Title Screen

The title screen shows a basic title image using the background and draws text asking the player to press A. Once the user presses A, it will go to the story screen.

Our title screen has 3 pieces of data:

• The “Press A to play” text
• The title screen tile data
• The title screen tilemap

INCLUDE "src/main/utils/hardware.inc"
INCLUDE "src/main/utils/macros/text-macros.inc"

SECTION "TitleScreenState", ROM0

PressPlayText::  db "press a to play", 255
 
titleScreenTileData: INCBIN "src/generated/backgrounds/title-screen.2bpp"
titleScreenTileDataEnd:
 
titleScreenTileMap: INCBIN "src/generated/backgrounds/title-screen.tilemap"
titleScreenTileMapEnd:

Initiating the Title Screen

In our title screen’s “InitTitleScreen” function, we’ll do the following:

• draw the title screen graphic
• draw our “Press A to play”
• turn on the LCD.

Here is what our “InitTitleScreenState” function looks like

InitTitleScreenState::

	call DrawTitleScreen
	
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    ; Draw the press play text
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

	; Call Our function that draws text onto background/window tiles
    ld de, $99C3
    ld hl, PressPlayText
    call DrawTextTilesLoop

    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

	; Turn the LCD on
	ld a, LCDCF_ON  | LCDCF_BGON|LCDCF_OBJON | LCDCF_OBJ16
	ld [rLCDC], a

    ret

In order to draw text in our game, we’ve created a function called “DrawTextTilesLoop”. We’ll pass this function which tile to start on in de, and the address of our text in hl.

DrawTextTilesLoop::

    ; Check for the end of string character 255
    ld a, [hl]
    cp 255
    ret z

    ; Write the current character (in hl) to the address
    ; on the tilemap (in de)
    ld a, [hl]
    ld [de], a

    inc hl
    inc de

    ; move to the next character and next background tile
    jp DrawTextTilesLoop

The “DrawTitleScreen” function puts the tiles for our title screen graphic in VRAM, and draws its tilemap to the background:

NOTE: Because of the text font, we’ll add an offset of 52 to our tilemap tiles. We’ve created a function that adds the 52 offset, since we’ll need to do so more than once.

DrawTitleScreen::
	
	; Copy the tile data
	ld de, titleScreenTileData ; de contains the address where data will be copied from;
	ld hl, $9340 ; hl contains the address where data will be copied to;
	ld bc, titleScreenTileDataEnd - titleScreenTileData ; bc contains how many bytes we have to copy.
	call CopyDEintoMemoryAtHL
	
	; Copy the tilemap
	ld de, titleScreenTileMap
	ld hl, $9800
	ld bc, titleScreenTileMapEnd - titleScreenTileMap
	jp CopyDEintoMemoryAtHL_With52Offset

The “CopyDEintoMemoryAtHL” and “CopyDEintoMemoryAtHL_With52Offset” functions are defined in “src/main/utils/memory-utils.asm”:

SECTION "MemoryUtilsSection", ROM0

CopyDEintoMemoryAtHL::
	ld a, [de]
	ld [hli], a
	inc de
	dec bc
	ld a, b
	or c
	jp nz, CopyDEintoMemoryAtHL ; Jump to CopyTiles if the last operation had a non zero result.
	ret

CopyDEintoMemoryAtHL_With52Offset::
	ld a, [de]
    add a, 52 
	ld [hli], a
	inc de
	dec bc
	ld a, b
	or c
	jp nz, CopyDEintoMemoryAtHL_With52Offset ; Jump to COpyTiles, if the z flag is not set. (the last operation had a non zero result)
	ret

Updating the Title Screen

The title screen’s update logic is the simplest of the 3. All we are going to do is wait until the A button is pressed. Afterwards, we’ll go to the story screen game state.

UpdateTitleScreenState::

    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    ; Wait for A
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

    ; Save the passed value into the variable: mWaitKey
    ; The WaitForKeyFunction always checks against this vriable
    ld a, PADF_A
    ld [mWaitKey], a

    call WaitForKeyFunction

    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

    ld a, 1
    ld [wGameState],a
    jp NextGameState

Our “WaitForKeyFunction” is defined in “src/main/utils/input-utils.asm”. We’ll poll for input and infinitely loop until the specified button is pressed down.

SECTION "InputUtilsVariables", WRAM0

mWaitKey:: db

SECTION "InputUtils", ROM0

WaitForKeyFunction::

    ; Save our original value
    push bc

	
WaitForKeyFunction_Loop:

	; save the keys last frame
	ld a, [wCurKeys]
	ld [wLastKeys], a
    
	; This is in input.asm
	; It's straight from: https://gbdev.io/gb-asm-tutorial/part2/input.html
	; In their words (paraphrased): reading player input for gameboy is NOT a trivial task
	; So it's best to use some tested code
    call Input

    
	ld a, [mWaitKey]
    ld b, a
	ld a, [wCurKeys]
    and b
    jp z, WaitForKeyFunction_NotPressed
    
	ld a, [wLastKeys]
    and b
    jp nz, WaitForKeyFunction_NotPressed

	; restore our original value
	pop bc

    ret


WaitForKeyFunction_NotPressed:

    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
    ; Wait a small amount of time
    ; Save our count in this variable
    ld a, 1
    ld [wVBlankCount], a

    ; Call our function that performs the code
    call WaitForVBlankFunction
    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

    jp WaitForKeyFunction_Loop

That’s it for our title screen. Next up is our story screen.