Game Boy

[TJA]

Hi all,
Well after almost 8 mounths without touching my emu, I finally finished uni and have a job lined up that doesn't start till next month. Therefore I have plenty of free time at the moment. Anyway, I have a question. I'm ready to implement some graphics for my emu but I'm unsure when I should be updating the display, do I update it straight after writing to video memory, any help on this and inplementing gameboy graphics in general would be appreciated.

Cheers, TJA.

 

[ShizZy]

huarifaifa - sounds great.

TJA - every 114 machine cycles, or 456 clock cycles, you draw one of the 144 screen lines - if I recall correctly. After that there is a vblank period for 10 scanlines.

 

[huarifaifa]

I'm ready to implement some graphics for my emu but I'm unsure when I should be updating the display

As ShizZy said. Draw each line every 114 machine cycles on your offscreen bitmap, and then blit it on screen during the V-Blank period, ie. something like:

while LCD Enabled do

  for LY = 0 to 143 do
     // OAM
     LCD Mode = 2
     wait 20 cycles

     // Transfer
     LCD Mode = 3
     wait 43 cycles

     // draw line using the current LCD registers (LY, LCDC, STAT, SCX, etc.)
     drawLine(LY, LCDC, STAT, ...)

     // H-Blank
     LCD Mode = 0
     wait 51 cycles
  end for

  // show the offscreen bitmap on screen
  showFrame()

  // V-Blank
  LCD Mode = 1
  for LY = 144 to 153 do
     wait 114 cycles
  end for

end while

It's usually implemented as a state machine (unless your programming language supports continuations .

There's some "weirdness" in the line 153 not shown here, and some games (like Prehistorik Man, Parodius, etc.) that need a more accurate way to draw each line, but most games should run fine.

 

[TJA]

Thanks, huarifaifa and shizzy. I understand this now thanks to your posts and examining the tech docs I have. Hopfully I shall have this implemented by the end of tommorrow, or if not the end of the weekend ( looking a my code I found some other things I need to implement first ).
Anyway wish me luck hopefully I should have a screenshot to show off sometime next week.

Cheers, TJA.

 

[TJA]

Hey guys I’ve implemented the start of my graphics, but are now stumped.

Here is the code I’m currently working on:

void draw_win()
{
unsigned char *tile_map, *tile_data;

if( LCDC&0x40) // select Windows tile map display
tile_map = &RAM[0x9C00];
else
tile_map = &RAM[0x9800];

if ( WX > 166 ) // makes sure window is visable
return;

if ( LY >= WY ) // only draw if LCD y position up to or equal the window position
{
what do I do now???
}
}
//draw line using the current LCD registers (LY, LCDC, STAT, SCX, etc.)
void h_blank()
{
//if (LCDC&0x01) draw_back();
if (LCDC&0x20) draw_win();
//if (LCDC&0x02) draw_obj();
}

It seems the document I have do not have enough detail to implement this from them.
For instance how do I know which pattern in the tile data table to draw?

Any help would be appreciated.

Cheers, TJA.

 

[huarifaifa]

It seems the document I have do not have enough detail to implement this from them.
For instance how do I know which pattern in the tile data table to draw?
Cheers, TJA.

Check out the Pan Docs:

http://www.work.de/nocash/pandocs.htm#vramtiledata

It's (almost) all explained there.

The tile data for BG and WIN starts at address 0x8000 or 0x8800, ie:

  if(LCDC&0x10)
    tile_data = &RAM[0x8000];
  else
    tile_data = &RAM[0x8800];

The format of the 8x8 / 8x16 tiles is explained in the Pan Docs.

Carlos

 

[TJA]

Thanks huarifaifa, I already had this document but had forgot about it, this should help alot, hopefully I will have some graphics displaying by next week, little busy at the moment as I are moving to a different city in a couple of weeks to start my new job.

TJA.

 

[Helius]

Hi all!!

After writing a Chip8 emulator for J2ME devices (http://www.emutalk.net/showpost.php?p=336235&postcount=697) I decided to start a GameBoy emulator using C#.

I have spent a month on and off to code all CPU opcodes and now I have finally completed it. Still buggy opcodes but it can load some games like Dr Mario to the splash screen.

A lot of work to do now, interrupts, io ports, input... but things are getting more interesting

Keep up the good work guys.

 

[ShizZy]

Nice job dude

 

[aprentice]

ah the good ol' gameboy coding days

 

[huarifaifa]

I have finally released my GameBoy emulator at SourceForge:

http://sourceforge.net/projects/mario

There're binaries for PalmOS, Windows and Java.

 

[hap]

Looks and sounds good! But playing games on it was annoying:
- The window is SO small (add a window zoom option, eg. mario xxx.gb -4 quadruples the window size, or a fullscreen option even, but that's a bit harder to implement).
- Buttons are placed wrongly, B should be to the left of A, Select to the left of Start (Select/Start shouldn't on top of eachother).

 

[huarifaifa]

> Looks and sounds good! But playing games on it was annoying:

Thanks!

> The window is SO small [...]

The Java version has the window size doubled (320x288). I'll check out if there's an easy way to scale the window in the C++ version (which uses SDL).

> Buttons are placed wrongly, B should be to the left of A, [...]

Yes, I "naturally" defined that layout (I'm left-handed) without knowing the actual button placement in a real GameBoy. I will change that to make it "not wrong."

Carlos

 

[EdgEy]

After looking at this topic for a while I decided to start coding a GB emulator.. so far I just have most of the core done (interrupt related opcodes aren't done).. nothing else really. this screenshot is me just dumping areas of ram to the screen ..

 

[ShizZy]

Nice job

 

[EdgEy]

Thanks ShiZzY

Unknown bugs are stopping me from running a lot of games

Tetris... doesn't seem to write tiles to vram or anything, but i have gotten a few demos to work (Big scroller works exactly the same as in VBA, landscape demo works, Merry Christmas demo works with a few bugs - neither have sprites yet)

:bouncy:

 

[Danny]

wow this thread hasnt seen activity in a while. Now that my chip8 emu is almost done im looking to port a gameboy emu.

bcbrew i noted that you used sdl in your emulator...

i wondered if i could have your latest source so i can port it to the psp?

or if anyone else has a early working emulator that would be great too

cheers

 

[hap]

or write one from scratch

1: You get to know the Gameboy ins and outs better: much easier to fix imperfections and bugs.
2: It's a lot more fun!

 

[Danny]

or write one from scratch

1: You get to know the Gameboy ins and outs better: much easier to fix imperfections and bugs.
2: It's a lot more fun!

yeah not a bad idea hap

Do you think i would be able for the challenge of making a gameboy emu from scratch after only porting a chip8 emu?

if so il start

i assume writing the cpu core is the first thing to start with

 

[cloudy]

GB emu opcodes question

Hey all,

I just need confirmation about how the gameboy opcodes work. All opcodes are 1 byte in size and the immediate data can be between 0 and 2 bytes correct?

So when i read the gb game into a byte array, i take it i should read the opcode first and the read the next index in the array as the immediate value (assuming of course that the opcodes immediate data is 1 byte).

So for instance the opcode 06 loads 1 byte of immediate data into register B.

So if for example i hexedited the gb game and seen the instruction 06AA. I would then load the value AA into register B?

So for example is this correct (psudo code):

opcode = byteMemory[instructionPointer] ;

switch(opcode)
{
 case 06: 
   register[REG_B] = byteMemory[instructionPointer+1];
   instructionPointer += 2;  // is this correct? for a load opcode anyway     
}

Thanks for any help

Edit:

Whats the deal with opcode 7F?

You have to Load RegisterA into RegisterA... whats the point?