On 2018-09-07 00:26, Eric Smith wrote:
On Thu, Sep 6, 2018 at 4:12 PM, Johnny Billquist <b...@softjar.se
<mailto:b...@softjar.se>> wrote:
The problem is that the MK11 memory boxes also have CSRs, and those
are living in that last 8K space. If you have memory mapping those
same addresses, I have no idea what happens, but either way is not
good.
Either you have the memory, in which case the CSRs are unaccessible,
and they control how the memory box works. Or else you have the
CS‰s, but what then happens to those memory cells?
You're right. The Unibus does not go to the MK11, which only connects
via the memory bus to the KB11-C cache, so at least some I/O page
accesses have to go across the memory bus for the MK11 CSRs. Rather than
decoding and only sending the specific MK11 CSR I/O page accesses to the
memory page, the KB11-C cache system most likely just sends all I/O page
accesses to both Unibus and the memory bus, expecting only one to
acknowledge.
Actually, it's more complex than that.
If you access the I/O page on the 11/70, it will only get directed to
the Unibus. No transaction happens on the memory bus.
To get to the MK11 CSRs, you have to be really tricky. You need to
enable the unibus map, pointing that to the I/O page, and then access
memory in the Unibus map space (the high 256K), with the map enabled,
which will then remap your memory access to the I/O page, but will run
it over the memory bus, thereby getting access to the MK11 CSRs.
Like I said - it's complicated... :-)
If I was desperate to have 4088KB/2044KW, I could probably kludge up
additional logic to the MK11 control modules to explicitly disable the
RAM operation for the entire I/O page range. However, I have only ten
M8722 modules (2.5MB/1.25MW), so it isn't something I actually need.
Well, you'd probably also need to deal with the Unibus map handling,
since those addresses also do not generate accesses directly on the
memory bus...
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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