Forums

Quick question before I start investigating.
Does anyone know about the differences between a SMS and a GG or Megadrive in SMS mode ?

Quote

> Quick question before I start investigating.
> Does anyone know about the differences between a SMS and a GG or Megadrive in SMS mode ?

Lucky you, I've been doing some research on this. :)

You can put Genesis VDP in SMS mode in two ways:

1. Clearing bit 2 of register #1. What's interesting is that
the way you program the VDP (register setting, memory access)
is like the Genesis, meaning you can do word-wide writes, etc.
The screen is like the SMS though, sized 256x192 in a 256x224
display. None of the Genesis specific registers work except
for #12, meaning you can have an interlaced, 320x192 display
if you really wish. :) I couldn't get the top/right side
lock, or left side blanking to work while in this mode.

To my knowledge *no* games use this feature.

2. Asserting the M3 line, which is pin 30 on side B of the
cartridge connector. This is what devices like the Super Magic
Drive, Power Base Converter, Phantasy Star MD, and pirate
carts with several SMS games do. This also puts the rest
of the system in SMS mode, meaning the Z80's memory map
is changed around and the 68000 is disabled.

What's really neat is that bit 2 of register #0 controls how
RGB values are encoded in CRAM. It switches between a SMS
and Game Gear / Genesis (the two have identical palette hardware)
mode. What sucks is that this bit doesn't work while in SMS
mode, if it did you could play Game Gear games on a Genesis
with correct colors. With no start button of course.

So, basically using SMS mode via software isn't quite like the
real thing, and using it through hardware is.

Other non-related points of interest; the YM2612 and apparently
the Z80 banking mechanism (swaps 32k chunks from 8000-FFFF)
are still usable while in SMS mode (the hardware way).
This doesn't conflict with any SMS games, because they are
in the ROM area which no game would write to.

I can run really limited tests for you if there's something
you want to check out. The unused SMS registers (3, 4) don't
do anything, btw.

Maybe I should include all this in the next VDP docs update. :)

Thanks for the infos. Not that useful in my research, but still interesting.
So basically we got a better SMS with the Megadrive in SMS mode, that's cool :)

I was asking because the Japanese version of Y's for SMS is known to have a bug when ran on Game Gear or Megadrive with adapter.

Quote

> I can run really limited tests for you if there's something
> you want to check out.

Still poking with GameGenie/ActionReplay?
bwahaha that rocks :)

Quote

> The unused SMS registers (3, 4) don't do anything, btw.

Yes it's probable. Video modes 0 to 3 we not implemented on the Megadrive, it would be only used by F16 Fighters anyway :)
Well, they *might* exists on Japanese Megadrive, after thinking about it. Gotta try one day.

Quote

> Thanks for the infos. Not that useful in my research, but still interesting.
> So basically we got a better SMS with the Megadrive in SMS mode, that's cool :)

> I was asking because the Japanese version of Y's for SMS is known to have a bug when ran on Game Gear or Megadrive with adapter.

>
> > I can run really limited tests for you if there's something
> > you want to check out.

> Still poking with GameGenie/ActionReplay?
> bwahaha that rocks :)

> > The unused SMS registers (3, 4) don't do anything, btw.

> Yes it's probable. Video modes 0 to 3 we not implemented on the Megadrive, it would be only used by F16 Fighters anyway :)
> Well, they *might* exists on Japanese Megadrive, after thinking about it. Gotta try one day.

Hum, so it because that f16 fighter doesnt works on a MegaDrive with sms adapter?

Author	Message
Zoop Joined: 24 Jun 1999 Posts: 1732 Location: Paris, France	SMS differences with GG/Megadrive Posted: Sat Apr 01, 2000 4:18 pm
	Quick question before I start investigating. Does anyone know about the differences between a SMS and a GG or Megadrive in SMS mode ?

Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Posted: Sat Apr 01, 2000 6:16 pm
Charles MacDonald Joined: 28 Sep 1999 Posts: 1197	Quote > Quick question before I start investigating. > Does anyone know about the differences between a SMS and a GG or Megadrive in SMS mode ? Lucky you, I've been doing some research on this. :) You can put Genesis VDP in SMS mode in two ways: 1. Clearing bit 2 of register #1. What's interesting is that the way you program the VDP (register setting, memory access) is like the Genesis, meaning you can do word-wide writes, etc. The screen is like the SMS though, sized 256x192 in a 256x224 display. None of the Genesis specific registers work except for #12, meaning you can have an interlaced, 320x192 display if you really wish. :) I couldn't get the top/right side lock, or left side blanking to work while in this mode. To my knowledge no games use this feature. 2. Asserting the M3 line, which is pin 30 on side B of the cartridge connector. This is what devices like the Super Magic Drive, Power Base Converter, Phantasy Star MD, and pirate carts with several SMS games do. This also puts the rest of the system in SMS mode, meaning the Z80's memory map is changed around and the 68000 is disabled. What's really neat is that bit 2 of register #0 controls how RGB values are encoded in CRAM. It switches between a SMS and Game Gear / Genesis (the two have identical palette hardware) mode. What sucks is that this bit doesn't work while in SMS mode, if it did you could play Game Gear games on a Genesis with correct colors. With no start button of course. So, basically using SMS mode via software isn't quite like the real thing, and using it through hardware is. Other non-related points of interest; the YM2612 and apparently the Z80 banking mechanism (swaps 32k chunks from 8000-FFFF) are still usable while in SMS mode (the hardware way). This doesn't conflict with any SMS games, because they are in the ROM area which no game would write to. I can run really limited tests for you if there's something you want to check out. The unused SMS registers (3, 4) don't do anything, btw. Maybe I should include all this in the next VDP docs update. :)

Zoop Joined: 24 Jun 1999 Posts: 1732 Location: Paris, France	Posted: Sat Apr 01, 2000 6:50 pm
	Thanks for the infos. Not that useful in my research, but still interesting. So basically we got a better SMS with the Megadrive in SMS mode, that's cool :) I was asking because the Japanese version of Y's for SMS is known to have a bug when ran on Game Gear or Megadrive with adapter. Quote > I can run really limited tests for you if there's something > you want to check out. Still poking with GameGenie/ActionReplay? bwahaha that rocks :) Quote > The unused SMS registers (3, 4) don't do anything, btw. Yes it's probable. Video modes 0 to 3 we not implemented on the Megadrive, it would be only used by F16 Fighters anyway :) Well, they might exists on Japanese Megadrive, after thinking about it. Gotta try one day.

PrOfUnD Darkness Joined: 29 Jun 1999 Posts: 383 Location: Brazil	Posted: Thu Apr 06, 2000 5:32 pm
	Quote > Thanks for the infos. Not that useful in my research, but still interesting. > So basically we got a better SMS with the Megadrive in SMS mode, that's cool :) > I was asking because the Japanese version of Y's for SMS is known to have a bug when ran on Game Gear or Megadrive with adapter. > > > I can run really limited tests for you if there's something > > you want to check out. > Still poking with GameGenie/ActionReplay? > bwahaha that rocks :) > > The unused SMS registers (3, 4) don't do anything, btw. > Yes it's probable. Video modes 0 to 3 we not implemented on the Megadrive, it would be only used by F16 Fighters anyway :) > Well, they might exists on Japanese Megadrive, after thinking about it. Gotta try one day. Hum, so it because that f16 fighter doesnt works on a MegaDrive with sms adapter?

Forums

View topic - SMS differences with GG/Megadrive