VDP questions

[Deleted]

After spending a good chunk of the afternoon looking over VDP documents, I'm now a little confused/unsure of a few things.

darkdust.net/writings/megadrive/initializing#VDP_registers

The person states that you can set the Genesis/Mega Drive into V28 mode, so Pal/Mega Drives will display 320x224 instead of 320x240, and this can be set through register 1 (#$81XX).

info.sonicretro.org/SCHG:VDP_Documentation/General/Registers
cgfm2.emuviews.com/txt/genvdp.txt

These 2 sites lead me to believe the same, though it could just be poorly written or I'm misunderstanding it.

So, by doing this:
move.w #$8120,($C00004)

I should be forcing PAL systems to display in 320x224 mode, yes?

If I compile that, I get a 68K error in Fusion and Regen. If I set everything to 00, and I run in PAL mode, it still shows artifacts on the bottom of the screen. Is this possible to do, or am I misunderstanding the whole thing?

Thanks!

[jlf65]

Yes, setting V28 mode makes both PAL and NTSC 224 tall. It works fine on any PAL system. The problem is the value you are storing is wrong. The MSB of $81 means write register 1, so that's okay, but the LSB of $20 has only bit 5 set, which is NOT okay. Look at the bits for register 1:

b7 - TMS9918 / Genesis display select
b6 - DISP (Display Enable)
b5 - IE0 (Vertical Interrupt Enable)
b4 - M1 (DMA Enable)
b3 - M2 (PAL / NTSC)
b2 - SMS / Genesis display select
b1 - 0 (No effect)
b0 - 0 (See notes)

You're only turning on the vertical blank interrupt, and NOTHING ELSE! The "typical" value for register 1 is $74, which enables the display (b6), enables the vblank int (b5), enables the DMA (b4), clears M2 (b3) which sets "NTSC" height (V28 mode), sets Genesis video mode (b2), and clears the two lsbs as needed.

[Deleted]

Ok, so I'm definitely lost now.

$81 is good, great. The typical value to write - $74, which I understand to be this:
01110100 for the bits

So, this should now be:

move.w #$8174, ($C00004)

Same results - illegal instruction

What am I missing? Boy am I starting to not like ASM

[jlf65]

Have you tried
move.w #$8174, $C00004

Parenthesis normally only go around address register indirect modes like (a0) or 4(a0).

[Deleted]

Yep, still the same results.

[Mairtrus]

That's because you are setting the Vertical Interrupt, but you haven't defined a section for it. Simply add a "On VBlank GoSub..." or just clear the bit 5 (it becomes $34).

[Deleted]

Tried move.w #$8134, 4(a4), still the same results.

Any other ideas?

[jlf65]

We need to see more of the code. With what little you provide, there's no way to tell you where the error is. If you don't want to show the code, you'll be stuck debugging it yourself.

[Deleted]

asm
;move.w #$8174, ($C00004)
move.w #$8134, 4(a4)
end asm


That's all the code. There wasn't much to really show. I figured posting the 1 line to make this work would be enough. Even that by itself causes the illegal error.

[Mairtrus]

May 31, 2011 15:50:45 GMT -5 @elusive said:

asm
;move.w #$8174, ($C00004)
move.w #$8134, 4(a4)
end asm

My bad. It's $54, not $34.

[Deleted]

Ok, so that works (I did try that), but maybe I'm confused on the way an emulator works. If you set the emulator to Autodetect, it'll detect it as NTSC. If you set it to PAL, it still shows artifacts at the bottom of the screen. Is this because the emulator overrides the VDP resolution and sets it at 320x240?

[jlf65]

It shouldn't. Maybe it's just the emulator - try another. I use Gens/GS.

[Tiido]

BEX automatically sets screen res to 320x240 if in 50Hz

[Deleted]

So there's really no way around this then in BEX?

[Deleted]

If you want to use 320x224, simply don't do anything.

PAL / NTSC doesn't have anything to do with region lock, neither can you select 50 / 60 Hz through software.