More than 32 sprites on a TMS9128

Resident (35)

12-07-2019, 13:33

MRC user Plainspooky found a very interesting feature in TMS9128 VDP that allows more than 32 Sprites to be displayed on the screen.

Curious about it? Then find the details on:

https://giovannireisnunes.wordpress.com/2019/07/12/more-than-32-sprites-on-msx1-screen

Por wimpie3

Champion (436)

12-07-2019, 13:44

As far as I understand those are the same sprites copied over. They will be identical to the first 32.

Por PlainSpooky

Resident (53)

12-07-2019, 13:59

Yes, all sprites from 8th level and at first thrid of screen are replicated in second and/or thrid thirds (depending of VDP's register 4 value).

Por wimpie3

Champion (436)

12-07-2019, 14:05

This limits the practical use. But nice find :-)

Por e1000

Resident (35)

12-07-2019, 14:05

Indeed, still over than 32 on the screen.

And their are even considered in the collision detection.

Por Grauw

Ascended (10818)

12-07-2019, 14:46

The ghosting of sprites is actually documented by TI themselves to some degree…

See the programmer’s guide section 8.4.2: “What this mode does is effectively shrink the Graphics II Mode Color and Pattern Table down from Hex 1800 bytes to Hex 800 bytes. [...] When in this mode 32 sprites can no longer be used. If you try to put more than eight sprites on the screen at once, they will start to duplicate themselves on the screen.”

Surprised this isn’t emulated by openMSX…?

Another way to increase the number of sprites beyond 32 is by using a screen split which changes the attribute table address mid-screen.

Por RetroTechie

Paragon (1563)

12-07-2019, 14:52

e1000 wrote:

Indeed, still over than 32 on the screen.

Since when are sprites limited to 32 per screen? I thought the more important limitation is # sprites per scanline.

No reason you can't change Sprite Attribute Table (or sprite patterns) several times per frame. Not to mention software sprites although that takes more effort programming wise.

And yes collision detection is a useful feature. But I'd think that still applies even if you're messing with SAT or sprite patterns halfway into a frame. That would just put some restrictions on what exactly is or isn't detected.

Por ARTRAG

Enlighted (6976)

12-07-2019, 19:18

It is the well known and documented cloning effect, as Gauw said
It is unstable and can vary from chip to chip even with the temperature
So it is hard to get something useful with it

Por hap

Paragon (2043)

12-07-2019, 21:01

Works fine on meisei:
https://i.imgur.com/ZacUmYW.png

Por alexito

Paladin (761)

12-07-2019, 21:09

Thanks hap, at least one emulator can imitate that very interesting TMS9918/9129 feature.

P.S.: how you get to run that MSX Basic program on meisei if I remember it just load from Tapes and Cartridges?

Por PingPong

Enlighted (4155)

12-07-2019, 21:19

Bleah! Do not expect nothing that is really useful from the crappy TMS VDP.
Even some undocumented modes that have some usefulness are plagued by some others side-effects that make them unusable as ARTRAG said