ADC
AND
ASL
BCC
BCS
BEQ
BIT
BMI
BNE
BPL
BRK
BVC
BVS
CLC
CLD
CLI
CLV
CMP
CPX
CPY
DEC
DEX
DEY
EOR
INC
INX
INY
JMP
JSR
LDA
LDX
LDY
LSR
NOP
ORA
PHA
PHP
PLA
PLP
ROL
ROR
RTI
RTS
SBC
SEC
SED
SEI
STA
STX
STY
TAX
TAY
TSX
TXA
TXS
TYA
| Branches | Decimal Mode | Interrupt Flag | Overflow Flag | Program Counter | Stack | Times | Wrap-around |
Affects Flags: S 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+ + add 1 cycle if page boundary crossed
ADC results are dependant on the setting of the decimal flag. In decimal mode, addition is carried out on the assumption that the values involved are packed BCD (Binary Coded Decimal).Return To IndexThere is no way to add without carry.
AND (bitwise AND with accumulator)
Affects Flags: S Z
MODE SYNTAX HEX LEN TIM Immediate AND #$44 $29 2 2 Zero Page AND $44 $25 2 2 Zero Page,X AND $44,X $35 2 3 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+ + add 1 cycle if page boundary crossedReturn To Index
Affects Flags: S 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 $2E 3 7
ASL shifts all bits left one position. 0 is shifted into bit 0 and the original bit 7 is shifted into the Carry.
Affects Flags: N V Z
MODE SYNTAX HEX LEN TIM Zero Page BIT $44 $24 2 3 Absolute BIT $4400 $2C 3 4
BIT sets the Z flag as though the value in the address tested were ANDed with the accumulator. The S and V flags are set to match bits 7 and 6 respectively in the value stored at the tested address.BIT is often used to skip one or two following bytes as in:
CLOSE1 LDX #$10 If entered here, we
.BYTE $2C effectively perform
CLOSE2 LDX #$20 a BIT test on $20A2,
.BYTE $2C another one on $30A2,
CLOSE3 LDX #$30 and end up with the X
CLOSEX LDA #12 register still at $10
STA ICCOM,X upon arrival here.
Return To Index
Affect Flags: noneAll branches are relative mode and have a length of two bytes. Syntax is "Bxx Displacement" or (better) "Bxx Label". See the notes on the Program Counter for more on displacements.
Branches are dependant on the status of the flag bits when the op code is encountered. A branch not taken requires two machine cycles. Add one if the branch is taken and add one more if the branch crosses a page boundary.
MNEMONIC HEX BPL (Branch on PLus) $10 BMI (Branch on MInus) $30 BVC (Branch on oVerflow Clear) $50 BVS (Branch on oVerflow Set) $70 BCC (Branch on Carry Clear) $90 BCS (Branch on Carry Set) $B0 BNE (Branch on Not Equal) $D0 BEQ (Branch on EQual) $F0
There is no BRA (BRanch Always) instruction but it can be easily emulated by branching on the basis of a known condition. One of the best flags to use for this purpose is the oVerflow which is unchanged by all but addition and subtraction operations.
Affects Flags: B
MODE SYNTAX HEX LEN TIM Implied BRK $00 1 7
BRK causes a non-maskable interrupt and increments the program counter by two. Therefore an RTI will go to the address of the BRK +3 so that BRK may be used to replace a three- byte instruction for debugging and the subsequent RTI will be correct. If BRK is used in place of a one- or two-byte op code, the controlling software will have to adjust the RTI appropriately.Return To Index
Affects Flags: S 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+ + add 1 cycle if page boundary crossed
Compare sets flags as if a subtraction had been carried out. If the value in the accumulator is equal or greater than the compared value, the Carry will be set. The equal (Z) and sign (S) flags will be set based on equality or lack thereof and the sign (i.e. A>=$80) of the accumulator.Return To Index
Affects Flags: S 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
Operation and flag results are identical to equivalent mode accumulator CMP ops.Return To Index
Affects Flags: S 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
Operation and flag results are identical to equivalent mode accumulator CMP ops.Return To Index
Affects Flags: S 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 7Return To Index
Affects Flags: S 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+ + add 1 cycle if page boundary crossedReturn To Index
Flag (Processor Status) Instructions
Affect Flags: as noted
These instructions are implied mode, have a length of one byte and require two machine cycles.
MNEMONIC HEX CLC (CLear Carry) $18 SEC (SEt Carry) $38 CLI (CLear Interrupt) $58 SEI (SEt Interrupt) $78 CLV (CLear oVerflow) $B8 CLD (CLear Decimal) $D8 SED (SEt Decimal) $F8
Notes:The Interrupt flag is used to prevent (SEI) or enable (CLI) maskable interrupts (aka IRQ's). It does not signal the presence or absence of an interrupt condition. The 6502 will set this flag automatically in response to an interrupt and restore it to its prior status on completion of the interrupt service routine. If you want your interrupt service routine to permit other maskable interrupts, you must clear the I flag in your code.
The Decimal flag controls how the 6502 adds and subtracts. If set, arithmetic is carried out in packed binary coded decimal. This flag is unchanged by interrupts and is unknown on power-up. The implication is that a CLD should be included in boot or interrupt coding.
The Overflow flag is generally misunderstood and therefore under- utilised. Following addition or subtraction, the overflow will equal the EOR (exclusive or) of the Carry and Sign flags. This flag is not affected by increments, decrements, shifts and logical operations i.e. only ADC, SBC, CLV, PLP and RTI affect it. There is no op code to set the overflow but a BIT test on an RTS instruction will do the trick.
Affects Flags: S 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
Affects Flags: none
MODE SYNTAX HEX LEN TIM Absolute JMP $5597 $4C 3 3 Indirect JMP ($5597) $6C 3 5
JMP transfers program execution to the following address (absolute) or to the location contained in the following address (indirect). Note that there is no carry associated with the indirect jump so:
AN INDIRECT JUMP MUST NEVER USE A
VECTOR BEGINNING ON THE LAST BYTE
OF A PAGE
For example if address $3000 contains $40, $30FF contains $80, and $3100 contains $50, the result of JMP ($30FF) will be a transfer of control to $4080 rather than $5080 as you intended i.e. the 6502 took the low byte of the address from $30FF and the high byte from $3000.
Affects Flags: none
MODE SYNTAX HEX LEN TIM Absolute JSR $5597 $20 3 6
JSR pushes the address-1 of the next operation on to the stack before transferring program control to the following address. Subroutines are normally terminated by a RTS op code.
Affects Flags: S 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+ + add 1 cycle if page boundary crossed
Affects Flags: S 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+ + add 1 cycle if page boundary crossed
Affects Flags: S 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+ + add 1 cycle if page boundary crossed
Affects Flags: S 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
LSR shifts all bits right one position. 0 is shifted into bit 7 and the original bit 0 is shifted into the Carry.
Use caution with indexed zero page operations as they are subject to wrap-around. For example, if the X register holds $FF and you execute LDA $80,X you will not access $017F as you might expect; instead you access $7F i.e. $80-1. This characteristic can be used to advantage but make sure your code is well commented.In cases where you are writing code that will be relocated you must consider wrap-around when assigning dummy values for addresses that will be adjusted. Both zero and the semi-standard $FFFF should be avoided for dummy labels. The use of zero or zero page values will result in assembled code with zero page opcodes when you wanted absolute codes. With $FFFF, the problem is in addresses+1 as you wrap around to page 0.