NEC V20 Question

Hardware questions and modifications

Re: NEC V20 Question

Postby Tempest » Wed Mar 18, 2015 1:33 pm

So what do those numbers mean?
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Re: NEC V20 Question

Postby James » Wed Mar 18, 2015 2:38 pm

Norton is wildly optimistic. I would say the V20 @ 4.77 mhz, more or less, gives the PCjr back the speed that video circuit robbed, making it about the same speed as the XT.
-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: NEC V20 Question

Postby Brutman » Wed Mar 18, 2015 2:55 pm

Guys/Gals ...

I feel like I'm talking in a vacuum sometimes.

The general consensus on the V20 is that improves the speed of things by about 5%. It depends on the instruction mix being executed; you can do better if the instructions favor the V20. It has a slightly larger instruction prefetch buffer which really helps on 8 bit buses. It also has more a more efficient implementation of some instructions, and it has the extra instructions. But it is not a miracle.

The video circuitry stealing/competing for cycles with the CPU is just a nasty design. That penalty makes a 128K PCjr feel like it is operating half or 60% of the speed of a real PC. You can do the math to work it out; read the tech ref to see how often the VGA chip steals cycles from the CPU. That is why reserving the first 128K for video and DOS data areas really improves things; you are no longer fighting with the VGA chip. (It's a pity the DOS and BIOS data areas are still affected.)

Adding a V20 is not going to make up for the fact that the VGA chip is stealing from the CPU.
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Re: NEC V20 Question

Postby Tempest » Wed Mar 18, 2015 6:10 pm

But it helps. :)

Is that V20 I linked to ok to use? http://www.ebay.com/itm/1pcs-D70108HCZ- ... 1349355121
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