[08] 6502 ASSEMBLY

The MOS 6502 Register Set

Unlike modern complex CPU architectures, the 6502 relies on an ultra-minimal set of registers:

• A (Accumulator): 8-bit primary register for arithmetic and logic.
• X & Y: 8-bit index registers for loops and indexed memory addressing.
• SP (Stack Pointer): 8-bit pointer locked to Page 1 ($0100–$01FF).
• PC (Program Counter): 16-bit register addressing the 64 KB memory space.
• Status Flags (P): Carry, Zero, Interrupt Disable, Decimal, Break, Overflow, Negative.

Zero-Page Optimization ($00–$FF): The first 256 bytes in RAM require only an 8-bit address operand, executing a full clock cycle faster per instruction.

Classic Rasterbar IRQ Routine

Changing the screen border color synchronized to the electron beam of the CRT monitor:

* = $c000          ; Origin in RAM (SYS 49152)

sei                ; Disable maskable interrupts
lda #$7f
sta $dc0d          ; Disable CIA1 timer interrupts

lda #$01
sta $d01a          ; Enable VIC-II raster interrupt signals

lda #100           ; Target rasterline (Line 100)
sta $d012
lda $d011
and #$7f           ; Clear 8th bit of rasterline
sta $d011

lda #<irq_routine  ; Low-byte of handler
sta $0314          ; Hook into KERNAL IRQ vector
lda #>irq_routine  ; High-byte of handler
sta $0315

cli                ; Re-enable interrupts
rts                ; Return to BASIC

irq_routine:
inc $d020          ; Flash border color register (Rasterbar effect)
dec $d020          ; Restore border color

asl $d019          ; Acknowledge VIC-II interrupt
jmp $ea31          ; Exit via default KERNAL IRQ handler