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Old 29 April 2019, 13:55   #1
Seiya
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68000 and 68881

just a curiosity about floating point with 68000.
i read that this processor had a emulation logique so, in theory, it can run floating point software without use real 68881 unit.
Are three some old fpu OCS software that work with 68000? As i said, just curiosity.
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Old 29 April 2019, 14:13   #2
Thomas Richter
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If you call "implementing floating point algorithms with integer logic" an "emulation logic", then even a 6502 has a "emulation logic" so it can run floating point giving the same results as a 68881. But this is not a miracle, this is just "implementing an algorithm".

So, the 68000 is in no way special - the mathieee libraries of the operating system just do that, use software to implement floating point.

Other than that, one can, in principle, also address the registers of a 68881/882 as IO-registers and by that run a 68881/882 FPU as an I/O extension of a 68000. That is neither special to the 68000. It is just slow because the 68000 does not have a coprocessor interface, and thus has to make all communications with the FPU by hand. The mathieee libraries up to release V37 supported that by means of the mathieeee.resource, but this was a fairly slow I/O bound process, so it was not very effective and about the same speed as computing the result "by hand" in the 68000.
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Old 29 April 2019, 14:20   #3
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To be fair, the 68000 (and up) do have line-F and line-A exception vectors that allow it to catch unimplemented FPU (or other co-processor) instructions and seamlessly execute the software emulation equivalent.

Properly used, this essentially allows even the 68000 to execute software that assumes a 68881 or 68882 is present (but not actually installed).

AFAIK FPU emulators like this did exist for the Mac, not sure about the Amiga though.
Edit: here is the Mac example I was referring to: https://www.macintoshrepository.org/...oftwarefpu-3-x

Last edited by roondar; 29 April 2019 at 14:29.
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Old 29 April 2019, 20:59   #4
Seiya
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thanks for yours informations
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