Co-Processor or Floating Point Unit (FPU)

    Co-Processor, Math-coprocessor, Math Accelerator or FPU. They enabled the operating system to deal more efficiently with large calculations like those involved in spreadsheets drawing programs and of course many games. In fact many programs refuse to even run if they do not detect the presence of a Co-Pro. 

    Older PC's typically the 386 and 486 often had a SX or SLC. e.g. 386 SX.  

This meant that there was no Co-Pro which originally would have been on a separate chip. The motherboard would have had a socket which would accept a Co-Pro chip. The name would have been the same except that the 6 on the end would be a 7. e.g. 387. Later CPU chips were made with inclusive Co-Processors. 

The 386 SX was the first 32 bit CPU and therefore the ancestor of the chips that lead to the Pentium. It was only 32 bit internally with the memory bus still only 16 bit. The 386 DX was fully 32 bit and therefore should have run faster than a 386 SX working with a 387 (Co-Pro) chip. 

    486 chips came as SX or DX.  Both were fully 32 bit and now the SX only meant that there was no Co-Processor incorporated into the CPU itself, whereas the DX did have one.  There was a story that the chips were being made from the same dies and masks but that the "SX" version was produced by burning out the Co-Pro. Leading to the nickname of "Brain-Damaged" chips. Speeds were determined by running them and measuring the temperature they ran at. So if for example the CPU was designed to run at 33 MHz. The ones that ran the coolest would be sold as 33 MHz and the hotter ones as 25 MHz. As manufacturing techniques improved and so many passed the quality control, there was more than enough for the faster speeds and the market for the slower speeds was satisfied with more than capable chips. Word got out and over-clocking became popular. Some suppliers would even do the testing for you. One could buy from them a chip guaranteed to run at a faster speed than the nominal one. Some manufacturers did not like this so "locked" chips which would only run at specified speeds were produced. This could be got round by plugging in a small adaptor board with jumpers on it between the CPU and the Motherboard. These were manufactured specifically for certain chips. Even today I am told that on some chips the higher speeds can be enabled by writing on the back of the chip with a pencil to make an electrical contact between two copper contacts. There was even a chip which could be piggy-backed onto the existing CPU to upgrade the Processor.  With such names as  "MakeIt486" these devices would convert your 386 SX to a 486 DX but there was still the limitations of the old motherboard. The economics were not good however as the cost was always about the same as changing the motherboard and buying a proper 486 CPU. It was however very easy and therefore more appealing to the not so technically minded.

     Nowadays it would be virtually impossible to get hold of a 387 chip. So I use a software emulator which does a good job of fooling the Operating System that the hardware is actually on board. It uses Extended memory leaving the base memory unaltered. Works with DOS, Windows 3.1 and also (though I have not tried it) on Windows 95. I benchmarked a few programs by timing how long they take to open in Windows 3.1 on a 386 SX and found that it varied from no difference to a reduction from 16 seconds to 9 seconds on a desktop publisher.

    Q87 is a Math-coprocessor emulator. It was originally distributed as a 20 minute limited demo. Q87Cr does not have any limit and is fully working. Download it from this site and copy it to a temporary directory. UnZip it and read the documentation.