Swiv undocumented score feature

[yaqube]

This question seems to be addressed to Toni.

Swiv uses 4-plane display (16 colours) except the scan lines where the score is displayed. During lines $34-$3A the 5th bitplane is switched on. But it's not ordinary 32-colour display. When a bit in the 5th plane is set the output colour is colour #16 regardless of the bits in planes 1-4.

The question is: what are the conditions to achieve such display?

I can't find anything relevant: bplcon0 = $5200 ($4200 outside lines $34-$3A), bplcon1 = $0000 and bplcon2 = $003F

[Toni Wilen]

It happens when BPLCON2 has "illegal" priority value, http://eab.abime.net/showpost.php?p=206345&postcount=5

[yaqube]

Thanks. I was searching for that post but couldn't find it.

[FrenchShark]

Originally Posted by yaqube

Thanks. I was searching for that post but couldn't find it.

Hello Yaqube,

I have noticed that the score in SWIV is wrongly displayed with Minimig.
Do you plan to fix it for the next Minimig core release ?
That would be great !

Regards,

Frederic

[yaqube]

That's why I was asking. But haven't done it yet.

[xaind]

Nifty trick.

I'm wondering, suppose you wanted to create this effect but you couldn't use this bug. How would you go about it? Is there a better way than blitting it onto the 4 regular bitplanes, which would be a PITA when the screen scrolls and you need to restore those sections of the planes...

Just curious...

[Leffmann]

If you're not using sprites in that part of the screen you could just set the upper 16 colors in the palette to the same value, and change them all on each line (if there's enough time) if you want the same coloring effect as in SWIV.

[Toni Wilen]

Originally Posted by xaind

I'm wondering, suppose you wanted to create this effect but you couldn't use this bug. How would you go about it? Is there a better way than blitting it onto the 4 regular bitplanes, which would be a PITA when the screen scrolls and you need to restore those sections of the planes...

"If you can't" means AGA is available = just use one extra plane

[dissident]

Originally Posted by Toni Wilen

It happens when BPLCON2 has "illegal" priority value, http://eab.abime.net/showpost.php?p=206345&postcount=5

Originally Posted by Toni Wilen

OCS/ECS-only "7-plane" mode.

If BPLCON0 is set to 7 planes following interesting things happen:

- Agnus DMA sequencer uses 4 planes mode
- Denise shows all 6 planes (can be EHB,HAM or DPF, all work)

BPL5DAT and BPL6DAT are "static" (16-bit pattern repeats) 5th and 6th plane pattern. (of course copper can also be used to update the pattern)

This trick was used in intro First Anniversary by Lazy Bones (perhaps only one program using it..) BPL5DAT was used for vertical lines. Normally you can't have >4 planes without slowing down the copper.

Perhaps it can also be used for some weird EHB/HAM tricks

Could it be, that these two undocumented effects have something in common? OCS/ECS only use BPU0-2-bits (bitplanes used) with the max-value of 6 to select the number of bitplanes.

As I learned from Toni's post, Agnus and Denise behave different, if we have BPU>6. What about Alice and Lisa. Do they have the same behaviour if BPU>8?

The next question is, why the BPLCON2-trick and the 7th-plane-trick don't work on the AGA chipset. Is it only because the number of planes has been increased upto 8 with four BPU0-3-bits? For the 7th-plane-trick this is obvious but also for the BPLCON2-trick? It would be interesting what really happens here, if PF2P>5. What is the logic behind it? And even more interesting, how did the programmers found out this effect for SWIV in 1991? They seemd to be the second ones, as Kefrens already used this trick in their Megademo7/Dragons-Part in 1989. They also worked with the value $3f for BPLCON2.

I suppose this "error" wasn't removed by the designers of the AGA chipset. This means that it could still work, if we adapt this logic to a higher number of planes.

What do you think about this, Toni?

[Toni Wilen]

It does not work in AGA. Also >8 planes = confirmed to always cause blank screen (= 0 planes)