[Simh] More on DMA to IO page

[Johnny Billquist]

On 2018-09-07 00:26, Eric Smith wrote:
> On Thu, Sep 6, 2018 at 4:12 PM, Johnny Billquist <bqt at softjar.se 
> <mailto:bqt at 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: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol