GameBoy Hello World

hello_world.ds

; See https://eldred.fr/gb-asm-tutorial/hello-world.html
;     https://github.com/ISSOtm/gb-asm-tutorial

INCLUDE "hardware.inc"

SECTION "Header", ROM0[$100]

EntryPoint: ; Where the code begins. 
    di ; Disable interrupts.
    jp Start 

; Fill the header.
REPT $150 - $104
    db 0
ENDR

; END SECTION Header.

SECTION "Game code", ROM0

Start: ; Main program.
    ; Turn off LCD.
.waitVBlank
    ld a, [rLY] ; Really ?
    cp 144 ; The screen is 144 px tall. After the LCD is past VBlank.
    jr c, .waitVBlank

    xor a ; The same as ld a, 0 ; We actually need to only reset the 7th bit.
    ld [rLCDC], a

    ; Copy the font to VRAM.
    ld hl, $9000
    ld de, FontTiles
    ld bc, FontTilesEnd - FontTiles
.copyFont ; For loop FontTiles <= de <= FontilesEnd.
    ld a, [de] ; Grab the 1st byte.
    ld [hli], a ; Place it. Actual copy.
    inc de ; Increment (will copy the next tile in the next iteration).
    dec bc ; Decrement count.
    ld a, b ; Check if count is 0 because dec does not update flags.
    or c ; If bc == 0 then Z will be set.
    jr nz, .copyFont

    ; Copy the string we want to display.
    ld hl, $9800 ; Print it at the top left.
    ld de, HelloWorldStr
.copyString
    ld a, [de]
    ld [hli], a
    inc de
    and a ; Check if the char is \0.
    jr nz, .copyString

    ; To turn the screen on, we need to set bit 7 of LCDC

    ; Initialize display registers.
    ld a, %11100100 ; Palette loading.
    ld [rBGP], a

    xor a ; ld a, 0
    ld [rSCY], A
    ld [rSCX], A

    ; Disable sound.
    ld [rNR52], a

    ; Turn the screen on.
    ld a, %10000001
    ld [rLCDC], a

    ; Trap it in an infinite loop.
.lockup
    jr .lockup

; END SECTION Game code.

SECTION "Font", ROM0

FontTiles:
INCBIN "font.chr"
FontTilesEnd:

SECTION "Hello World string", ROM0

HelloWorldStr:
    db "Hello Prout!", 0

; END SECTION Font

Makefile

GB_NAME := hello_world

all: $(GB_NAME).gb

$(GB_NAME).gb: $(GB_NAME).o
	rgblink -o $@ $<
	rgbfix -v -p 0 $@

$(GB_NAME).o: $(GB_NAME).ds
	rgbasm -o $@ $<

clean:
	rm -f *.o *.gb