[Simh] RK Word Count register question

[Will Senn]

On 12/26/15 4:26 PM, Johnny Billquist wrote:
> On 2015-12-26 22:56, Johnny Billquist wrote:
>> Short, obvious question. Is simh expecting 16-bit addresses, and running
>> all addresses through the mmu in the command mode, or does it actually
>> use 18-, or 22-bit addresses, in which case you commands are accessing
>> memory, and not the I/O page, which is actually 17 777 777 410, if we
>> talk 22-bits.
>
> Doh! Too many bits in there. 22 bit address should be 17 777 410 of 
> course. :-)
>
>     Johnny
Johnny,

Well. You were right (and others offline) about the address needing to 
be 17777410:

PDP-11 simulator V4.0-0 Beta        git commit id: 0f43551d
sim> d 10000 012737
sim> d 10002 177440
sim> d 10004 177410
sim> d 10006 000000
sim> g 10000

HALT instruction, PC: 010010 (HALT)
sim> ex 17777410
17777410:    177440

I don't really understand what you are saying about the mmu and 18 or 22 
bits other than getting that there are PDP-11's with 16, 18 and 22 bit 
memory sizes. I am guessing that the reason it's 17777410 in this case 
is because the default machine in SimH is 22 bits and the IO space is 
reserved in the top 4k on this machine. If I had been more specific and 
limited SimH to a smaller memory footprint, less bits would be needed 
(the upper 4k would be addressable with fewer bits).

Ah, well, off to read Chapter 6 - Memory Management out of the Processor 
Handbook, again... I have to admit to barely skimming this chapter the 
first few times through because it didn't really make any sense, without 
context and I didn't think I needed to know. Now that it's relevant, and 
with a bit more background, I'm sure it'll make a lot more sense and be 
that much more interesting.

Thanks,

Will