starpath compatibility question

[TheShanMan]

In a thread in the main 2600 forum I had this question with this answer:

 

I loaded the bin onto my harmony and I'm seeing that the game over screen is corrupted. I don't remember where I got my cart images from because it was several years ago, but is there a good source for this that doesn't have the corrupted game over screen? Maybe the one on atarimania?

Your binary probably is fine. Some of the Starpath games rely on the fact that the SC BIOS sets the TIA into a certain state during the loading screen. In the case of the Escape from the Mindmaster game over screen it's the position of the player graphics. If the Harmony cartidge doesn't simulate the SC BIOS, you'll run into problems like the one you describe.

So is it true that the SC BIOS is not simulated, and if not, can it be? My problem isn't major by any means but it makes me wonder if there's any chance that this problem could affect gameplay at some point with any of the Starpath games.

[+batari]

In a thread in the main 2600 forum I had this question with this answer:

 

I loaded the bin onto my harmony and I'm seeing that the game over screen is corrupted. I don't remember where I got my cart images from because it was several years ago, but is there a good source for this that doesn't have the corrupted game over screen? Maybe the one on atarimania?

Your binary probably is fine. Some of the Starpath games rely on the fact that the SC BIOS sets the TIA into a certain state during the loading screen. In the case of the Escape from the Mindmaster game over screen it's the position of the player graphics. If the Harmony cartidge doesn't simulate the SC BIOS, you'll run into problems like the one you describe.

So is it true that the SC BIOS is not simulated, and if not, can it be? My problem isn't major by any means but it makes me wonder if there's any chance that this problem could affect gameplay at some point with any of the Starpath games.

I had heard of the potential issue before but until now nobody had mentioned what binaries were affected.

 

This can be fixed. The Harmony has a barebones SC BIOS built into the bankswitching routine. It could be modified to include whatever setup is needed. Note that I "rolled my own" BIOS to avoid any potential copyright or licensing issues, so if I am to fix this, it would help to know exactly what setup is needed so I can try to duplicate it, such as the horizontal positions of players.

[TheShanMan]

I'm not sure what you mean by "horizontal positions of players", but the problem happens with "Escape from the Mindmaster" when it shows the game over/final score screen. Characters are corrupted so you can't see what it says, nor your score. If there's anything I can do to help test a fix for this, I'd be more than happy to do so.

[+batari]

I'm not sure what you mean by "horizontal positions of players", but the problem happens with "Escape from the Mindmaster" when it shows the game over/final score screen. Characters are corrupted so you can't see what it says, nor your score. If there's anything I can do to help test a fix for this, I'd be more than happy to do so.

I guess that was more directed toward a general audience, so if anyone knows the answer (perhaps Eckhard does, or someone knows of a post in the old Stella mailing list?) I'd like to know.

[Nukey Shay]

Gotta picture of the glitch?

[dwane413]

Gotta picture of the glitch?

 

These pics were made with a copy I hacked for this test. I got rid of the 9 extra lives (so the game would end quicker) by changing 1 byte. Here is a result of a DOS fc (file compare) between the original and my hack:

00001106: 09 00

 

(NOTE: The RGB colors on my TV aren't aligned correctly. I don't know if you can tell it in these pics, but wanted to mention it in case that might cause confusion.)

 

Here's a closer pic of the text.

 

Here's what it looks like in Stella 2.4.2.

[Nukey Shay]

Harmony having problems with HM's being called too soon?

[Eckhard Stolberg]

I guess that was more directed toward a general audience, so if anyone knows the answer (perhaps Eckhard does, or someone knows of a post in the old Stella mailing list?) I'd like to know.

It's been a long time since I looked into that for z26 and Stella, but IIRC it were mostly the multiload games and the playable demos that caused problems. Here is the player positioning code we use in z26:

 

                lda     #$10
               sta     HMP1
               sta     WSYNC
               ldx     #$07
               dex
Pos:            dex 
               bne     Pos
               lda     #$00 
               sta     HMP0
               sta     RESP0
               sta     RESP1
               sta     WSYNC
               sta     HMOVE

We added it because of the problem in the Escape from the Mindmaster game over screen. I don't think any game relies on specific positions for the ball or the missiles.

 

The x-register and the stack pointer are $ff, the y-register is $00 and IIRC the accu contains a fast running counter that gets used to seed a random number generator by some games.

 

The TIA registers get cleared as does the RAM from $81 to $9d. RAM from $fa-$ff contains CMP $fff8 / jmp Start_of_new_load

 

The last page of the first bank of SC-RAM is used by the starfield effect. So if a game doesn't reload it, it should contain random data. None of the existing games have problems with this, though.

 

I think that's all we do in the emulator, and AFAIK all Starpath games work fine.