Subject:
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Re: Some comments (long) (was: Something else is needed, I think...)
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 7 May 1999 00:47:08 GMT
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Viewed:
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996 times
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Kekoa Proudfoot <kekoa@pixel.Stanford.EDU> wrote:
> Chris Phillips <chris.phillips@computerboards.com> wrote:
> > I'm not surprised that MulDiv isn't supported directly by the H8.
> > But do you know if the H8's MULtiply instruction returns a 32-bit
> > value? (On 16-bit 80x86 processors, for example, this is done using
> > two registers: AX for the low word and DX for the high word of the
> > result.) I believe that this would be a requirement for implementing a
> > MULDIV byte code on the RCX.
>
> Actually, the H8 is worse than this. It only supports 8-bit multiplies,
> storing the result in a 16-bit register. 16-bit and 32-bit operations are
> supported in software by routines located in ROM.
I should clarify this. The 16-bit software multiply takes two 16-bit
numbers and stores their product in a 16-bit number. The 32-bit software
multiply takes two 32-bit numbers and stores their product in a 32-bit
number. The operations are intended to support C-style short and long
multiplies. The upshot is that to do 16-bit fixed point, you have to use
32-bit multiplies. No biggie, I just didn't want to mislead you into
thinking you could get 64-bit temporaries out of any of this.
-Kekoa
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