Gameboy Hello World

main.asm

INCLUDE "hardware.inc"

; Beginning of the rom
SECTION "Header", ROM0[$100]

di ; Disable interrupts since we don't want to be interrupted
jp Start ; Jump to the Start label
nop ; No nothing for 1 cycle (approx: 1MgHz)
ds $150 - $104 ; Write nothing for $150 to $104 which is the rom header information

; This is the start of the program
SECTION "Program", ROM0

Start:
.waitVBlank
    ; We need to wait for the VBlank state to be over
    ; During a VBlank, we can't access anything in the RAM
    ; The VBlank duration is from 144 - 153 then flips back to 0
    ld a, [rLY] ; Get the address value of the LCDC (LCD Controller)
    cp 144 ; Is the value 144?
    jr c, .waitVBlank ; Keep checking if it's not 0

    ; We also can't draw tiles to the screen while it's on
    ; So now we turn off the LCD screen
    xor a ; Use a logical OR with register a against itself and assign the value back into a
          ; When using a XOR (Exclusive OR), anything XOR'd with itself will be 0
          ; Effectively changing whatever the value of a was to 0
    ld [rLCDC], a ; Set the LCDC to a, which is 0

    ; What we need to do now is to get the string data that we'll be printing out to the display
    ; We set this up before going into the next label, .copyFont
    ld hl, $9000 ; Assign the literal value of $9000 to the hl register
                 ; $9000 is the address of the first section of the third part of the VRAM tiles
                 ; This section is empty when you boot the rom
    ld de, FontTiles ; Get the data from the FontTiles label into de
    ld bc, FontTilesEnd - FontTiles ; Get the count of all of the tiles
                                    ; This will act as the loop counter for .copyFont
.copyFont
    ; Now we need to get the font out of the font file hand into memory
    ld a, [de] ; Get the address value of the data in the de register
    ld [hli], a ; Insert the byte into the hl register and increase it's value by 1
    inc de ; Increase the value of the de register by one
           ; We have to increase this manually since the hl register is the only
           ; register that can use the hl(i) shorthand syntax
    dec bc ; Decrease the value of the bc register
    ld a, b ; Put the value of the b register into the a register
    or c   ; Assign the value of `or c` to register a
    jr nz, .copyFont ; If the value in the a register is not zero, go back to .copyFont

    ; Now that we have the data from FontTiles into VRAM, we need to now get it onto the display
    ld hl, $9800 ; This will print the data of the hl register in to the
                 ; memory address of $9800 which is the top left of the Gameboy's display
    ld de, HelloWorldStr ; This puts the data of the HelloWorldStr label into the de register
.copyString
    ld a, [de] ; Put the address value of the de register into the a register
    ld [hli], a ; Insert the byte into the hl register and increase it's value by 1
    inc de ; Increase the value of the de register by one
    and a ; Check to see if the byte we copied to a is a 0
    jr nz, .copyString ; Do the instruction set inside .copyString if a is not zero

    ; Whew, now that we have the data onto the screen, now we have to turn it back on!
    ; Remember that we had to turn off the screen to write anything to it.
    ; Reference: http://gbdev.gg8.se/wiki/articles/Video_Display#FF47_-_BGP_-_BG_Palette_Data_.28R.2FW.29_-_Non_CGB_Mode_Only
    ld a, %11100100 ; This will set the binary value for background pallete color
                    ; It doesn't matter how we set this i.e. it could be hex $E4
                    ; I chose binary in this case because it makes more sense visually
                    ; when looking at the above reference
    ld [rBGP], a ; set the BGP (Background Pallete) to the color we selected

    ; We will turn the sound off since we don't have sound in this program
    xor a ; A value XOR'd by itself is 0 - you could write `ld a, 0` as well.
    ld [rNR52], a ; We set the address value of register NR52 to a, which is 0
                  ; Register NR52 is the register that determines if the sound is on or off
                  ; 1 = on, 0 = off
    
    ; We also don't want to have any scrolling in this program so we will turn that off too
    ld [rSCX], a ; Turn off horizontal scrolling by setting the address value to 0
    ld [rSCY], a ; Turn off vertical scrolling by setting the address value to 0

    ; Now we can turn the screen back on!
    ; Reference: http://gbdev.gg8.se/wiki/articles/Video_Display#LCD_Control_Register
    ld a, %10000001 ; Set the value of register a to a binary value
                    ; Reminder that this could be hex: $81
    ld [rLCDC], a ; Put the value in register a into the address value of the LCDC register

; Now we are done with all of our instructions! But to stop the CPU from processing all the
; remaining memory addresses, we will lock the program here infinitely
.lockup
    jr .lockup


SECTION "Font", ROM0

FontTiles:
INCBIN "font.chr" ; INCBIN will take the bytes of the file and store it in the label, in this case
FontTilesEnd:

SECTION "Hello World String", ROM0

HelloWorldStr:
    db "Hello World!", 0 ; We add the 0 so that we know that the string is over