Huge Insect does not fully start

[zeroone]

As shown below, my emulator makes it to the Stage 1 - 1 screen, the player can control the sprayer (left, right and fire works) and the music and sound effects work. But, the Stage 1 - 1 message never vanishes and no enemies ever appear.



I carefully implemented the PPU sprite evaluation incorporating the ideas from these links: Obscure details of OAMADDR and Where to start DMA. But, that was not enough.

On even cycles (65--256), my code does this:

Code: Select all

            int address = (n + oamAddress) << 2;
            spriteY = primaryOAM[0xFF & (address + m)];
            spriteT = primaryOAM[0xFF & (address + m + 1)];
            spriteA = primaryOAM[0xFF & (address + m + 2)];
            spriteX = primaryOAM[0xFF & (address + m + 3)];

On odd cycles, it does this:

Code: Select all

            if (secondaryOamWritesEnabled) {
              int address = s << 2;
              secondaryOAM[address] = spriteY;
              if (!(spriteY > scanline 
                  || spriteY + spriteBottomOffset < scanline)) {
                if (n == 0) {
                  sprite0Used = true;
                }                
                secondaryOAM[address + 1] = spriteT;
                secondaryOAM[address + 2] = spriteA;
                secondaryOAM[address + 3] = spriteX;
                if (++s == 8) {
                  secondaryOamWritesEnabled = false;                  
                }
              }
              n = 0x3F & (n + 1);
              if (n == 0) {
                oamTransferCompleted = true;
              }
            } else {
              if (!(spriteY > scanline 
                  || spriteY + spriteBottomOffset < scanline)) {
                spriteOverflowDetected = true;
                for(int k = 0; k < 4; k++) {
                  m = 0x03 & (m + 1);
                  if (m == 0) {
                    n = 0x3F & (n + 1);
                    if (n == 0) {
                      oamTransferCompleted = true;
                      break;
                    }
                  }
                }
              } else {
                m = 0x03 & (m + 1);
                n = 0x3F & (n + 1);
                if (n == 0) {
                  oamTransferCompleted = true;
                }
              }
            }
          }

Those pieces of code execute while oamTransferCompleted is false. n, m and s are initialized to 0. secondaryOamWritesEnabled is initialized to true. spriteBottomOffset is either 8 or 16 depending on sprite size mode.

oamAddress is set to 0 during each of ticks 257--320 (the sprite tile loading interval) of the pre-render and visible scanlines.

At cycle 257, the PPU status register ($2002) is updated with the value of spriteOverflowDetected. I found that if the code updates the status register during cycles 65--256, some games had issues. For instance, in Battletoads level 2, the player could not injure the yellow venus fly trap creatures and vice versa; the player and the enemy would pass through each other.

[Zepper]

EDIT: fixed the problem. According to Nintendulator:

If Sprite address is not zero at the beginning of the pre-render scanline, then copy the contents of its 8-byte page into the first 8 bytes.

B00M, working like a charm.
---------------------------------------
Hehe, a Sachen game. In my emulator, it's almost the same problem.
I can move the shooter (&shoot), but that's it. If your sprite evaluation code is wrong, mine is wrong too. ^_^;;

[zeroone]

I missed this on the wiki:

On the 2C02, writes to OAMADDR reliably corrupt OAM. This can then be worked around by writing all 256 bytes of OAM. It is also the case that if OAMADDR is not less than eight when rendering starts, the eight bytes starting at OAMADDR & 0xF8 are copied to the first eight bytes of OAM; it seems likely that this is related. The former bug is known to have been fixed in the 2C07; the latter is believed to be.

And, yes, that solved the problem:

Code: Select all

if (oamAddress >= 8) {
  for(int i = 0; i < 8; i++) {
    primaryOAM[i] = primaryOAM[(oamAddress & 0xF8) + i];
  }
}

Thanks.

[tepples]

I just wonder what the h███ Sachen engineers were thinking when they decided to rely on this bug.

[Zepper]

No no... there's a japanese Capcom game that's glitched if such thing isn't emulated...

but... what's its name? A side scroller, a guy with a sword and a shield walking in a city in background, night time.
After the title screen, a map is displayed, much like Ghost'n Gobblins.

[lidnariq]

I just wonder what the h███ Sachen engineers were thinking when they decided to rely on this bug.

"Free sprite rotation!"
1- Assuming OAMADDR & 7 = 0, the first two entries you upload always become Sprite 0 and Sprite 1
→ You don't need to restructure your OAM page at all
2- They probably didn't notice the loss of the two sprites that were obliterated by the copy.


Zepper: One of the latter Ironsword games?

[Zepper]

Tatakai no Banka (J).

It's glitched if sprite rotation isn't emulated.

[zeroone]

Tatakai no Banka (J).

That game seems to work. But, it's horrible. It looks like some kid's high school game project. How did something like that get through Nintendo's quality control.

[koitsu]

Tatakai no Banka (Eulogy of War) is also known as Trojan in the US and other countries. It's an arcade port. It's also not that bad of a game -- really. It doesn't feel like a "high school project" to me. :-)

That game is also useful in some way for testing certain mirroring quirks. I forget what the details are, sadly, but I remember some emulators not implementing something correctly, so during the map screen ("Let's try the first stage, good luck!") text would get printed oddly or in the wrong on-screen location. Ugh, I wish I could remember what the issue was. IMO this, and the use of $2003/$2004, make it a good subject for "emulator testing".

And if I remember right, the quirks with $2003/$2004 are why everyone advocates using $4014 (DMA) instead.

[Zepper]

I remembered the japanese name (!) after a nap during the afternoon. ^_^;; Go figure...

[SedrynTyros]

Tatakai no Banka (Eulogy of War) is also known as Trojan in the US and other countries. It's an arcade port. It's also not that bad of a game -- really. It doesn't feel like a "high school project" to me.

That game is also useful in some way for testing certain mirroring quirks. I forget what the details are, sadly, but I remember some emulators not implementing something correctly, so during the map screen ("Let's try the first stage, good luck!") text would get printed oddly or in the wrong on-screen location. Ugh, I wish I could remember what the issue was. IMO this, and the use of $2003/$2004, make it a good subject for "emulator testing".

And if I remember right, the quirks with $2003/$2004 are why everyone advocates using $4014 (DMA) instead.

I played this game on my NES with my cousins back in the late 1980s when it was new. I remember it was a lot of fun at the time.

[zeroone]

By the way, was Huge Insect a ripoff of Buzz Bombers?