Tetris Title Screen

Gameboy Emulator Progress Update:

Copyright Screen

Getting to the copyright screen was straight forward for the most part. There was however the problem on the tileset being drawn in signed mode. This was resolve later. As a temporary fix I forced drawing in unsigned mode.

Getting past the Copyright Screen

After getting to the Copyright screen nothing happened. I knew at this point it should move into the menu screen. So clearly there’s a problem!

After some debugging I identified, what I believe is the Tetris “Main Loop”.

; --- MAIN LOOP ??? ---
0343: call 29FA     ; sets FF in FF80 location -- potentially need to do something with joypad registers?
0346: call 0377
0349: call 7FF0     ; this path did not set FF80 location
034C: ldh A,(80)
034E: and 0F
0350: cp 0F
0352: jp Z,029A    
0355: ld HL,FFA6
0358: ld B,02
035A: ld A,(HL)
035B: and A
035C: jr Z,01
035E: dec (HL)
035F: inc L
0360: dec B
0361: jr NZ,F7
0363: ldh A,(C5)
0365: and A
0366: jr Z,04
0368: ld A,09
036A: ldh (FF),A  ; Enable Serial and VBlank Interrupts
036C: ldh A,(85)  ; load some value from ram
036E: and A       ; and with self
036F: jr Z,FB     ; go back if 0. Basically loop until non-zero value.
0371: xor A       ; clear A
0372: ldh (85),A  ; rest flag
0374: jp 0343    ; --- MAIN LOOP ??? ---

The program was looping at this section:

036C: ldh A,(85)  ; load some value from ram
036E: and A       ; and with self
036F: jr Z,FB     ; go back if 0. Basically loop until non-zero value.

Clearly a flag needs to be set and can only happen in a interrupt!

The only active interrupts were Serial and VBlank. Since it most likely wasn’t the serial interrupt setting the flag I started debugging my VBlank interrupt handler.

After fixing interrupt I still wasn’t at the menu screen. But some progress as the program moved pasted the loop.

The copyright screen started to blink:

To make a long story short: The joypad register was giving the wrong values.

The Joypad inputs are pulled high by a resistor. So there default value should be all ones.

The Tetris game was seeing all the inputs held down, which I believe is meant to cause a soft reset of the program.

Look here in the main loop:

034C: ldh A,(80)
034E: and 0F
0350: cp 0F
0352: jp Z,029A

If the flag located at $FF80 is not set correctly a jump to $029A occurs.

Here’s a snippet of what happens at this location:

029A: ld A,01        ; -----------------------------
029C: di             ; Disable interrupts
029D: ldh (0F),A     ; Enables VBlank
029F: ldh (FF),A     ; Enable VBlank
02A1: xor A         
02A2: ldh (42),A
02A4: ldh (43),A
02A6: ldh (A4),A
02A8: ldh (41),A
02AA: ldh (01),A
02AC: ldh (02),A
02AE: ld A,80
02B0: ldh (40),A     ; Enable LCDC
02B2: ldh A,(44)
02B4: cp 94          ; Wait for LY register to be 94
02B6: jr NZ,FA
02B8: ld A,03        ; LCDC config
02BA: ldh (40),A
02BC: ld A,E4
02BE: ldh (47),A
02C0: ldh (48),A
02C2: ld A,C4
02C4: ldh (49),A
02C6: ld HL,FF26
02C9: ld A,80
02CB: ld (HL-),A
02CC: ld A,FF
02CE: ld (HL-),A
02CF: ld (HL),77
02D1: ld A,01
02D3: ld (2000),A   ; ROM bank switch
02D6: ld SP,CFFF    ; set stack pointer

A lot of register initialization and setup code. Note specifically the last two line. A ROM bank switch and setting the stack pointer.

Anyway thought that was interesting..

As I haven’t implemented the Joypad interface yet. I ensure the first 4 bits of the Joypad register are always equal to 0xF.

With this the soft reset no longer occurred and the program proceeded to the Title screen!

Drawing Sprites

Currently I am trying to draw sprites to the screen. Current progress:

Issue with aligning the sprites correctly.