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To V20 or not to V20...

voidstar78

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My 5150 is working fine for all the things I want to do with it.

So, while I have a V20 in hand, I'm debating on actually installing it.


I'd like to understand more about the period-context of the V20: how much did it cost, around 1985? Was it readily available, by mail order or what stores would have carried it? Was it really a "drop in replacement" (not even a chip puller needed?)? Was it something that home users typically acquired? Or was it more just niche users? (maybe software developers who want faster compile times, or Lotus number crunchers?)


I am starting to get into DESQView and multi-tasking options. But I'm content to just explore what the stock 4.77 8088 could do (e.g. I ran BEAST while playing a MOD audio in the background, that was neat). BEAST is great, it seems like most people don't know you can PULL the blocks, not just push them.



I know there was some Intel litigation related to the V20 - I wasn't sure if it was continued to be sold or made available during those trials?

I also came across an old article naming the V20 as equivalent to the "8088-2" (used in the Tandy 1000). Is that accurate?


And, it's an easy enough thing to undo? Just swap the 8088 back in? Just I kind of like the idea of a known-stable-original motherboard that's never had any chips pulled out of it.

Thanks!
 

Ruud

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It is faster than a 8088 because it needs less cycles to execute instructions than the 8088 does (I don't know which ones). As an extra: it can run 8080 instructions and it is able, for example, to run CP/M 2.2. Please see: https://hackaday.io/project/170924-v...ebrew-computer
It has a small disadvantage as well: some older games will run too fast.

Speaking from my own experience: I was quite happy with the extra speed in those days.
 

Timo W.

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It has a small disadvantage as well: some older games will run too fast.
And that is why I'd say: no, leave the 5150 as is. Some games rely on the 4.77 MHz 8088 and run too fast or break otherwise (California Games comes to my mind). There's no point in breaking compatibility by adding a V20.

The V20 makes sense if you need 286 instructions on that machine.
 

Chuck(G)

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I've got a parts drawer full of 8088 CPUs and a couple of XT clones, both of which have been running a V20 since about 1987. No problems so far, but I don't play computer games.
 

Eudimorphodon

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I'd like to understand more about the period-context of the V20: how much did it cost, around 1985? Was it readily available, by mail order or what stores would have carried it? (*snip) Was it something that home users typically acquired? Or was it more just niche users? (maybe software developers who want faster compile times, or Lotus number crunchers?)

An ad in a 1986 Byte magazine lists the 5mhz chip for $14, $16 for 8mhz. I don't think swapping them out was really "mainstream" before 1986 or so for a number of factors; first may have been issues with Intel's lawsuit, and another may have been simply that it's around this time that the CP/M dihards were getting serious about moving on and the NEC V20 offered a unique solution for being able to migrate at least some software over to a PC via its ability to natively emulate the 8080. I think that its 8080-emulation stupid pet trick was really its first selling point, although as word spread that it would give at least some benchmarks a boost more people bought it solely for the that.

As to it being "readily available" it depends on your definition of that term. It is definitely *not* something that was sold over the counter in a mainstream computer store. Some of those independent hole-in-the-wall places might have sold it, possibly bundled with an overclock kit like the PC-SPRINT. The upgrade was name-dropped in mainstream magazines occasionally but it wasn't really ever a must-have. (I remember reading some articles that were really down on it, basically declaring that it'd improve your score in Norton SI and little else and you should look elsewhere if you really want a noticeable speed increase.)

Was it really a "drop in replacement" (not even a chip puller needed?)?

"Drop in" means you can replace one CPU with the other without doing anything else. Unless you had a ZIF socket installed in your computer it doesn't literally mean "it drops right in!" (* holds the chip carefully a few inches above the socket, trying to line the pins up just right... *) Pulling a 40 pin DIP without a chip puller is "doable" with a little levering from a screwdriver or something (or sheer HULK SMASH grip strength if you have long fingernails) but it's a pretty stupid idea to try the latter in particular and even if you're careful there's a good chance you're going to bend some pins with the screwdriver method.

I know there was some Intel litigation related to the V20 - I wasn't sure if it was continued to be sold or made available during those trials?

Intel actually sued them twice, which *may* have significantly impacted its ability before 1985 in the US? By 1986 it was easy to get one out of the ads in the back of mainstream magazines. Many late XTs used it from the factory, especially laptops.

I also came across an old article naming the V20 as equivalent to the "8088-2" (used in the Tandy 1000). Is that accurate?

Not really. The 8088-2 was just the 8mhz rated flavor of the 8088. The V20 came in a slew of speed grades down to 5mhz. Maybe the article was trying to claim that a V-20 at 4.77mhz would be "equivalent" to an 8088-2 at its usual speed of 7.16mhz? How true that is is, well, very debatable. The aforementioned Norton SI would probably give about the same score to those two machines, but that's going to give a really misleading impression as to how those machines actually perform in the real world.

And, it's an easy enough thing to undo? Just swap the 8088 back in? Just I kind of like the idea of a known-stable-original motherboard that's never had any chips pulled out of it.

Most factory machines from the era are going to have cheap wiper sockets that are good for maybe a dozen insertion/removal cycles if you're gentle. So don't make a career out of swapping the two unless you're fine desoldering and replacing your socket once and a while. I suppose if you have room enough around it you could throw a ZIF socket in and go nuts. Just make sure you don't drop it in backwards.

On the broader question of whether it's worth doing or not: For most programs you're not really going to "feel" the speed difference. A significant reason the V20 is faster is instead of completely using the same ALU hardware for instruction processing and effective address calculation (IE, segment arithmetic) it has additional hardware to help with the latter. This makes it internally a little closer to the rarely-used-in-PCs Intel 80186/80188 chips and generally lets it save a few cycles on loops, etc. It also has somewhat more efficient implementations of some instructions; for instance, slightly faster multiply and divide instructions almost solely account for its Norton SI benchmark wins. It also incorporates some instruction enhancements from those machines. (When people say the V20 has "286 instructions" that's what they mean; from a software perspective an 80186 is basically a real-mode-only 80286, and the same real-mode instruction set was standard for later models of the x86 family.) These instructions include some block move I/O commands that can significantly accelerate some tasks; for instance, the PIO-only (no DMA) versions of the XTIDE/XT-CF cards can be sped up quite a lot by running a V20 version of the BIOS. (We're talking about the difference between 300k per second and 500k per second maximum transfer rates.) But this only affects software that specifically uses these instructions. And, as mentioned, if you want to see your 5150 "directly" running a CP/M version of Wordstar or something the V20 can do this with the help of software like 22nice thanks to its ability to be switched in and out of an alternate 8080 binary compatibility mode. It's not a very useful thing today but it did the needful for some people back in the day.

So let's make this easy and lay out the pros and cons:

Pros:
  • At least some benchmarks will show your machine runs faster. You probably won't notice it much (some games might feel "off" compared to how they ran before), but the bigger number can give you warm fuzzies. These benchmark wins will look particularly huge if you're running an XT-CF-lite card; your hard disk performance will go from "a lot faster than a real IBM/XT hard disk" to "literally about as fast as the ISA bus can go". (The XT-CF in my V20-upgraded Tandy 1000 benchmarks at almost precisely the same read speed as an EMS RAMDISK. Without the V20 it'd only be a little over half that fast. But that's still *way* faster than your typical MFM controller, especially since the big thing that's going to matter is access time, not transfer rate.) In the real world, it isn't going to matter. But, yeah, it looks good.
  • You can do the run-CP/M-programs-natively thing. It's not really good for anything, but it's a fun stupid pet trick. I've done it for just that reason. Unfortunately you still won't be able to run later CP/M programs that were compiled for Z-80 CPUs instead of the 8080, but unless there's some significant thing you already care about that falls into that category you're not going to care.
  • Sometime around the early 1990s DOS software publishers started assuming that everyone had a 286 or better CPU and started compiling their software based on that assumption. This created a special category of software which despite being "normal" Real Mode programs wouldn't run on XT-class computers with 8088/8086 CPUs. (My vague recollection is the "big one" in terms of instruction incompatibilities is the 186 and later have a single instruction that speeds up saving and restoring register contents during subroutine calls.) Having the V20 will let you run some of this late stuff. There are also expansion cards with ROMs, like some VGA cards, that make this assumption, having a V20 can fix problems with that sometimes. (Assuming the hardware on the card is otherwise able to fall back to an 8-bit bus.) If you want maximal compatibility with the totality of real-mode DOS software a V20 probably helps more than it hurts; what you lose in compatibility with early games you gain back at the end, although of course some of the software you now technically can run is realistically too slow on your machine.
Cons:
  • The cycle-time differences will screw up some early PC software, mostly games, along with recent impressive tech demos like 8088mph. If your main use of the machine is gaming you may want to avoid it for just this reason. (*)
  • There is a very small class of software that trips up on the V20 for other reasons related to relying specifically on some obscure/undocumented aspects of how the 8088 behaves. I can't name anything specific off the top of my head but I know it exists. (This same software is also likely to break on a 286 or later machine, period.) Again you're probably more likely to find this with game software, anything copy protected, whatever.
The (*) above is to note this: I vaguely recall you have a VGA card in this machine. That's already harming compatibility with hardcore bit-twiddling game software, so a V20 is less likely to be the straw that breaks the camel's back. If you care less about early CGA software and are instead concentrating on late barely XT-compatible software in EGA/VGA modes then take the V20, you need all the help you can get.

Considering all the time you've been spending trying to port early games onto cassette tapes I'd probably recommend against the upgrade, but if you're planning to change focus then it probably won't hurt.
 
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deanimator

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Eudimorphodon said:
Considering all the time you've been spending trying to port early games onto cassette tapes I'd probably recommend against the upgrade....

Also, 5150CAXX will detect that the 5150 does not contain a genuine 8088 and will fail to load :)
 

vwestlife

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The Windows 95/98 version of MS-DOS EDIT is one example of a real-mode program that uses 186/286 instructions and thus will lock up on an 8088 or 8086 but will run fine on a NEC V20/V30/V40. As long as you have a supported CPU, it runs fine in DOS 3.x or higher and is much smaller and faster than the QBASIC-based version of EDIT.
 

Eudimorphodon

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80186 instructions also occur in:
  • Newer device drivers (the aforementioned ZIP driver, network card drivers, some versions of CuteMouse)
  • Some 90's vintage or later communications and network software.
  • Later builds of WordPerfect 5.1 (* pretty sure?)
  • Windows 3.0's color VGA driver
  • (some versions at least of) Programs like DOSMAX which are useful/needed for UMB support, if your machine is so equipped.
  • FreeDOS (Many/most builds are broken on 8088/86)
 

bladamson

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The only reason I installed one is because some TSR (I think it was the PLIP) driver needed the 186 instruction set support. Otherwise I don't think the 10% speed boost amounts to much.
 

Chuck(G)

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My exposure to the NEC V20 was through the NEC USA in Natick, MA. I was developing software for the 8080 emulation mode and got some help from them. I may even have an issue or two of their MicroNOTES publication. I've used it with NICs that required the 80186 instruction set quite successfully.
 

TheDrip

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I installed a V20 in my 8088 machine to run a different version of the XT-IDE firmware. IDE_AT instead of IDE_XT which made for a 50% boost in throughput for my CF card.

IDE_AT only requires 186, not 286 it turned out.
 

Eudimorphodon

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I installed a V20 in my 8088 machine to run a different version of the XT-IDE firmware. IDE_AT instead of IDE_XT which made for a 50% boost in throughput for my CF card.

IDE_AT only requires 186, not 286 it turned out.

I'm pretty sure in the pre-built binaries the "_XTP" ones are the canonical versions you want to use on V20/30/40s, not _AT. The docs say:

USE_AT

Assembles code targeted for AT systems. For example, AT-builds always operate in Full Operating Mode. Another difference is that AT-builds use some BIOS functions that are not available on XT systems

But I guess if it's working for you that thing about using AT-only BIOS functions must not be entirely true. I can attest that the _xtp version gives me the same speed boost over a plain _xt build.
 

voidstar78

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Thanks guys! I'll save my V20 for if I ever pick up another 5150, and I'll leave this one alone as-is. I can do some electronics work (see here) and I've lapped a few CPUs in my day during my "extreme PC" adventures (and they all still work to this day! also took my Pentium D to 4.1 GHz in '06 ). Swapping this 8088 seems like easy work. But that was a good way to put it: losing some earlier compatibility to gain some later capability.



For instance, SimCity is basically unrunnable for me at 4.77mhz. Same for King's Quest IV. They do run, but painfully slow. Lemmings is sluggish, but tolerable. And I'm ok with all that, as any title past about 1988 is just lucky that it runs at all IMO. I'm not sure if the V20 would make these all more tolerable. Besides, absolutely, I can't give up my 5150caxx compatibility :) And I'm ok with my MODs being played at 9-11 khz instead of 20-22.



Thanks Eudimorphodon for covering all the questions! Wow, ~$15 for a V20 even then !? (well, and then shipping - another $15 maybe?). About the cost of one $29.95 software title :) I was thinking it would be over $100, possibly 2. But I suppose at that price, you'd better just upgrade to an AT. I already had a Tandy 1000 in '87, but if I had a stock 5150 then I'm not sure if I would have made the call to try the V20. I might have, as a BBS operator, to give DESQView a little better chance (as I'd like to use Turbo Pascal while folks were connected - and the idea back then of having TWO computers, no way, could barely afford one!). But I can't remember if I got a '486 before 1990 or not (I know I skipped the 386s, spoiled jumping straight to a '486dx50).


Re, VGA in the 5150: You're right some early titles don't work with the VGA card. Specifically, Astro Doge won't work with my VGA card - I'm really curious why though, and bummed since it's a pretty decent game for '82. And King's Quest 1 switches color palettes between when I have the CGA card (yellow/green) and then the VGA card (cyan/white), so little things like that happen. [ there was a video once on "what CGA color palette did Sierra intend for KQ1 to be in?" and I'd say the answer was none-of-the-above :) but if I had to pick, I prefer the yellow/green for that one ]. You're right also that curiously, some .COM programs I've tried just freeze up my 5150 - and I assume it's some "errant" opcode, some '186 thing. Nothing real sacred about what instructions can end up in a .COM. I should have kept a better list/track of those incompatible titles, I tended to just delete them.


I don't mind the MDA/CGA/VGA adapters (and using the CGA card), but the phase isn't perfect and the font ends up slightly wavy (maybe it's the cable?). Plus when I shut things down, that adapter is one extra thing to remember to turn off. I'm kind of on the hunt for some more MDA game titles (besides text adventures). Something like Kroz should have been possible on the MDA early on. I suppose Castle Adventure should support MDA? [ was thinking of adding a section about what titles to use if all you had was MDA - '82 WordPerfect is fun for a bit, and BEAST was actually '84 - surely there was some interesting non text adventure game in '83 ? ]



Although, the increased performance for the XT-IDE with a V20 is appealing. But that's a good point, it is already faster than "stock"/contemporary performance, so I can just appreciate that. I hope the XT-IDE doesn't also simulate the gradual accumulation of bad sectors :) Speaking of that - is DEFRAG necessary or helpful on the XT-IDE ?




And I especially liked the point about not taking for granted how many times you can swap chips in and out (and yes, each time is a risk of some bent pins -- which aren't the end of the world, unless they snap off -- and the end falls somewhere down on the motherboard). I was nervous about even swapping expansion cards on this antique, but I'm mostly over that now. I guess an extreme thing to do is to have a toggle switch between the 8088 and V20? Seems like some elaborate board could be made, to slide into the 40-pin DIP - put your two processors on that board, have a little switch to alternate between them? Would just that extra little distance skew timing? Hmm... Won't be my project, but would be interesting to see :) [ the chips could even be vertical to save space? maybe something like this was already done? ]





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Krille

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I installed a V20 in my 8088 machine to run a different version of the XT-IDE firmware. IDE_AT instead of IDE_XT which made for a 50% boost in throughput for my CF card.

IDE_AT only requires 186, not 286 it turned out.

You must be misremembering. Not only do the AT-builds, as Eudimorphodon already mentioned, call system BIOS functions not available on XT BIOSes, they are also using undocumented CPU instructions not supported by the NEC V20.
 

compaqportableplus

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I generally install a V20 in the machines I use frequently. It’s just enough performance increase without being too much (like a full-blown accelerator). I want my PC or XT to still run at a realistic speed, and the V20 is pretty damn perfect for that in my opinion. I’ve never had any compatibility issues with it personally.
 

kdr

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I have a 5150 that I keep in its original condition (8088 CPU, MDA/CGA video, no fixed disk) and a generic Turbo XT clone that gets kitted out with all the extras like a V20 CPU, EGA video, etc. Best of both worlds!
 

digger

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I don't know if this also applies to the V20, but I clearly remember that once we'd replaced the 8086 CPU in my Dad's Olivetti M24 with a NEC V30, some games started showing certain animations that were disabled before the upgrade. I remember this being the case with Space Quest III and the CGA/Tandy/EGA version of The Secret of Monkey Island. In the latter game, when you walked into the SCUMM Bar™, you would see a pirate swinging back and forth on the chandelier if the game was running on a V30 CPU. Before the upgrade, the chandelier would be hanging still without a pirate in it.

I'm still not sure whether these games enabled these graphical details on the basis of timing loops, or by checking whether certain 186+ instructions were present. Probably the former.
 

Patrick.B (TTR)

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well, I have a tandy 1000sx and before I was running an 8088, I could barely run some games (like battle chess for example) I put a D70108C-8 (aka v20-8Mhz and a D8087-1 (aka 8087-10Mhz) and its the best thing I could do to this old box, now I have not found a period game I am not able to play so far. so I am all for the authenticity, but with usability, no one can see the chip and if someone bought it from me, I would provide the original chip for them to downgrade the machine if they so desire.
 
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