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Thread: Matching clock speed of 8087 with 8088

  1. #1

    Default Matching clock speed of 8087 with 8088

    So, I’m looking at filling the empty 8087 socket in my LSI Octopus and as its 8088 runs at 8MHz, I assume it would require (at least) an 8MHz 8087-2.

    My question is, would I be able to fit the faster (and slightly cheaper) 10MHz 8087-1 coprocessor? I only ask because the Wikipedia details for the Intel 8087 states “Unlike later Intel coprocessors, the 8087 had to run at the same clock speed as the main processor”.

    Oh, and does anyone know of a source of genuine 8087s or are those Far East outfits on ebay OK?
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  2. #2
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    10 MHz chip on 8 MHz motherboard will happily run at 8 MHz so it will work.

  3. #3

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    I have an Intel D8087 with box and manual if that's of interest.
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  4. #4

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    Quote Originally Posted by krebizfan View Post
    10 MHz chip on 8 MHz motherboard will happily run at 8 MHz so it will work.
    Thanks, that's what I thought (hoped).

    Quote Originally Posted by Stone View Post
    I have an Intel D8087 with box and manual if that's of interest.
    Thanks Stone, but I'm about to loan a 10MHz chip, with the option to buy it if I want.
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  5. #5

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    Quote Originally Posted by SteveH View Post
    “Unlike later Intel coprocessors, the 8087 had to run at the same clock speed as the main processor”.
    that part is VERY poorly worded and misleading. It has to RUN at the same clock speed as they're fed by the same bus clock. They don't have to be RATED the same speed so long as they are both rated as fast or FASTER than the bus.

    The rating on a CPU isn't the fixed speed it has to run at, it's just the top speed it is designed for. Hence why most 8088's were rated from 5 to 8mhz even when they were put into 4.77mhz machines. The AMD 8088-2 that went into the lions share of real PC's (XT's in particular) I've ever opened are all capable of 8mhz easy, and the real Intels were rated to 5. It's just that the 4.77 clock was under that 5 of the intel and easy to grab off the 28.62mhz NTSC colour clock crystal.

    Same for Tandy's 7.16mhz which was simply 1/4th the colour clock instead of 1/6th. (though there were other factors that let Tandy pull that off)
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