Sunday, January 20, 2019

The IBM PC 5150: Part 3 - The CPU and FPU

Continuing on from Part 2, we're now looking deeper into the IBM PC 5150 I bought off Craigslist.

The CPU and FPU sockets are behind the right floppy, underneath the two grey hard drive ribbon cables and colored power supply wires. As this PC was pretty "decked out", having an i8087 floating point chip was a real possibility. And, as my first PC, a Compaq Deskpro, had an 8086 processor, this was actually my first 8088 machine purchase. Ok, to nudge my cell phone camera in there and take a shot......

What?? The two long sockets behind the keyboard and cassette powts hold the processing chips, CPU on the pictures left, FPU on the picture right. There is no 8088 chip! We know the machine runs, what gives??

The chip in the socket is a NEC V20. I know what that is but I looked it up to get up to speed. Via Wikipedia:
The NEC V20 (μPD70108) was a processor made by NEC that was a reverse-engineered, pin-compatible version of the Intel 8088 with an instruction set compatible with the Intel 80186. The V20 was introduced in 1982, and the V30 debuted in 1983.
The chip featured much more than the 29,000 transistors of the simpler 8088 CPU, ran at 5 to 10 MHz and was around 30% faster (application dependent) than the 8088 at the same clock speed, primarily due to faster effective address calculation, along with faster loop counters, shift registers and multiplier.
Ok, so at some point the owner swapped CPUs to get more horsepower. Not bad. But....

I was very disappointed I didn't have the original CPU. I complained to my family and stomped around. It wasn't until later when a thought occurred to me: check out that bag of extras he gave me! In the bag was:

  1. One gender changer from a RJ type connector to serial port, maybe handy for some terminal type keyboards.
  2. An in-packaging DB9 to DB15 adapter. That _might_ allow the CGA display adapter to connect to a compatible multisync VGA+ display. With a MicroCenter receipt for this dated 1996. Nice.
  3. Two paper manuals he threw in, Quatro Pro for Windows and Toolworks Backup Pro. That software may not be on this PC but old documentation is getting scarce.
  4. A parallel to pin header cable, which should belong to one of the expansion cards.
  5. A plastic box I hadn't seen until looking.
The plastic box was labelled V20 on the front. Opening it provided great joy:

There was the 8088 processor. I assume the PC owner had gotten the V20 as an upgrade at some point and put the original CPU in the V20 box. Yay! We can see the CPU is dated 1982.

But what? That's not an Intel chip, it's an AMD. Aren't AMD and Intel enemies? Didn't IBM only use Intel chips? No worries for me. Wikipedia helps here too:
In early units, the Intel 8088 used was a 1978 version, later were 1978/81/2 versions of the Intel chip; second-sourced AMDs were used after 1983 (no citation). Some owners replaced the 8088 with an NEC V20 for a slight increase in processing speed and support for real mode 80186 instructions. The V20 gained its speed increase through the use of a hardware multiplier which the 8088 lacked.
At the time this IBM 5150 was built, the Intel 8088 was being "second sourced" or licensed to other companies to manufacture. This was often done to reassure users that a steady stream of chips may be available, even if one company (like Intel) had a production issue. Taking a look, Wikipedia says of the 8088:
Common manufacturer(s)
Wow, that's a lot of licensing. There were many PCs and clones made and the 8088 was put into many designs so it's not surprising. There wasn't the friction between AMD and Intel that we've had since the Athlon days.

Ok, we have two CPUs (one in-place, one in-hand), what about the 8087?

The picture shows that an 8087 is not in the socket, the socket is empty. The dust would indicate the original owner probably never bought one and put it in (or had one for any length of time if they did). That's ok with me for now. Why?

  1. Unlike the floating point units in later PCs, the 8087 was not used widely in software. The math operations could be simulated with 8088 based routines but the calculations would be slower. For any software I may be using, having an 8087 would probably not matter. 
  2. I looked on eBay and 8087 chips appear to be a lot less expensive than in the 1980s. Less than $10 US although that would be from China and one risks getting a fake chip.
Ok, so much for a more adventurous look at the processors in this IBM PC. I'll get to the expansion cards next which will allow a better look of the motherboard beneath. Thanks for reading along, if you all would like more or less detail, etc. let me know in the comments.

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