#include "ti83plus.inc"

    .org $9d93
Start:
    ; Clear the screen
    bcall(_ClrLCDFull)

    ; Install interrupt for the counter
    di              ; Disable interrupts while installing

    ld a, $9a       ; Fill $9900 to $9A00 with $9a
    ld ($9900), a   ; ^
    ld hl, $9900    ; ^
    ld de, $9901    ; ^
    ld bc, 256      ; ^
    ldir            ; ^

    ld a, $99       ; Set the interrupt HSB to $99
    ld i, a         ; ^
    ld hl,9a9ah
    ld (hl),Interrupt
    im 2            ; Set interrupt mode 2
    ei              ; Re-enable interrupts

    ; do stuff

    im 1

    ret

d_tick:
    .dw $00
Interrupt:
    di              ; Disable interrupts to prevent interruptception

    ex af, af'      ; Swap all registers with their shadow registers
    exx             ;   so we don't mess up data

    ld hl, (d_tick)
    inc hl
    ld (d_tick), hl

    ex af, af'      ; Swap back the registers
    exx             ; ^