[Info-vax] OpenVMS.Org quick pool

[Johnny Billquist]

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.

> It is feasible to exceed that limit; to address more memory than what
> that 16-bit addressing would allow, from within an application.

It is feasible to access more than 64K of memory. It is not possible to 
address more than a 16-bit address range. And that is what the PLAS 
directives gives you, when running under RSX (for example), or directly 
through changing the MMU registers, if you have access to the I/O page.

>> And yes, I've done plenty of coding under RSX, using large memory
>> regions. Your virtual address is still only 16 bits. What that 16-bit
>> virtual address resolves to in the 22-bit physical address space is a
>> different story, and don't change that my code only deals with a
>> 16-bit virtual address.
>> I can probably recall and present exactly how the PDP-11 works
>> straight from my head without checking the documentation.
>>
>> Maybe you need a refresher on what a virtual address is? :-)
>
> I'll leave it to you to decide which one of us is more familiar with
> virtual addressing, and how to use virtual addressing (limits and all)
> to advantage.

Good. If you feel confident that you understand virtual addressing, then 
we should not need to argue any more over this. :-)

	Johnny