6502 STUDIO

; ═══════════════════════════════════════════
; Fibonacci Sequence Generator
; Computes Fibonacci numbers in zero page
; For the MOS 6502 processor
; ═══════════════════════════════════════════

.org $0600          ; Program start address

; Zero page variables
.define fib_lo  $00 ; Current fib number (low byte)
.define fib_hi  $01 ; Current fib number (high byte)
.define prev_lo $02 ; Previous fib number (low byte)
.define prev_hi $03 ; Previous fib number (high byte)
.define temp_lo $04 ; Temp storage
.define temp_hi $05
.define count   $06 ; Loop counter

start:
    CLD             ; Clear decimal mode
    LDA #$01        ; Initialize first Fibonacci = 1
    STA fib_lo
    LDA #$00
    STA fib_hi
    STA prev_lo     ; Previous = 0
    STA prev_hi
    LDA #$18        ; Compute 24 Fibonacci numbers
    STA count

loop:
    ; Save current fib to temp
    LDA fib_lo
    STA temp_lo
    LDA fib_hi
    STA temp_hi

    ; fib = fib + prev
    CLC
    LDA fib_lo
    ADC prev_lo
    STA fib_lo
    LDA fib_hi
    ADC prev_hi
    STA fib_hi
    BCS overflow    ; Branch if > 65535

    ; prev = temp (old fib)
    LDA temp_lo
    STA prev_lo
    LDA temp_hi
    STA prev_hi

    ; Decrement counter
    DEC count
    BNE loop        ; Continue if count != 0

done:
    BRK             ; Stop execution

overflow:
    ; Handle overflow - just stop
    BRK