NEC V20 Question

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Re: NEC V20 Question

Postby Brutman » Wed Mar 18, 2015 9:17 am

I've had a V20 in my PCjr since 1985 when the ExecPC BBS was selling kits. It adds about 5% onto the overall performance of the machine. My game playing skills are not finely tuned enough to notice a 5% difference.

The real value is in the slightly expanded instruction set. With a V20 you can run some software that requires an 80286. The V20 has the newer non-protected mode instructions. Some of the Zip drive device drivers and NCSA Telnet can take advantage of this.

The only game that I have ever heard of not liking the V20 was Lode Runner, and it can be patched.
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Re: NEC V20 Question

Postby Tempest » Wed Mar 18, 2015 9:26 am

I don't see myself doing a lot of 286 style programs. I use the jr mostly for old stuff that runs too fast on the DOS 6.22 box and for things that support Tandy graphics.

It would be interesting to see two systems running the same program side by side with and without a V20 just to get a sense of the speedup. I've looked around but it appears no one has done that.
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Re: NEC V20 Question

Postby Brutman » Wed Mar 18, 2015 9:47 am

What exactly is a 286 style program? I can't tell by looking at them. They use text. They are used in lots of places. The only difference is that somebody flipped a compiler flag and said to generate instructions for the 286, forgetting that there are a lot of earlier machines out there.

There are device drivers out there that use the 80286 instructions. You won't know it unless you disassemble them. I never suspected that NCSA Telnet did not work on an 8088 either, and that hardly seems like powerful software that one would never run on a Jr.
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Re: NEC V20 Question

Postby jmetal88 » Wed Mar 18, 2015 10:26 am

Brutman wrote:What exactly is a 286 style program? I can't tell by looking at them. They use text. They are used in lots of places. The only difference is that somebody flipped a compiler flag and said to generate instructions for the 286, forgetting that there are a lot of earlier machines out there.

There are device drivers out there that use the 80286 instructions. You won't know it unless you disassemble them. I never suspected that NCSA Telnet did not work on an 8088 either, and that hardly seems like powerful software that one would never run on a Jr.


Yep, IIRC my V20 is the whole reason I was able to run Iomega's zip drivers and access my Zip 250 drive on my PCjr before I got my jrIDE.

It also allowed me to run one of my old favorite games that I bought from the $1 store for my 286 back when I was a kid, Overkill.

http://www.mobygames.com/game/dos/overkill

With the Tandy mod and enough memory installed, that thing has about the best graphics I've seen on my PCjr. Unfortunately, it needed a 286 for the extra speed in addition to the extra instructions, so it runs really, really slowly on the PCjr.
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Re: NEC V20 Question

Postby Tempest » Wed Mar 18, 2015 11:03 am

Brutman wrote:What exactly is a 286 style program? .

I mean I don't see myself trying to run programs designed for 286/386 machines on my jr because even if you get them running they'll be too slow to play. I suppose the V20 is a boon for people who want to run 286/386 applications though.
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Re: NEC V20 Question

Postby Brutman » Wed Mar 18, 2015 11:24 am

The point is, you don't know what is designed for a 286 or 386. The Zip drivers are a great example; there is no reason on Earth for them to require those instructions. There are packet drivers for Ethernet cards and other software that have the same problem.

If you want a stock machine, fine. 128K and a floppy drive is for you. The NEC V20 is such a good improvement that I don't understand why people would not get one, except for extreme cost.
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Re: NEC V20 Question

Postby Tempest » Wed Mar 18, 2015 11:50 am

Brutman wrote:If you want a stock machine, fine. 128K and a floppy drive is for you. The NEC V20 is such a good improvement that I don't understand why people would not get one, except for extreme cost.

Because even a 5% increase in speed can mess up the timing in older games making them harder to play. Yes that same 5% speed increase can let the jr run some games that maybe it couldn't normally, but I'm looking at both sides.

Then again, didn't I read that the jr was actually a smidgen slower than a stock AT? Would the V20 bring it up to about the same speed? For $8 it might be worth trying:

http://www.ebay.com/itm/1pcs-D70108HCZ- ... 1349355121

I wish I had the skills to make a switch that could switch between the two processors. Sadly I have zero soldering skills.
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Re: NEC V20 Question

Postby Brutman » Wed Mar 18, 2015 12:07 pm

You know, unless you are playing for money in a tournament and the game has to be perfect, I don't think 5% is going to matter. Unless it crashes it is not going to be noticeable.

A PCjr is nowhere near the speed of even the slowest PC AT. A 6Mhz PC-AT compared to a PC at 4.77Mhz is pushing close to 3x faster just simply because the CPU is so much better. The poor PCjr has to deal with the video circuitry stealing cycles from the CPU. It's not a fair fight.
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Re: NEC V20 Question

Postby Tempest » Wed Mar 18, 2015 12:18 pm

Brutman wrote:You know, unless you are playing for money in a tournament and the game has to be perfect, I don't think 5% is going to matter. Unless it crashes it is not going to be noticeable.

A PCjr is nowhere near the speed of even the slowest PC AT. A 6Mhz PC-AT compared to a PC at 4.77Mhz is pushing close to 3x faster just simply because the CPU is so much better. The poor PCjr has to deal with the video circuitry stealing cycles from the CPU. It's not a fair fight.


Bah, I meant XT. I always get those mixed up. How does the PCjr fair against the XT?


Also, will that chip I linked to work?
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Re: NEC V20 Question

Postby MikeModified » Wed Mar 18, 2015 1:04 pm

Norton's wildly accurate :lol: SI, Advanced Edition 4.50 reports (in part):

Computer Name: IBM PCjr
Main Processor: NEC V20
Computing Index (CI), relative to IBM/XT: 1.9 (or 2.0)
Disk Index (DI), relative to IBM/XT: 6.2 (or 6.3)
Performance Index (PI), relative to IBM/XT: 3.3 (or 3.4)

Your mileage may vary.

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