PCjr speedup

Hardware questions and modifications

PCjr speedup

Postby KenG » Wed Nov 16, 2016 11:33 am

The old options for speeding up the PCjr seem to be extreemly rare. Has anyone considered trying to come up with a solution, similar to what was done to develop the jr-IDE? It seems to me that , using modern parts, a board with a variable clock speed and cache ram could be produced to replace the 8088/V20. I was thinking the cache controller would have logic to take care of wait states and write through things that need to be immediate. The Tandy video mode might also be able to be handled by the cache controller. Any write to B800-BFFF would be redirected to wherever the screen memory is and any writes directly to screen memory or a port would also write through and not be cached.

I don't have any of the skills necessary to try this so I am just throwing the idea out there to get a discussion going.
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 alanh » Wed Nov 16, 2016 1:04 pm

Step 1 is adding a JR-IDE. A large reservation of memory is made effectively moving the start of any DOS programs into the faster card memory and not the slower on-board 128K.

Step 2 is a V-20 drop-in replacement. About a 10% speed-up for most instruction mixes.

I've explored what a step 3 would look like. I just don't have time to pursue it atm. I've always wanted to make a PCB with pins for a 40 pin dip socket with on-board level translation, SRAM, and an FPGA. One could then add an open-source x86 core and clock both it and the SRAM as fast as possible. Only go to the external bus interface unit for other accesses. However this is starting to encroach upon the question of if you change more than half the machine to a modern substitute, how much does it still resemble a PC-JR? Another alternatives is build a 286 or 386sx adapter board like some of the in-board processor upgrades of the era. A few companies back then also made replacement CPU boards that contained an V20 clocked faster. They almost always contained a speed compatibility switch for the finicky edge cases.
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Re: PCjr speedup

Postby KenG » Wed Nov 16, 2016 1:41 pm

I've done steps 1 and 2. The machine is still too slow for doing much with. If I had the space and money I would buy and set up machines to cover the spectrum, but I don't have that much of either. My first PC compatible machine was a Commodore Colt. It had selectable speeds of 4.77, 7.16, and 9.54 mhz. After I added a VGA graphics card, this was a very capable machine for the times.

On a side note. I was cleaning out some shelves in my garage this week-end and found a laptop computer I didn't know I had. It is an Elite from Midwest Micro. One of the first laptop computers with built in sound. It is a 486 100 mhz. Being a laptop and small at that, I think I will keep it. Now if I just had 286 and 386 machines. LOL Getting the jr has gotten me interested in re-playing some of the old games so I am looking for hardware to use. A turbo version of the PCjr with Tandy graphics should cover most software up to around 1988 or so. The Elite was built in 1994.
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 jmetal88 » Wed Nov 16, 2016 5:34 pm

In one of Mike's file archives, there are instructions for a 'speed up' mod, which I have performed on my own PCjr. It involves stacking two 8284s with certain pins cut and certain jumpers put in, as well as a new clock crystal so you can run the CPU faster than the rest of the system. I think I'm using a 24MHz crystal in mine? Which should be an 8MHz clock. I found it kind of tough to install it in a way that's removable, but it does include a switch in the design to run at the stock speed if necessary.
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Re: PCjr speedup

Postby Brutman » Wed Nov 16, 2016 7:46 pm

There was even a kit made for PCjrs at one point for the PC Sprint mod - Matt Carpenter had done the kit. I remember it because I just ran across the bare board just a few days ago. (Matt - are you out there?)

I've always been leery of speed kits like that because I don't want my system to become unstable in any way. And the kits can only speed the CPU up - you still have the bottleneck of the very limited bandwidth to memory, and the original memory which is terribly slow.

Some minor modifications are fine. But at what point does the machine stop being a PCjr? I've found that it's just much easier to have a variety of machines around for all of my different needs instead of trying to turn a PCjr into a Turbo XT.
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Re: PCjr speedup

Postby James » Thu Nov 17, 2016 11:52 am

I have the PC Enterprises jrExcellerator in one of my PCjrs and have found that it works pretty much flawlessly. Some people have complained of beeps but there is a patch for that. The only downside is there are a few early PC programs that run too fast for my like, eg Bouncing Babies, Killer Bees. I believe it is software switchable. Mike you have one of those I believe.

And of course there's DOSBox. There was a patch available for the AGI based Sierra Games that let you play the PCjr/Tandy sound with a Sound Blaster compatible sound card. I find it works in DOSBox as well as with a 1995 Pentum desktop with a ProAudio Spectrum MediaVision card. The PAS MediaVision card was one of the first Sound Blaster compatible cards. While the MediaVision functions are stereo the Sound Blaster compatible functions are only mono like original Sound Blaster.
-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 KenG » Thu Nov 17, 2016 1:07 pm

If I could get a jrExcellerator for a reasonable price, I would probably go that way. My thought though was: using cache ram and a programmable device (CPLD or FPGA) as a cache contoller to you might be able to switch speeds and implement the Tandy graphics mod in one kit that would replace the 8088/V20 cpu. A little more speed and Tandy graphics should let the jr run most software written in the 80s. Another reason I floated this idea was to try to liven up this forum. It looks to me like there was a lot of exciting things going on a few years ago, like the jr-IDE project, but not much lately. This is , by far, the best source of PCjr information I have found, but it is pretty static.
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 Brutman » Thu Nov 17, 2016 2:22 pm

It's been a long time since I looked at the Tandy mod, but I think it is more than just swapping the CPU. I think there is a motherboard modification that needs to be made as well.

Just to level set things, this particular message board has always been 'slow/low traffic.' My PCjr site dates back to around 2000 or 2001. Brian Heise (Brian where are you!) started micro-zone.com a few years after that, which was the original PCjr forum. I worked with him on that and even renewed the domain once or twice while he was out of the country. After a while we decided to move things back to my site, but the original forum software was not portable so we basically started again from scratch. That was in 2008. Keeping a limited focus and having a limited membership might make things seem slow, but it also improves the signal to noise ratio. I value the signal to noise ratio a lot.

We had a burst of activity in 2012 when I grafted an XT IDE card onto a Jr and made it work. That inspired Alan to work on the jrIDE sidecar, which turned out to be far better than I ever imagined. And there is still a lot of function left to unlock in it. With Alan's current project we're going to get a full blown Linux coprocessor hidden inside of a sidecar. That has a lot of potential, but it's also going to be a lot of work.

The biggest problem we have is that people generally don't have the skills to do software or hardware hacking on these old machines. We're extremely lucky when people do step up - never skip a chance to show them your appreciation.


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Re: PCjr speedup

Postby James » Thu Nov 17, 2016 2:54 pm

KenG wrote:If I could get a jrExcellerator for a reasonable price, I would probably go that way.


Not if I see it first. Seriously, I have never seen one in the wild. I got mine back in 1991 from PC Enterprises. I think Mike found his in a computer, if memory serves.

The Tandy Mod involves piggy backing a chip, some trace cuts and selective soldering. Definitely beyond my skill set at this time. I spoke to Gary at Computer Reset and he seemed willing to give it a try while mentioning he hasn't done it in 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 KenG » Thu Nov 17, 2016 6:49 pm

If I understand it right, the Tandy mod extends the range of addresses that are automatically redirected to screen memory so that 32k modes are in continuous address space. In the Jr, 16k is redirected and the rest must be written to directly. This would make it harder to program. If you used a programmable device as a cache controller, it could presumably be programmed to redirect all 32k to wherever screen memory is. This would implement the Tandy mode without cutting traces on the motherboard. I have extra motherboards and am on vacation next week, so am planned on trying it next week. I already have the chips. If this tests out well, I will probably do it to my main machine.

The Raspberry PI may be able to do the same thing since there is now memory in the space you want to redirect. I think that would be harder to implement and slower though because it would be harder to know when contents have changed and screen memory needs to be updated.
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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