[Simh] Custom ROMs on PDP-11 sim

[Johnny Billquist]

On 2017-12-17 01:20, Tim Stark wrote:
> Folks,
> 
> Well, there are some PDP-11 tech docs indicates that higher models like 
> 11/53, 11/73, 11/74, 11/83, 11/84, 11/93, and 11/94 have built-in own 
> firmware ROMs at starting 160000 with switch bits for banking.

As Christian said, the 11/74 is not correct to include in this 
discussion, as it is in a different family.
All the others are J11 based machines, and somewhat similar.

Also, some variant of the 11/73 used a double height module, which did 
not include any boot roms, so that one does not have anything around this.

I can't remember how the 53, 83 and 84 works straight up, but I just 
went back and checked the 93 and 94. I think the others work the same, 
but someone should check.

Anyway, the 93 and 94 have an 8K EEPROM, which obviously is too big to 
all fit at the same time in the I/O page. However, it does not map into 
160000 ever. There are two memory areas in the I/O page set aside for 
boot roms.
165000, holding 512 bytes.
173000, holding 512 bytes.

Usually 165000 holds self diagnostics, and the common code for booting, 
while 173000 holds device specific booting code.

The KDJ11 module can use neither, one, or both of these areas. If 165000 
is enabled, it holds the basic, common code, which can then also boot 
using traditional M9312 boot roms, located in 173000. For this, the 
KDJ11 needs to disable its own use of 173000. If you have some module 
with its own booting in general, which uses 165000, you also should 
disable the KDJ11 use of that area (obviously). The enabling and 
disabling of the use of these areas are controlled by a CSR at 177520 
(there don't seem to be a specific name for that CSR).

If the KDJ11 use either of these areas, they are bank switched. The 
KDJ11 creates the full address into the EEPROM by using the low 9 bits 
from the physical address, and the high bits come from a register called 
PCR (177522). One byte is used for the 165000 area, and one byte for the 
173000 area.

All this is documented in the 11/93 and 11/94 manuals.

   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