hello-world.gb

INCLUDE "constants.asm"

SECTION "rst 00", ROM0 [$00]
ClearVRAM:
.clearVRAMLoop
    ld [hld], a
    bit 7, h
    jr nz, .clearVRAMLoop
    ret
SECTION "rst 08", ROM0 [$08]
HandleCommand:
    and b
    jr z, .done
    cp b
    ret z
    add l
    ld l, a
    ld [hl], b
    ret
.done
    xor a
    ld [$98a0], a
    ld hl,rLCDC
    ld [hl], %11010011
    jr @

SECTION "rst 20", ROM0 [$20]
    push de
    ld de, $20
    add hl, de
    pop de
    ret

SECTION "Entry", ROM0 [$100]
    db $18,$4e ; jr Start0150

SECTION "Header", ROM0 [$104]

    ; The header is generated by rgbfix.
    ; The space here is allocated to prevent code from being overwritten.

    ds $150 - $104

Start0150:
    di

    ld sp, $fffe
    ; palettes
    xor a
    ld [rSCX], a
    ld [rSCY], a
    ld a, $e4
    ld [rBGP], a

.waitForVBlank
    ld a, [rLY]
    cp $90
    jr nz, .waitForVBlank
    xor a
    ld c, a
    ld [rLCDC], a
    ld hl, $fe9f
.clearOAMLoop
    ld [hl], a
    dec l
    jr nz, .clearOAMLoop

    ld hl, $9fff
    rst $00
    ld hl, $800f
    dec a
    rst $00

    ld hl, $9800
    ld de, HelloWorldOffsets
    ld b, $f
    ld [hl], b

    push hl
.loadBytemapIntoVRAMLoop
    ld a, [de]
    ld c, a
    swap a
    rst $08
    jr nz, .notNextLine
    pop hl
    rst $20
    push hl
    ld [hl], b
.notNextLine
    ld a, c
    rst $08
    inc de
    jr nz, .loadBytemapIntoVRAMLoop
    pop hl
    rst $20
    push hl
    ld [hl], b
    jr .loadBytemapIntoVRAMLoop

HelloWorldOffsets::
; commands:
; $01 - $0e = add offset to current pointer
; $00 = stop execution
; $0f = go to next line
    dn $02,$02, $01,$02, $03,$03, $01,$01
    dn $0f,$02, $02,$03, $03,$03, $02,$0f
    dn $01,$01, $02,$01, $02,$03, $03,$02
    dn $0f,$02, $02,$03, $03,$03, $02,$0f
    dn $02,$02, $01,$02, $01,$02, $01,$02, $01,$01
    dn $0f,$0f
    dn $02,$02, $02,$01, $01,$02, $01,$03, $03,$01
    dn $0f,$02, $02,$02, $02,$02, $02,$02, $03,$02
    dn $0f,$02, $02,$02, $02,$02, $01,$03, $03,$02
    dn $0f,$01, $01,$01, $03,$01, $01,$02, $02,$02, $01,$02, $01,$00