The instruction set
Grouped by what you are trying to do, rather than alphabetically, because that is how you look for one. Cycle counts are for the plain form of each instruction; add one for a page crossing on an indexed read, and one more for a taken branch.
Moving bytes about
LDA LDX LDY | Load a register. 2–5 cycles. |
STA STX STY | Store one. 3–6 cycles. |
STZ | Store zero without loading anything first. 65C02. |
TAX TAY TXA TYA | Copy between registers. 2 cycles. |
TSX TXS | The stack pointer, in and out of X. |
Arithmetic
ADC | Add with carry — CLC first. |
SBC | Subtract with borrow — SEC first. |
INC DEC | Add or subtract one, in memory or (65C02) in A. |
INX INY DEX DEY | The same for the index registers. |
CMP CPX CPY | Compare. Sets the flags, keeps the register. |
CMP is a subtraction that throws the answer away. After it: Z means equal, C means the register was greater or equal, and N is the sign of the difference — which is why bcs and bcc are the right branches after comparing unsigned numbers, and bmi/bpl are the wrong ones.
Bits
AND ORA EOR | The three you expect. |
ASL LSR | Shift left or right, dropping a bit into the carry. |
ROL ROR | Rotate through the carry — how you shift a 16-bit value. |
BIT | Test bits without changing anything: Z from A AND memory, and N and V straight from bits 7 and 6 of the memory byte. |
TRB TSB | Clear or set every bit in a mask, in one instruction. 65C02. |
BIT reads two flags for free
Bits 7 and 6 of the byte land in N and V without touching A. Hardware status registers are often laid out with the two most urgent flags in exactly those positions, and this is why.
Deciding and going
BEQ BNE | Zero flag set / clear |
BCS BCC | Carry set / clear |
BMI BPL | Negative flag set / clear |
BVS BVC | Overflow set / clear |
BRA | Always. 65C02. |
JMP | Anywhere — direct, indirect, or (65C02) through a table with (abs,X). |
JSR RTS | Call and return. |
The stack
PHA PLA | Push and pull A |
PHX PHY PLX PLY | The same for X and Y. 65C02. |
PHP PLP | The flags |
The stack is 256 bytes at $0100, and it wraps rather than overflows: push 257 things and you are back where you started, quietly writing over your own return addresses. In practice this only ever bites recursive code.
Flags, interrupts, and stopping
CLC SEC | Carry |
CLD SED | Decimal mode |
CLI SEI | Interrupts on / off |
CLV | Clear overflow |
BRK | Software interrupt — on this machine, a breakpoint that reports the registers |
RTI | Return from an interrupt |
NOP | Nothing, for two cycles |
WAI STP | Wait for an interrupt / stop until reset. W65C02S. |
BRK is two bytes, not one
The processor pushes the address of BRK plus two, so a one-byte BRK would return into the middle of whatever followed it. Assemblers know this; what it means for you is that a BRK used as a breakpoint should have a spare byte after it — and on this machine that byte is reported as the break number. BRK stops the program and prints where it stopped and every register, which makes it the cheapest debugging tool there is — see Reaching the machine.
Decimal mode
SED makes ADC and SBC work in packed decimal: $09 + $01 gives $10 rather than $0A, and each costs a cycle more than it does in binary. It is genuinely useful for a score you intend to print digit by digit, and genuinely dangerous if you forget to CLD afterwards, because every ADC and SBC you reach later is still doing arithmetic in the mode you left set.
An interrupt is the one thing that does not inherit it. The processor clears D as it takes the vector, so a handler always starts in binary — and the mode you were in was pushed first, so RTI gives it back. That is one of the 65C02's fixes: on the original 6502 a handler ran in whatever mode it interrupted, which is why 6502 handlers open with CLD. Here that CLD is redundant.
The Kernal clears it at power-on and never sets it. If you use it, bracket it tightly.
The full tables
For cycle counts down to the opcode, in every addressing mode:
- masswerk's instruction set — every opcode, every mode, every cycle count
- The W65C02S datasheet — including
WAI,STPand the Rockwell bit operations - Easy 6502 — if you want to be walked through it with an assembler in the page
Next: where everything lives.