IBM 5160 math copro switch

Jules Richardson jules.richardson99 at
Mon Sep 7 08:49:59 CDT 2020

Sifting through my XTs and clones... I've had three so far with no math 
copro (8087) fitted, but switch 2 of SW1 is set to off.

IBM docs say off is "math copro installed" and on is the "installed" 
position, i.e. the reverse of what I'm seeing (the minuszerodegrees site 
repeats this, although I expect that just copied what's in the manual, too).

These three machines (two are 5160's, the other a clone) came from 
completely different sources - it seems unlikely that all three had 8087's 
fitted which were pulled at some point. Googling a few more system board 
images, ones with 8087's have that switch on - but for boards without, the 
setting seems to be pretty much 50/50.

Did the meaning of the switch perhaps change at some point (both of my 
5160's are late model 256-640 boards), and rather than being a simple 
installed / not installed it was more along the lines of "software should 
use it if present / software should never use it"? After all, it's not like 
the BIOS will do anything with an 8087; it's only there for software 
specifically coded to make use of it.

I suppose it's also possible that users were in the habit of first setting 
all the switches to off when configuring a machine, then setting the 
relevant ones to on according to their memory/floppy/video config - and the 
copro setting just got overlooked.

Weird, anyway.


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