[Simh] Custom ROMs on PDP-11 sim

[Timothe Litt]

On 16-Dec-17 15:42, Paul Koning wrote:
>
>
>> On Dec 16, 2017, at 6:14 AM, Timothe Litt <litt at ieee.org
>> <mailto:litt at ieee.org>> wrote:
>>
>> On 15-Dec-17 22:14, khandy21yo wrote:
>>> Can't you just load them into ram and *run them* from there?
>>> Rom is just non writable memory.
>>>
>>>
>> He could, except that these ROMs are probably in I/O space, so would need
>> to be part of a simulated device for any code to execute properly[1]. 
>
> It would make sense (and it would be very easy) to add a feature to
> SIMH to load a memory image file into what is nominally ROM in I/O
> space.  It could be made ROM from the program point of view (i.e.,
> it's loaded from the user interface but the CPU can't store there).
>
> paul
>
>
Sure, that's trivial.  But a ROM in a peripheral is going to want to
talk to that peripheral, so to "run the code", as the OP said he wanted
to do, the CSRs need to be implemented too.  ROM or RAM, a memory
location doesn't act like a CSR, so the likely result is that the code
will loop, hang waiting for an interrupt, or do something else strange
when the locations that expects are CSRs don't act like a device.

If the goal is to disassemble, loading into I/O space would ensure that
all the addresses are correct.  But if he really wants to *execute* the
code, it's rather more involved.

Again, ROMs embedded in peripherals (he said a robot) are part of a
device, and the code will expect the other parts to be there and to be
functional.  His device's CSRs aren't emulated.  So this is necessary,
but not sufficient.

If it were just a BOOT rom (e.g. a M9301-xx), your suggestion would
work, as it's separate from the device emulation.   (You can read the
M9301 maintenance and operation manual on bitsavers.)

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