[Simh] I/O Registers in SimH PDP-11

[Will Senn]

All,

In looking at bootloaders I have observed some interesting SimH behavior 
and I am hoping y'all might have something to say about it.

In sum, is it possible to deposit values directly into the I/O registers 
from the sim> prompt, if so, can you provide an example using the RK or 
RP command register?

I have tried the following in the default PDP-11/73 with 4MB Memory:

PDP-11 simulator V4.0-0 Beta        git commit id: 0f43551d
sim> d 17777410 177540
Address space exceeded

This would seem to answer the question in the negative. However, if I 
then put info into the buffer by typing in some machine code and 
executing it directly:

PDP-11 simulator V4.0-0 Beta        git commit id: 0f43551d
sim> id 10000-10006
10000:    012737
10002:    177540
10004:    177410
10006:    000000
sim> g 10000

HALT instruction, PC: 010010 (HALT)

and then reference the I/O buffer in SimH:

sim> ex 17777410
17777410:    177540

The I/O buffer now appears to be available for examine.

sim> ex 17777410
17777410:    177540

And for deposit:
sim> d 17777410 177530
sim> ex 17777410
17777410:    177530

Interestingly, Address  space exceeded doesn't appear to be a problem. 
What is going on with SimH's PDP-11 simulation. Is the device not really 
"there" until it is referenced from a machine code program, or is the 
17777410 address not initialized until the device is accessed, or what?

Thanks,

Will