PDP11 and Simh Floating point

Paul Koning paulkoning at comcast.net
Tue Jul 18 10:38:24 CDT 2017

> On Jul 18, 2017, at 11:04 AM, Douglas Taylor via cctalk <cctalk at classiccmp.org> wrote:
> How does SIMH perform floating point operations for the PDP-11 simulation?
> If I run a numerical computation on real PDP-11 hardware and also on Simh how do you talk about the comparison?

You can look for yourself: PDP11/pdp11_fp.c.  The code clearly implements the floating point operations the hard way: picking apart the pieces and manipulating them as needed. 

For stuff like this, the devil is always in the details.  Some floating point implementations have odd corner cases that aren't necessarily well documented, if at all.  But it's clear the intent is that SIMH should produce the same answers as a real PDP-11.

An example of floating point emulation that would be wrong is to convert to IEEE and operate on "double" values, then convert back.  I've done that in the past as a first stab, but you can't do that if you want the emulation to be anywhere near accurate.


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