Snooping on the A/B

From: Michael Grove <mgrove_at_iwvisp.com>
Date: Wed, 24 Mar 1999 10:55:02 -0800

Can anyone explain how the A/B logic handles 68040 snooping?

One problem I still have with the 060 on the bus is a memory
controller error. This error is responsible for the machine
failing the video timing test (diagnostics reports this as a
counter error for its own timing routine check), and SCSI
DMA. The COMBEL is not returning an address for the
diagnostics to perform its operation (AFAICT). I think that
when the machine is reset, the DRAM is not being
initialized. The diagnostics handle this by performing the
initialization itself, bypassing the COMBEL. However when
booting from ROM, the COMBEL is failing to initialize the
system. That is why video test, MFP, IDE, dRAM all check out
ok because the COMBEL is not being used for addressing. That
is my theory.

There are some options for the 060 socket that I feel might
effect how this is occurring. The 060 UM states that if an
040 system utilizes the MI pin, this pin must be pulled HI
with the 060 as the 060 does not drive it (MI inhibits
memory devices from responding to an alternate master during
snooping operations). This is true, with the 060 installed,
without the MI pin being pulled HI, the diagnostics will not
work. (However, without the diagnostics, and MI low, I do
get a screen display. The display is consistent with the
troubleshooting guide, looking as if there is a DRAM
problem, and BTW, the processor will accept a warm reset).
It is interesting also that the A/B will not function with
the 040 installed, and the MI pin pulled continually HI
(negated), which is a legal condition for the 040 (according
to MOT).

Back to snooping. Maybe the way snooping is implemented on
the A/B, it is not (for lack of a better word) allowed to
have the MI pin continually negated (it's active low), and
snoop disabled (as it is with the adapter socket). The 060
has only one snoop control pin, snoop invalidate. The 060 UM
says that the SC# line from the 040 bus can be mapped to the
060 snoop control pin, but after looking at the 040UM, I am
a bit confused which snoop control (SC1 or SC0) should be
mapped to the 060 as the 040 (and possible the AB logic) has
four different encoding possibilities. It would appear to me
that SC1 should be tied to SNOOP on the 060, but only if the
last two control encodings for the 040 bus are not used on
the A/B.

Maybe the COMBEL is setting up the RAM, but the data is
dirty. As I understand this, it shouldn't matter if CDIS is
asserted. What ever is occurring, the COMBEL isn't able to
do its job in this application.

Mike
Received on to. mars 25 1999 - 19:51:06 CET