; This is the main loop.
; Of course it could be done without a buffer, but now we
; are building the scroll element the way it will look in the
; finished game....

mloop  halt                ; start main loop with vblank.

; Update vdp right when vblank begins!

       ld a,(scroll)       ; 1-byte scroll reg. buffer in ram.
       ld b,9              ; target VDP register 9 (v-scroll).
       call setreg         ; now vdp register = buffer, and the
                           ; screen scrolls accordingly.

; Blah blah ... in the game, lots of stuff goes on here....
; and then, towards the end...

; Scroll background - update the vertical scroll buffer.

       ld a,(scroll)       ; get scroll buffer value.
       sub vspeed          ; subtract vertical speed.
       ld (scroll),a       ; update scroll buffer.

       jp mloop            ; jump back for another round.

; --------------------------------------------------------------
; SET VDP REGISTER.
; Write to target register.
; A = byte to be loaded into vdp register.
; B = target register 0-10.

setreg out ($bf),a         ; output command word 1/2.
       ld a,$80
       or b
       out ($bf),a         ; output command word 2/2.
       ret