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running cpu faster then the math co processer

per said:
Epson still keep support pages for their old machines! Have a look here:
http://www.epson.com/cgi-bin/Store/support/supDetail.jsp?BV_UseBVCookie=yes&infoType=Doc&oid=14214

According to the user-manual, the speed is selectable between 8MHz and 10MHz. If I understand correctly, the 8087 will only be active in 10MHz mode.
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That's so...bizarre. I specifically remember reading that same doc and seeing 5/10mhz and I looked at it more then once. god, i'm getting old, my mind is going.
 
per said:
According to the user-manual, the speed is selectable between 8MHz and 10MHz. If I understand correctly, the 8087 might only be active in 10MHz mode.
Click to expand...

Possibly a translation error. I think they meant that 8087-1 is required to support 10MHz frequency. I see no reason why 8087-1 won't work at 8 MHz.

From the User Manual:
Math coprocessor 8087-1 microprocessor (optional) for use at 10 MHz

From one of support bulletins:

Q21. Which math coprocessor will work in the Equity Ie?
A. 8087-1 (10MHz. The co-processor is installed by simply plugging it into the socket.
No Setup is required because the ROM BIOS automatically checks for the
presence of a co-processor when the system boots up.
 
I later saw the product sheet states 8/10MHz 8087 NPU, so I guess it follows the CPU speed then.

How I read the quote from the user manual was as if it exclusively ran at the 10MHz mode.
 
I don't think you *can* run your CPU faster than your FPU on most systems, because they are just clocked off the same base frequency.
The only exception I know of is the IBM PC/XT 286, which clocks the FPU at 4.77 MHz even though the CPU runs at 6 MHz.
So apparently it is possible, but only if your motherboard was designed to do so, which afaik 99% of them are not.
 
Many 286s, 386SX and 386DX systems are capable of running the FPU at a different speed than the CPU (can be slower OR faster). However, on 386 systems it will only work with intel FPUs I believe. I don't believe any 808x systems are capable of doing it...maybe if they were adapted to a 287, but I doubt that is even possible.
 
modem7 said:
[IBM 5162 CPU/NPU diagram] (5162 = XT/286)
[IBM 5170 CPU/NPU diagram]
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Interesting how they wired it up. So there is actually a different clock divider in the FPU?
This site also mentions that: http://www.cpu-world.com/CPUs/80287/
The 80287 may run asynchronously from the main processor. The 80286 processor divides external clock by 2, while the co-processor divides it by 3. If both the 80286 and 80287 use the same external clock it results in the co-processor running at the 2/3 speed of the main processor. Nevertheless, the ability to run asynchronously can be used to its advantage. Using dedicated external clock for the Intel 80287 it's possible to run the co-processor at higher frequency than the main processor.
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Quite interesting. Seems to be a feature unique to the 80287. I wonder why they did that. Perhaps because they had difficulty scaling up the FPU speeds to the same speed as the CPU? Perhaps that would make 8+ MHz 287s too rare/expensive?

This is also depressing to read:
However, due to the additional overhead of
the 80287's CPU/coprocessor interface (at least ~40 clock cycles), an 8 MHz
80286/80287 combination can have lower floating-point performance than an
8086/8087 system running at the same speed.
Click to expand...
 
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