Load & Store

• LDA — Load into A
  LDA #$05 (immediate)
  LDA $D020 (absolute)
• LDX / LDY — Load into X/Y
• STA / STX / STY — Store to memory

Arithmetic

• ADC — Add with carry
  CLC then ADC #$10
• SBC — Subtract with borrow
  SEC then SBC #$01
• INC / DEC — Increment/decrement memory
• INX / DEX / INY / DEY — Inc/dec registers

Logic & Shifts

• AND / ORA / EOR — Bitwise AND/OR/XOR
• ASL / LSR — Shift left/right
• ROL / ROR — Rotate through carry

Compare & Branch

• CMP / CPX / CPY — Compare (sets flags)
• BEQ / BNE — Branch if equal/not equal
• BCC / BCS — Branch if carry clear/set
• BMI / BPL — Branch if minus/plus

Subroutines & Stack

• JSR — Jump to subroutine
  Pushes return address (PC+2) to stack, then jumps
• RTS — Return from subroutine
  Pops address from stack, jumps to address+1
• PHA / PLA — Push/pull A to stack
• PHP / PLP — Push/pull flags to stack

Stack lives at $0100-$01FF, grows downward. SP starts at $FF.

Common Patterns

; Loop 10 times
    LDX #$0A
loop:
    DEX
    BNE loop
    RTS

Interrupts

Interrupts pause the CPU to run a handler routine, then resume — used for timing, keyboard, and raster effects.

• SEI — Disable interrupts (set I flag)
• CLI — Enable interrupts (clear I flag)
• RTI — Return from interrupt

IRQ vector at $0314-$0315 (KERNAL), hardware at $FFFE.

CIA Timer (IRQ)

• $DC04/$DC05 — Timer A latch (lo/hi)
• $DC0D — Interrupt control
  Write #$81 to enable Timer A IRQ
  Write #$7F to disable all IRQs
• $DC0E — Control register A
  Bit 0: Start timer, Bit 3: One-shot mode

VIC-II Raster IRQ

• $D012 — Raster line (bits 0-7)
• $D011 — Bit 7 = raster line bit 8
• $D019 — IRQ status (write #$01 to ack)
• $D01A — IRQ enable (bit 0 = raster)

Raster IRQ Setup

    SEI
    LDA #$7F
    STA $DC0D     ; Disable CIA IRQ
    LDA #$32
    STA $D012     ; Raster line 50
    LDA #$01
    STA $D01A     ; Enable raster IRQ
    LDA #<handler
    STA $0314
    LDA #>handler
    STA $0315
    CLI