Forums

A question posed via PM to me by someone who doesn't want to say something foolish...

What differences are there between the SMS VDP and the TMS9918a/TMS9918?

I'll post an answer shortly but feel free to add more.

The first bit is the TMS9918 vs. the TMS9918a. The latter added another graphic mode as its main feature. From the point of view of a Master System user, however, all of the TMS9918a modes are a bit low-tech.

What Sega did with their VDP was (presumably) to take the basic chip design and add a bunch of features, in a new graphic mode: mode 4. This is the regular Master System (and Game Gear) video mode used by almost all games. This offers:

- definable palettes (two 16-colour palettes from a choice of 64 colours), compared to the TMS9918a's fixed palette (which is kind of muted)
- character tiles can use any colours per pixel from a 16-colour palette (TMS9918a tiles were mostly 2-colour)
- more sprites on the screen at the same time (64 total, 8 per line compared to 32/4)
- hardware scrolling (a big deal for games, making scrolling smooth and "free" for games, whereas on TMS9918a and home computers it was usually "expensive" (requiring a lot of resources))
- it also has a sound chip built in
- it still supports all the TMS9918a features, for compatibility with SG-1000 games (including a bunch of stuff never used in those games). The only downside is that the TMS9918a palette is (badly) mapped onto the colourful SMS palette, making older games look a bit weird.

Sega built on this to make Mode 5 in the Mega Drive. The Saturn video chips seem a bit more of a generation leap.

Maxim wrote

Sega built on this to make Mode 5 in the Mega Drive.

And they dropped the 'legacy' video modes too, thus keeping a 99% compatibility with SMS games and 0% compatibility with SG games.

Also, the Second revision of the SMS VDP(Used in the vast majority os SMS 2) dropped the ability to force some bits of video entities addresses to zero. This ability was useful for TMSs with 4kb of memory, so I dont think that any sg1000 game used that.

Author	Message
Maxim Site Admin Joined: 19 Oct 1999 Posts: 14751 Location: London	SMS VDP differences to TMS9918a Posted: Thu Apr 07, 2016 9:50 pm
	A question posed via PM to me by someone who doesn't want to say something foolish... What differences are there between the SMS VDP and the TMS9918a/TMS9918? I'll post an answer shortly but feel free to add more.

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14751 Location: London	Posted: Thu Apr 07, 2016 10:11 pm
	The first bit is the TMS9918 vs. the TMS9918a. The latter added another graphic mode as its main feature. From the point of view of a Master System user, however, all of the TMS9918a modes are a bit low-tech. What Sega did with their VDP was (presumably) to take the basic chip design and add a bunch of features, in a new graphic mode: mode 4. This is the regular Master System (and Game Gear) video mode used by almost all games. This offers: - definable palettes (two 16-colour palettes from a choice of 64 colours), compared to the TMS9918a's fixed palette (which is kind of muted) - character tiles can use any colours per pixel from a 16-colour palette (TMS9918a tiles were mostly 2-colour) - more sprites on the screen at the same time (64 total, 8 per line compared to 32/4) - hardware scrolling (a big deal for games, making scrolling smooth and "free" for games, whereas on TMS9918a and home computers it was usually "expensive" (requiring a lot of resources)) - it also has a sound chip built in - it still supports all the TMS9918a features, for compatibility with SG-1000 games (including a bunch of stuff never used in those games). The only downside is that the TMS9918a palette is (badly) mapped onto the colourful SMS palette, making older games look a bit weird. Sega built on this to make Mode 5 in the Mega Drive. The Saturn video chips seem a bit more of a generation leap.

sverx Joined: 05 Sep 2013 Posts: 3841 Location: Stockholm, Sweden	Posted: Fri Apr 08, 2016 8:07 am
	Maxim wrote Sega built on this to make Mode 5 in the Mega Drive. And they dropped the 'legacy' video modes too, thus keeping a 99% compatibility with SMS games and 0% compatibility with SG games.

gvx32 Joined: 17 Sep 2013 Posts: 128 Location: Gravataí, RS, Brazil	Posted: Fri Apr 08, 2016 12:17 pm
	Also, the Second revision of the SMS VDP(Used in the vast majority os SMS 2) dropped the ability to force some bits of video entities addresses to zero. This ability was useful for TMSs with 4kb of memory, so I dont think that any sg1000 game used that.

Forums

View topic - SMS VDP differences to TMS9918a