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

[Mark Pizzolato]

Hi Will,

This seems to be a legitimate problem given the inconsistencies that you are seeing.

Please create an issue at: https://github.com/simh/simh/issues outlining what you've said below and that will be used to track this to a resolution.

Thanks.

- Mark

On Monday, December 28, 2015 at 3:58 PM, Will Senn wrote:
> 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?