9 Jan
2002
9 Jan
'02
10:47 a.m.
On Jan 9, 18:19, Tom Leffingwell wrote:
I think I finally get the picture on the addressing...However,
that raises two more questions. If my program is trying to talk to a
22-bit address, and I have 18-bit addressing, will it not work, or will
it
be converted?
It will be converted. When the CPU is running a program, it only uses
16-bit addresses in the program. The MMU treats anything above 160000 as
an access to the I/O page, and remaps it.
Also, if my backplane becomes 22-bit (by replacing it
or
adding the jumpers for the other 4 bits) does everything automagically
change to 22-bit, or do you change a jumper on the M8186, or on the
MSV11,
or both?
It automagically works, except for a very few cases (and I can't even think
of an example at the moment). The reason is that most I/O devices actually
decode a signal called BBS7 (Bus Bank Select 7) instead of the highest
address bits. The signal gets its name from the fact that the original
LSI-11 used 16-bit addressing, and bank 7 was the I/O page. It's still
only activated for I/O page access, regardless of whether your processor
uses 16- 18- or 22-bit addressing.
--
Pete Peter Turnbull
Network Manager
University of York