Sorry it has been so long since my last update. I’ve had projects that took higher priority than my emulation development. I still have very little free time to work on emulation but I will make more of an effort to make time for it and this site.

I will start looking back into gameboy sound emulation and will write a tutorial on it when I finish.

Also I wanted to say thanks to those of you who have left positive comments, it is appreciated. Ive been searching the internet and  found a few people using my tutorials for their own emulators which is great to see.

Thanks

CS

CS.

On a slightly annoying note my trunk svn gameboy dev seems to have a few bugs in the vdp and cpu that have appeared out of nowhere. I might look into redeveloping my Gameboy emulator when I have finished the sound as the knowledge I have gained from developing the SMS I feel I could take it to the next level. Still I’m getting impatient to start my mega drive/genesis emulator…

CS

I feel more motivated lately to continue wih my emulation work after a few months break. I’ll more than likely optimize my gameboy emulator and add sound to it next.

CS

Whilst writing these tutorials I have had one eye on the sega mega drive/genesis hardware notes so hopefully soon I can start emulating this system. This system looks very interesting and should be a good step up in difficulty.

CS

The SMS has been a really fun project and I’ve learnt a lot more about emulation programming from it, but have also realised how much left there is to learn. Hopefully my next emulator (Sega Mega Drive/Genesis) will have more advance emulation techniques.

Keep watching this space for the release of my emulator along with source code and the “how-to” tutorials.

CS

I have found a good article on the SMS sound chip SN79489 written by Maxim which explains how it works, and I have been programming in the dark trying to implement this into my emu. I believe my sound emulator is now 100% complete with the tiny problem that it doesnt seem to be synchronised with the cpu so the frequency of the music is incorrect. Apart from this it seems to be working fine.

Now im on my christmas holidays I should have enough time to get my SMS emulator finished by the end of the week which will be a great achievement. As soon as it’s finished I’ll write the tutorials for it and then go back to my gameboy emulator to tweak it and emulate sound before moving on to the sega mega drive, which im really looking forward to.

Merry Christmas all

CS.

I finally managed to get mode 2 sprite emulation working which was down to a really stupid error on my part.

Finally I’ve been debugging why my code masters larger res screen doesnt work and why the graphics are screwy in mortal kombat.

Luckily for me they were both the same error. The error was with my 8 and 16 bit opcodes for adc and sbc. Now this is how I emulated 8 bit adc (for when adding register c to register a):

BYTE before = m_RegisterAF.hi ;
BYTE toAdd = m_RegisterBC.lo ;

if (isCarrySet)
toAdd++ ;

m_RegisterAF.hi += toAdd ;

if ((before + toAdd) > 0xFF)
SetCarryFlag() ;
else
ResetCarryFlag() ;

// code to work out the other flags

Can you spot whats wrong with the above code?

Solution:
At first glance this looks fine however think what would happen if register C was 0xFF? When the carry gets added the toAdd variable wraps back round to 0. So register a stays set to 0xFF which is correct, however the setting of the c flag never gets set. This was causing the following instruction in mortal kombat (jump if carry is set) to fail.

That took me ages to find!

However both mortal kombat and micro machines are playable. My main concern now is why half of the games I play suffer from graphic flicker…. Better get my debugger back out.

Fortunately this is the last of the big bugs that need debugging and then it’s on to sound emulation (oh joy). However the end feels just round the corner.

CS

I managed to fix a bug in my emulator without zexalls help which was causing a few games to display the background incorrectly. Mainly Lemmings and Land of Illusion. This bug has been haunting me for awhile so im quite relieved to have fixed it.

I have recently found out that some games run at the wrong speed with my emulator so after checking all my opcodes to see if they are emulating their cycles correctly I was left with the conclusion it must be an error with my vsync emultion. Sure enough I wasnt moving on to the next scanline quick enough which was causing too few vsync interrupts to occur which is one of the main way speed is controlled in games. This is now fixed (after a day of headache) and all games play at the correct speed ( or should I say all tested games? )

Over the past few days I have also emulated the none maskable reset interrupt, the different screen resolution controlled by the current vdp mode and the codemasters mapper. Although the codemasters mapper is implemented correctly the majority of the games show issues with background rendering. However I believe this to be a problem with the background rendering routine rather than the mapper iteself because there is not much room for error with the mapper (compared to the sega mapper anyway).

CS

Easy:

• All control pad buttons including player 2 (Implemented)
  
• Correct Game Speed (Fixed)
• Reset Interrupt (Fixed)
• CodeMasters Mapper (Fixed)

Medium:

• Zexall test completion (Fixed)
  
• Speed optimizations (Implemented)
  
• Render background correctly for larger screen res (Fixed)
  
• Render mode 2 sprites correctly (Fixed)
  
• Change screen res for codemaster games (Fixed)

Difficult:

• Sound (Implemented)
  
• Fix graphical bugs found in Land of Illusion, Lemmings (Fixed)
• Fix bug Street Fighter (Fixed)
• Find out why some roms dont show first screen (Fixed)
• Fix bug in mortal kombat (Fixed)
• Fix black graphic flicker in some games (Fixed)
  

Whenever I fix one of these items I’ll edit this post and label them as fixed.

Doesn’t look too bad does it?

CS