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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro.68k,net.micro.16k
Subject: Re: Re: PDP11s vs the micros
Message-ID: <5900@utzoo.UUCP>
Date: Thu, 22-Aug-85 13:08:55 EDT
Article-I.D.: utzoo.5900
Posted: Thu Aug 22 13:08:55 1985
Date-Received: Thu, 22-Aug-85 13:08:55 EDT
References: <1617@hao.UUCP> <847@mako.UUCP> <2422@sun.uucp>, <223@uthub.UUCP>
Organization: U of Toronto Zoology
Lines: 18

> [discussion about saving floating-point-unit state only when needed]
> Careful reading of the specs for the National FPU, plus a little experimenting,
> shows that they also provided this capability, though their implementation
> has the look of being a fortuitous accident (hint: check the behaviour of the
> Trap Type field of the Floating Status Register).
> Unfortunately, whoever wrote the documentation made no mention of this
> use, which suggests that they don't realize what they have and are in danger
> of making it not work in future releases of the hardware.

It should also be possible to get a similar effect by using the SETCFG
instruction to tell the cpu "no floating point", which will produce a
trap when the user tries to use floating point.  Save the state and then
turn floating point on again.  When I asked the local National man about
this, he said it would work.  Beware that I have *not* tried it on real
hardware yet.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry