[Info-vax] OpenVMS.Org quick pool

[Stephen Hoffman]

On 2012-08-24 16:47:40 +0000, Johnny Billquist said:

> On 2012-08-24 18:32, Johnny Billquist wrote:
>> On 2012-08-24 17:56, Stephen Hoffman wrote:
>>> On 2012-08-24 14:43:55 +0000, Johnny Billquist said:
>>> 
>>>> On 2012-08-24 14:48, Stephen Hoffman wrote:
>>>>> On 2012-08-24 02:58:03 +0000, Johnny Billquist said:
>>>>> 
>>>>>> Oh. I should probably point out that "22-bit address" is the physical
>>>>>> addressing of the machine. Virtual addresses are always 16 bits, even
>>>>>> on modern machines with 22-bit physical addresses.
>>>>> 
>>>>> So you don't know how to or haven't had to program a PDP-11 box past
>>>>> 16-bit addressing, or another analogous use of (lowercase-W) windows?
>>>>> Ok.
>>>> 
>>>> Huh? Are you saying that I'm wrong... I am not.
>>> 
>>> What you're reading in the PDP-11 manuals is correct, as far as it
>>> goes.  A PDP-11 does have 16-bit addressing.
>> 
>> A *virtual* address on a PDP-11 is *always* 16 bits. It cannot be
>> anything else. No matter which model, which features you turn on, nor
>> which kind of other tricks you think you came up with.
> 
> This is actually just the same as on a VAX, where a *virtual* address 
> is always 32 bits, even though a physical address is either 30 bits, or 
> 34 bits, depending on the CPU, and a setting in a register.
> 
> I would assume you feel way more comfortable with the VAX, Hoff?

You're arguing specs.

Now please read what I am writing.

You can exceed those specs.

Can you do it all at once?  No.  But - with virtual memory - and 
there's a whole lot of support activity going on underneath, within the 
realm of the virtual memory system - to emulate the appearance of 
logically contiguous physical memory.  (I was going to write 
"non-segmented", but that's not the case.)  Nothing precludes you, the 
programmer, from doing the same thing within your application as is 
going on within the operating system and the hardware, and referencing 
(though not all at once) more than what the address space allows you.  
Hence you can keep more than 16-bits or 32-bits around, between 
whatever available unmapped physical memory is present, and available 
disk storage.

Implementing this addressing is rather more work, but - if you're stuck 
on a PDP-11, VAX or Apple II, for that matter - it's one of the ways 
you write a larger application, or reference larger sets of data.

But it means that - as I keep referencing - you can have more than 
16-bits of "stuff" around.


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