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pnr

99/2 difference between 24K en 32K versions

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I was reading up on the 99/2 and wondering what the difference is between the 24K en the 32K versions of the ROMs. What added functionality is in the extra 8KB?

 

(all the documents on whtech in the 99/2 directory seem to refer to the 24KB version only).

 

 

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The documents are utterly inconsistent; for instance, they said it had 2 KiB RAM, which is clearly wrong for both 24K and 32K versions; 4 KiB is the minimum. I tried it in MAME; you may want to have a look, both 24K and 32K are emulated. The 24K version was incomplete in some places; in particular, the Hexbus support was not completed (or was broken).

 

Reminds me of the issue with the HX5102 with programs that take more than one data sector. I have it on my list.

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Thanks for that. I re-read the Byte article on WHTECH and there is a sentence that I missed earlier. It says they went from 16KB ROM to 32KB ROM in order to be able to include a full file system. The storage required for that also caused the RAM to be increased from 2K to 4K. As you say, the first design docs actually planned for 12K ROM and 2K RAM, so there was definitely some feature creep there.

 

The Byte article also says the the last 8K are bank switched. Did you figure out how the bank switching works? From a quick glance at the schematics I get the impression that the bank switch is a hack using one of the keyboard lines... On the other hand, the schematics appear to use a single 32KB ROM, which does not appear to match with the actual (pre-production) hardware.

 

Any insights about this would be welcome

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Posted (edited)

Never mind: I found the MAME source files (https://github.com/mamedev/mame/blob/master/src/mame/drivers/ti99_2.cpp and https://github.com/mamedev/mame/blob/master/src/devices/bus/ti99/internal/992board.cpp) and that documents quite a bit.

 

I do wonder, though, why so much ROM space was needed. Standard 1980 MS Basic fitted in 8KB, maybe 12KB once you added a BIOS.

Edited by pnr
added 2nd link

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Hadn't you found the links by yourself, I would have pointed you that way. 🙂 The 99/2 is actually working (so far as I can see) in MAME, so I think I was not entirely wrong.

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