island-story/src/util.s

SECTION "Utilities", ROM0

; Busy-wait until vertical blank occurs
wait_for_vblank::
    ld a, [$ff41]
    and 3
    cp 1
    jr nz, wait_for_vblank
    ret

; Copy data between two regions (takes 40*N+12 cycles)
; @param bc Pointer to the destination region
; @param hl Pointer to the source region
; @param d  Size (in bytes) to copy. Must be >0
; @destroy a, b, c, d, h, l
memcpy::
    ld a, [hli] ; 8
    ld [bc], a ; 8
    inc bc ; 8
    dec d ; 4
    jr nz, memcpy ; 12/8
    ret ; 16

; Fill a memory region with a value
; @param hl Pointer to the destination region
; @param a  Byte to fill with
; @param c  Number of bytes to fill. Must be >0
; @destroy a, c, hl
memset::
    ld [hli], a
    dec c
    jr nz, memset
    ret

; Divide a value by 10 (http://homepage.divms.uiowa.edu/~jones/bcd/decimal.html#division)
; @param b Dividend
; @return a Quotient
; @destroy b
div10::
    ld a, b
    srl a
    srl a
    add b
    srl a
    add b
    srl a
    srl a
    srl a
    add b
    srl a
    add b
    srl a
    srl a
    srl a
    srl a
    ret