PCjr speedup

Hardware questions and modifications

Re: PCjr speedup

Postby GHiero » Sat Nov 19, 2016 5:09 pm

James wrote:
alanh wrote:It would be a fairly simple mod if so. That's what I would do if I we're making a Tandy mod. I still don't understand how that would break compatibility with anything.


The Tandy Mod doesn't break compatibility with anything (AFAIK) but using the jrExcelleratror's on board video memory breaks PCjr-CGA Compatibility when its internal video buffer is used. My understanding is the PCjr software writes directly to the PCjrs video buffer and Tandy software writes to B800h. The jrExcellerator, with the video buffer on, takes over the B800h address and ignores the PCjr video writes, which would just be ordinary DOS memory. It also allows you to boot 640K without any video management software, eg jrconfig. My understanding from documentation I read from PC Enterprise, from 25 years ago, so forgive me if my memory is faulty, was that a Tandy program writes to B800h but only 16k is redirected to the PCjrs video buffer, hence the Venetian blind affect when running Tandy software without the Mod. I was never sure why it had a Venetian blind effect vs half the screen being filled.


I found that the Tandy mod had trouble with Scubaventure, it would interject a line about time the screen scrolled vertically by 200 lines. The game deemed this line a solid line, making it harder for your character to move around. I disabled the mod in my system to restore my PCjr. to stock.
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Re: PCjr speedup

Postby KenG » Sun Nov 20, 2016 9:20 am

That is the only program I have heard of that doesn't work. Sounds like a programming error where it writes to a place where there is no/unused memory without mod and memory with mod.

Are there any other programs know to be incompatible with the mod?
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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Re: PCjr speedup

Postby KenG » Sun Nov 20, 2016 9:35 am

The description of the GameMaster sidecar says it implemented the Tandy mod without motherboard modification and i think it had an on/off switch. The ad lib/sound blaster and other parts of this sidecar aren't that interesting. I am fine with the PCjr sound, but having the Tandy mod without modifying the motherboard would be great. If soundbaster could be implemented also, I certainly wouldn't mind.
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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Re: PCjr speedup

Postby James » Sun Nov 20, 2016 3:52 pm

My main reason for getting the Tandy Mod was to run Prodigy in 16 colors. In CGA Prodigy ran in grayscale. PC Enterprise later modified the Tandy driver to run Prodigy in 16 colors on an unmodified PCjr. There were also a couple of Tandy branded games, Risk and Clue that ran great in Tandy on a modified PCjr. PC Globe (1990) runs in 16 colors but I am not sure If it supports PCjr-CGA or is running in Tandy. Amazing how much the world has changed in the past 26 years.
-PCjr (1989)-DOS 3.3, Racore Drive II, jrIDE-128 MB DOM, jrExcellerator, Tandy Mod, Serial Card, Combo v2, Config Plus, IBM Parallel Port, Megaboard, Xircom PE3-10BT
-PCjr (2001)-DOS 3.3, jrIDE-128 MB DOM, NEC v20, Parallel Port, Comswap, Xircom PE3-10BT
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Re: PCjr speedup

Postby OldComputerGuy » Sun Jan 01, 2017 9:02 am

Would a board that plugs in where the 8088 goes that has both a V20 and a 8087 on it be something that would be doable or would there be issues?
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Re: PCjr speedup

Postby James » Sun Jan 01, 2017 3:33 pm

I don't believe the 8087 math coprocessor is compatible with the NEC V20. There was a way to add an 8087 to the PCjr but I don't remember the details. Most likely a daughter card with both an 8088 and 8087. How else? Why would you want to add an 8087?

The only software I ever found a coprocessor useful for was Lotus 123. When I added a 80387 to my 80386 computer the calculations took 1/3 of the time. This was using Lotus 123 v2.2. Autocad would also be helped by a coprocessor.
-PCjr (1989)-DOS 3.3, Racore Drive II, jrIDE-128 MB DOM, jrExcellerator, Tandy Mod, Serial Card, Combo v2, Config Plus, IBM Parallel Port, Megaboard, Xircom PE3-10BT
-PCjr (2001)-DOS 3.3, jrIDE-128 MB DOM, NEC v20, Parallel Port, Comswap, Xircom PE3-10BT
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Re: PCjr speedup

Postby alanh » Sun Jan 01, 2017 4:03 pm

I believe the 8088 has to be in maximum mode for 8087 operation. Tandy 1000s and PCjrs are minimum mode only motherboards. It would take more than just jumpering a few pins.
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