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From: Milo Velimirovic <milov@toes.its.uwlax.edu>
Date: Wed, 22 Apr 98 10:33:34 -0500
To: edgee@cyberpass.net
Subject: Re: Floating Point-How Important
Cc: pups@minnie.cs.adfa.oz.au
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Hi Ed,

Begin forwarded message:
[snip]
>
>> Sounds reasonable to me, but rather than worry about the sign of the data, I'd
>> just decode the instruction far enough to see how much data followed, and skip
>> over it.
>
>Your suggestion, of course, is emminently reasonable.  Sometimes it's 
>easier to argue about an old approach than think up a new one.
>
>I'm looking at page 2 of Bob's pdp11_cpu.c source which has a nice 
>summary of the various instruction formats for the PDP-11.  The 
>summary includes octal ranges for double, single operand instructions etc.

any of  digital's pdp11 processor handbooks would be a good resource too.

>
>How about this approach to determine how many words to skip when 
>scanning for floating point op codes:
>
>Assuming Bob's summary is complete, my program could classify
>instructions as either double, single or no operand based on the
>range they fall in. No operand cases could be discarded.  Single and
>double operands would have to be further decoded to see if they use
>any of the PC addressing modes (immediate, absolute, relative, and
>relative deferred) and the appropriate number of words skipped.
>
>Would this work?  Have I forgotten anything important?

Yes, other registers can be used with indexed addressing modes (6 and 7)
and these too would place additional words in the instruction stream.

>
>Ed
>
>
--Milo
---
Milo Velimirovic       <Milo.Velimirovic@uwlax.edu>
Unix Computer Network Administrator  (608) 785-8030
Information Technology Services -- Network Services
University of Wisconsin - La Crosse
La Crosse, Wisconsin 54601 USA    43 48 05 N 91 14 22 W