jonathan
Tue Jun 25 01:47:33 EDT 1996
Update of /a/cvsroot/src/sys/arch/pmax/pmax
In directory pain.lcs.mit.edu:/a/tmp/cvs-serv14634

Revision/Branch: netbsd-1-2

Modified Files:
	machdep.c 
Log Message:
Fix 4.4bsd/pmax memory-sizing bug:

physical memory is sized by a loop that writes data to the first
word in a page, (writes something else to settle the bus) and then reads
back the word it wrote. If the read succeeds, the amount of physical
memory is increased by one page.

This fails on a 5000/1xx with a memory subsystem filled with 8 low-density
(4Mbyte) SIMMs. The memory-decoding hardware  aliases the 32Mbytes of
physical memory  at physical addresses 0, and at 32M (and presumably
at 64 and 96Mbytes.)  The contiguous aliasing causes the memory-sizing
loop to continue  at 32 MBytes, testing the memory that's really
at address 0,  overwriting and crashing the kernel.

Fixed (for 1.2) by reading the SIMM-decoder stride size from the
motherboard, and reducing the loop bound to 32Mbytes on a 5000/1xx
with low-density SIMMs.  (Other models have a non-power-of-2 maximum
memory and so are not subject to _contigous_ aliasing of physical memory).