[Simh] More on DMA to IO page

[Mark Pizzolato]

On Thursday, September 6, 2018 at 1:00 PM, Bob Supnik wrote:
> Looking at the DEC ChipKit documentation, 

[...]

> So my conclusion is that IO mapped PDP11s (which are, of course, always
> Unibus) do support DMA to IO space, so you could put a GT40 (in theory) on a
> Unibus J11, and it would work.
> 
> Okay, here's the bottom line, then.
> 
> 1. There's no need to support DMA to IO space on Qbus systems. It wasn't
> done.

OK.

> 2. Therefore, the ba limit test can be for the 18b Unibus limit on Unibus
> systems, that is,
> 
> if (UNIBUS && (ba >= unibus_io_base_page)) {then read/write IO space}

Done now.

> 3. It's still necessary to keep DMA access away from the CPU and system
> registers, with all the hair that implies.

Done previously.

> That would certainly work for the GT40 and its ROM too.

The register interface to the VT11 really has only one register which is a 16 bit
value that sets the PC of the graphics processor to an address in the PDP11's 
memory where the graphics program that it was to run existed.  Once the 
graphics PC is set, it fetches instructions from PDP11 RAM (or ROM in the
I/O page) and continues on its merry way.  I've changed the code which 
performs graphics processor memory fetch to sign extend all addresses >=
0160000 before being passed into the Map_ReadW().

- Mark