but:

    PLA  <-  0 0 1 1 0 0 1 1  =  $33


2)
    LDA #$32 ;00110010

    PHA  ->  0 0 1 1 0 0 1 0  =  $32

    PLP  <-  0 0 - - 0 0 1 0  =  $02


3)
    LDA #$C0
    PHA  ->  1 1 0 0 0 0 0 0  =  $C0
    LDA #$08
    PHA  ->  0 0 0 0 1 0 0 0  =  $08
    LDA #$12
    PHA  ->  0 0 0 1 0 0 1 0  =  $12

    RTI
         SR: 0 0 - - 0 0 1 0  =  $02
         PC: $C008

6502 Instruction Layout

Note: The operand of instructions like "ASL A" is often depicted as implied, as well.
Mind that, for any practical reasons, the two notations are interchangeable for any
instructions involving the accumulator. — However, there are subtle differences.

A rotated view, rows as combinations of c and b, and columns as a:

"Illegal" Opcodes Revisited

So, how do the "illegal" opcodes fit into this decoding scheme?
Let's have a look — "illegals" are shown on grey background.

The first view — rows by c and a and columns as b — reveals a strict relation
beetween address modes and columns:

And, again, as a rotated view, rows as combinations of c and b, and columns as a.
We may observe a close relationship between the legal and the undocumented
instructions in the vertical (quarter-)segements of each column.