[Simh] More on DMA to IO page

[Eric Smith]

On Thu, Sep 6, 2018 at 1:59 PM, Bob Supnik <bob at supnik.org> wrote:

> Now, as to bus maps. The 11/70 bus map has a jumper-adjustable upper limit
> for what parts of Unibus address space go through the map, and what parts
> are resolved on the Unibus itself. The maximum value is 760000; that is,
> the IO page can never be mapped to memory.
>

By coincidence I was recently looking over the 11/70 (KB11-C CPU) and MK11
(semiconductor memory box) documentation. The latter claims that if you
fill an MK11 with the M8722 storage array boards (256KB/128KW each), to
have 4MB/2MW of memory in a single MK11 box, the 11/70 will not work. I was
surprised; I expected that the KB11-C would simply not perform memory
requests for physical addresses in the I/O page, whether those accesses
originated from the CPU or the Unibus. The MK11 docs say that you have to
omit two storage array boards (if using interleaving) or one board (if
not), so the maximum is 3584KB/1792KW interleaved or 3840KB/1920KW
non-interleaved, rather than 4088KB/2044KW that would otherwise be possible.

Eric
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