Add on Video Card

[Bill H]

Okay - I understand.
It is just because you said following:

No changes would have to be made to the zx81, everything could be done with an interrupt.

So at least you need a modified ROM version.

Not to sure about that, dont quote me on it but I remember reading about that when starting a Z80 processor, if you hold down a line it will vector to a specific address that is on the the dataline (or something similar) and I believe this is what the TS1500 cartdrige system does to allow to take over the system when you install the cartridge (this was discussed alot on the old ts1000.us board) . So the thought is, the add on card takes over the system and installs its interrupt that points to the new video processing rom, then lets the system go and the standard installed rom takes over without never knowing things changed. The new rom could sit at 8K or in himem.

[PokeMon]

Yes you can do that, set interrupt mode to 0 or 2 (IM 0 or IM 2) to place either an instruction or an address during the interrupt acknowledge cycle to let the isr point to another location. I think the instruction mode is used only in combination with RST (restart vector) and address table in ROM but which is already occupied. So you can put an address (lower part) to the data bus during IACK cycle and I register (higher part) is used to create a 16 bit address where to jump.

If you provide this technique you could read the D-FILE and transfer it to the video chip. But this will take some time also, so it's not 100% of the interrupt time you can provide for user programs. Maybe you find some trick to check if content of DISPLAY_FILE changed since last transfer - would take some time too. I think the main advantage is, that you have to transfer only characters (not pixels) to a hopefully in video chip integrated char generator - but not sure about this. In that case you have to do the job only every eight scan lines again which could add significantly calculation time in slow mode.

So sounds interesting, anyway good luck for realization.

[Bill H]

Maybe you find some trick to check if content of DISPLAY_FILE changed since last transfer - would take some time too. I think the main advantage is, that you have to transfer only characters (not pixels) to a hopefully in video chip integrated char generator - but not sure about this.

I thought of this also - I was thinking of calculating a simple checksum on what is in the DISPLAY_FILE and comparing to last one, if changed dump the video to the VDP. I think since you are running in fast mode all the time you may not notice much of a slow down, then again maybe you would?

[PokeMon]

It's maybe complicate to give a new address on the bus during interrupt acknowledge cycle.
What you think about activating your external ROM just in case of interrupt (disable internal ROM with high at /ROMCS and activating your supplement ROM only during display period (interrupt). Would make control easier and could be handled as non-modified ZX81 with non-modified ROM. With good will.

[siggi]

Why not use Wilf's design and use a ZX81 as "display controller"?
http://www.user.dccnet.com/wrigter/index_files/HS4.htm

See also his older designs:
http://www.user.dccnet.com/wrigter/index_files/HS2a.htm
http://www.user.dccnet.com/wrigter/index_files/HS3.htm

Siggi

[Bill H]

Why not use Wilf's design and use a ZX81 as "display controller"?
http://www.user.dccnet.com/wrigter/index_files/HS4.htm

See also his older designs:
http://www.user.dccnet.com/wrigter/index_files/HS2a.htm
http://www.user.dccnet.com/wrigter/index_files/HS3.htm

Siggi

Siggi - i like that design- I was the one who asked how they accomplised what the ad said it oculd do and Wilf figured it out.

[siggi]

Had anyone ever considered creating an add on video card for the ZX81/TS1000?

Yes: Kai Fischer in 1997.
See his description of his LCD-Interface card for the ZX81:
http://forum.tlienhard.com/magazin/1997/magazin_197.pdf (pages 11ff., schema at page 16/17)

Siggi

Edit: Here is a description in English:
http://www.fischerkai.de/zxteam/zx96_e.htm

[PokeMon]

Had anyone ever considered creating an add on video card for the ZX81/TS1000? I was looking at the specs on the TMS9928, a video processor chip that was used in the Coleco game, Texas Instruments 99/4 and MSX1 systems.

The last proposal is is not exactly what you asked for, but up to you. Of cource you can create a ZX81 cluster with one for video display and one for calculating.
But I think it is not the same as a video board.

If we put all ZX81 of the whole world together, maybe we could beat one of the supercomputers ?
What you think, 1 Gigaflop, possible ?
Better we send them to Andy first for pimping up.