🚀 go-pugleaf

in article <2135@pur-phy>, ng@maxwell.physics.purdue.edu (Nicholas J. Giordano) says:

> Thanks for the tip - I ran setcpu and as you suspected the system is
> not seeing the 68881.

> It is seeing the 68020 though.  I didn't think that the Lucas board would
> boot up if the 68881 doesn't show some signs of life, but maybe that
> is not the case.  Also, I do not get a GURU "B" - instead I get an type
> "D" GURU.

I'm kind of surprised it works myself.  I guess when Exec does it's '881
test, it's just checking to see if an '881 instruction actually works, not
what kind of exception is actually taken if it doesn't work.

Here's the trick.  Normally, the 68020 expects a 68881 to respond to any
68881 instruction, whether it's there or not.  If someone else responds, or
if the 68881 hookup is somehow in trouble, you'll get the "D" GURU, which is
a Coprocessor Protocol Violation exception.  If you really meant to run
without a 68881 in a system, you'd have to decode for a 68881 and generate
a bus error when the '881 is accessed.  That causes the '020 to generate the
F-line exception (a "B" GURU if there's no handler for it) just like a
68000 would.

Since you're trying to use the '881, you need to get it actually working.
There's really not all that much I can tell here about what your '881 is
doing wrong.  First thing I'd check would be the connections to it and
to the logic that creates the '881 chip select.  Something is generating 
the DSACK(s) for that '881 cycle.  First thing I'd look for is the 68881
chip select being made OK.  Next, make sure that both DSACKs and all the
proper address lines are connected.  If it still doesn't work, check all
other connections.  I really can't suggest much else without more info.

> Any more help or suggestions would be greatly appreciated.

> Nick
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