NEC V20 CPU: A bit of pep for an XT

Last Updated on May 18, 2023 by Dave Farquhar

The NEC V20 was an Intel 8088 compatible CPU that ran slightly faster. It was a niche CPU in the 1980s and 1990s but had a following as a cheap upgrade for power users, especially in instances where motherboard swaps were impractical. It’s popular with retro computing enthusiasts today, as a period-correct upgrade.

The NEC V20 was pin-compatible with the Intel 8088 but included some unique forward and backward compatibility features. It included the 80186 instruction set and could also emulate the Intel 8080, in addition to being faster than the 8088.

The NEC V20 as a cheap upgrade

An NEC V20 is a cheap and easy upgrade for 8088 systems when you can’t or don’t want to swap a motherboard.

The NEC V20 plugs directly into the Intel 8088 socket with no modifications necessary. It’s slightly more efficient than an Intel 8088 running at the same clock speed. That means an NEC V20-upgraded machine performs anywhere from 8 to 30 percent faster than an 8088, depending on the application. For most applications, the improvement is around 20 percent. That’s modest but may be enough to be noticeable.

The NEC V20 was a good processor upgrade option in machines that used nonstandard motherboards like the Tandy 1000 and IBM PCjr, and inexpensive PC/XT clones like the Leading Edge Model D. In an IBM PC/XT or other clone that followed the IBM design, swapping in a small form factor AT motherboard provided more performance. But it cost more too.

Today, replacing the 8088 in an IBM PC or XT with a NEC V20 remains a popular upgrade option. It improves performance while permitting the owner to continue using the original IBM motherboard. Plus it’s a period correct upgrade involving a vintage part from 1984. And a fair number of people in the know made the swap back then.

NEC V20 variants

The NEC V20 was available at speeds of 5 MHz, 8 MHz and 16 MHz. An 8 MHz version is suitable for upgrading PC and XT machines running at 4.77 or 7.16 MHz. The 16 MHz version is suitable for 9.54 MHz XT clones.

The V20 carried a part number of D70108C, so you sometimes hear it referred to by that name. NEC had three licensees who also made the chip in the 1980s. So if you find a Sharp LH70108, Sony CXQ70108, or Zilog Z70108, they are all functionally the same chip.

The V20HL is a slightly later revision that runs cooler than the original.

Even though I frequently see retro computer enthusiasts on Youtube putting heatsinks on their V20s, it’s not really necessary. That’s especially true of the later V20HL variant. The chips run cool anyway, and in PC applications they’re always running at lower than their rated speed. It doesn’t hurt anything, but it’s overkill.

What’s the point?

When you install an NEC V20 in an IBM PC 5150 or XT 5160, it’s still an IBM. Although a faster motherboard will fit in an XT, it’s not an IBM anymore once you do that. The V20 lets you retain most of the machine’s originality and it runs a little bit faster. And if you ever need to revert the mod, it’s easy. It only takes a couple of minutes to swap the 8088 back into the CPU socket.

In a PCjr or Tandy 1000, the NEC V20 gives you a slightly faster CPU to complement the machine’s enhanced graphics and sound. Some of Sierra’s later titles run poorly on a stock 8088 and benefit from the boost. A Tandy 1000 TX or TL with a 286 will give better performance. But that also means you have to find one. The 8088-based Tandy 1000s were more plentiful.

The other nice thing about a NEC V20 is that some network drivers that say they require a 286 will work on a V20. If you want to network your XT-class machine to make it easier to load software on it, this is helpful.

Speeding up your XT IDE interface

If you have an XT IDE interface and use a compact flash card with it, installing a V20 CPU and reassembling the XT IDE BIOS with V20/80186 instructions speeds up IDE access almost 90 percent. If you use a Tandy 3 in 1 card, Rob Krenicki has already built and provided a suitable V20 optimized BIOS for that purpose. On my Tandy, disk throughput went from around 250K per second to around 500K per second.

So even if the benefits when running software can be inconsistent, an NEC V20 has a very noticeable impact on your loading times. And everyone likes faster loading times.

The NEC V20 as an Intel 286 alternative

The NEC V20 implements the Intel 80186 instruction set. The 186 is a largely forgotten CPU that saw much more use in embedded applications than as a CPU in PC compatibles. The later 286 and 386 processors proved far more popular and enduring. But since a fair bit of software that claims to require a 286 processor actually only uses 8086 and 186 instructions, the V20 has reasonable compatibility with the 286.

The V20 isn’t as fast as a 286 partly because it’s limited by the 8088’s 8-bit data bus. A V20 running at 7.16 MHz benchmarks about half as fast as the original IBM PC/AT, with a 6 MHz 286. But at least it allows some software to run that wouldn’t run before.

The NEC V20 in the 1980s and 1990s

The big-name PC vendors like IBM and Tandy stuck with Intel 8088s and its authorized second-source providers like AMD and Siemens. However, by the late 1980s it wasn’t hard to find XT clone boards with factory-installed V20 processors. These found their way into many white-box PC clones. They provided an alternative for people who needed something faster than an 8088 but couldn’t afford a 286.

Power users were aware of the V20, and I knew of a few people in the St. Louis area who took advantage of the V20’s Intel 8080 compatibility mode to run CP/M software on their PC clones.

It wasn’t necessarily something you’d find at just any computer store. It was more of a back pages of Computer Shopper thing.

NEC V20 vs Intel 8088

When it comes to an NEC V20 vs Intel 8088, the chips have obvious similarities. But while there were lots of second-source 8088s, made with Intel’s blessing, the V20 wasn’t one of them. The V20 is more like the spiritual ancestor of Cyrix processors: independently reverse-engineered and pin-compatible with the dominant Intel processor of the time, with some more advanced technology bolted on.

The 8088 wasn’t the most efficient CPU of its era and NEC tried to address that, which is why a V20 is a bit faster than an 8088 when running at the same clock speed. The V20 gets about 20% more work done per clock cycle than its more famous competitor.

The V20 also ran at higher clock rates than the 8088, generally at 8, 10, and 16 MHz, but in PC applications it was generally clocked at 7.16 or 9.54 MHz. When you swap it in for an 8088, it runs at the same clock speed as the 8088 it replaced.

The V20 is still an XT-class CPU, like the 8088. But if you have an 8088-based system and want to put a little more spring in its step, the V20 is a reasonably priced upgrade to do so. It was affordable in the 1980s. And it’s still readily available and affordable today.

The PC-Sprint and the NEC V20

In the mid 1980s, there was a hack going around called the PC-Sprint. This was (and is) a free project that made a cheap accelerator for an IBM PC or XT. It also works in some of the more popular compatibles like the original Compaq Portable and the early Tandy 1000/1000A. In theory it might work with some other XT clone motherboards too. By adding the PC-Sprint daughterboard to a PC or XT and swapping the CPU, you can run a PC or XT at 7.16 MHz instead of 4.77 MHz. Some people call this the first overclocking project, but it’s not really overclocking. You’re swapping the CPU for one that runs at the rated speed, and the whole point of the mod is running the rest of the motherboard at 4.77 MHz.

In theory you probably could run the stock 8088 at 7.16 MHz. And that would be overclocking. But it’s also less reliable.

The PC-Sprint of course works fine with the V20. The V20 is designed to run at 8 or 16 MHz, so 7.16 MHz doesn’t hurt it at all. A PC or XT running a V20 at 7.16 MHz will run noticeably faster than the original. It should give a 50-75% improvement. It still won’t be as fast as a 6 MHz 286, but it will make a PC or XT more enjoyable to use, while retaining the soul of the machine. And like the V20, you can revert the PC-Sprint mod in a matter of minutes.

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One thought on “NEC V20 CPU: A bit of pep for an XT

  • April 13, 2021 at 6:02 am

    I’ll never forget my first build in 1989. 10MHz 8088. 1MB RAM, 10MB MFM HD, Paradise SVGA w/512K RAM. Very first game I bought for my newly created rig – Sierra Online’s Hero’s Quest, later renamed So You Want to be a Hero. Kept getting artifacts on the screen with the game. Did some reading, discovered my CPU was too slow. Did some research, found my solution in a Jameco catalog I got from tech school. Filled out the order form, and 2 weeks later, my upgrade arrived. WOW I WAS AMAZED at the huge speed boost. It wasn’t “a bit” or “a little”, it was a very large noticeable difference. I later sold that system (because I upgraded to an 80386-16 computer) to the head of security at work. The next day he commented “You’re right that computer’s fast, it’s almost as fast as a ‘286!”. Those were the days.

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