[Simh] DMA access to the IO page

[Angelo Papenhoff]

On 06/09/18, Lars Brinkhoff wrote:
> > Apparently, the GT40 does this. So... problems.
> 
> Yes, the boot ROM includes instructions for the VT11 display processor.
> 
> Admittedly, I'm quite clueless when it comes to details about the PDP-11
> and Unibus.  The vt_fetch function in pdp11_vt.c does go through the I/O
> map, but apparently that by itself isn't enough to access the ROM
> region.  The GT40 is built on a 11/05 which doesn't have an I/O map,
> right?

Having written a PDP-11/05 emulator recently I can explain how things
work there.
As Bob said, internal registers are not visible on the Unibus,
the CPU handles the internal addresses itself and "simulates" a Unibus 
transaction.
Everything else it puts on the Unibus, the top page of the 16 bit
address space is mapped to the top page of the 18 bit address space,
the rest is identity mapped. This happens, as Bob said, on the CPU.
The internal registers are:
the PSW, the 16 scratchpad registers (8 GPRs + some temporaries
used by the μcode. words addressed at even and odd locations!!),
the switches, the line clock, the serial console (KL11 look-alike).
On top of that the internal "KL11" and(?) the line clock could be disabled
so you could use a real Unibus peripheral instead.

aap