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IntroJR demo modified to work properly on Tandy 1000s

NewRisingSun said:
The TX has the Big Blue graphics chip, but only supports 128K of video RAM. The chip alone does not determine the video RAM size; it also has to be connected in the proper way. The Big Blue chip even has provisions for 512K of video RAM, but no system into which it is built connects it that way.

Re-reading the SX technical reference manual, I'm not even sure if the SX supports 256K video RAM. On the EX and HX, it's clear: the standard 256K on the system board are all video memory. All expansion RAM is separate and inaccessible to the video chip.

The TX on the other hand has fixed 128K video memory and 512K or 640K of system memory, for a total of 640K or 768K of memory. Enabling the various 256K video RAM bits does nothing on the TX. How the SX performs in this regard can only be found out from experimentation --- the TX Technical Reference manual reuses portions from the EX manual without noting that they don't apply, and I would suspect the same is the case for the SX. What makes me think that the SX can't do 256K video RAM either is that on page 22 of the Technical Reference, it says "The Tandy 1000 SX video interface circuitry controls 128K of memory.", just like the TX', while the EX' and HX' clearly state 256K on the respective page.
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Having reviewed the schematics for the EX and SX, I would definitely agree that the EX's Big Blue controls 256KB of RAM and the SX's Big Blue controls 128KB of RAM. Both systems use the same DMA Custom Chip, but the DMA chip controls different amounts of RAM for the EX and SX. Obviously the HX functions just like the EX. The TX's schematic shows a function identical to the SX as far as Big Blue is concerned. Big Blue controls 128KB and the other 512/640KB is controlled by the DMA and 82A205 buffer/driver chip.
 
Trixter said:
Does that mean the video memory on an EX and HX is in a fixed location between segments 0000h-3FFFh?
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No, any expansion RAM automatically starts at 0000h. As I understand it, BIOS disables video RAM (by writing 0E to I/O address A0), scans how much expansion RAM is installed (i.e. find the highest address that retains any value written to it), then writes the appropriate value to A0 to put the 256K of video RAM directly on top of expansion RAM.
 
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I've just tried the intro in my Tandy 1000 HX (256kb RAM)... and after it said "decrunching... ", it hanged.
Should it work on a HX? Does it need more than 256km RAM? thanks.
 
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