[Info-vax] VAX VMS going forward

[Simon Clubley]

On 2020-07-28, Arne Vajhøj <arne at vajhoej.dk> wrote:
> On 7/28/2020 3:09 PM, Stephen Hoffman wrote:
>> On 2020-07-28 15:42:44 +0000, John Reagan said:
>>> On Monday, July 27, 2020 at 5:55:52 PM UTC-4, Stephen Hoffman wrote:
>>>> Getting a flat address space and getting rid of the mumble() and 
>>>> mumble64() and 64-bit objects is no small development project, and 
>>>> that's past the remediations for the forever-32-bit-app dependencies 
>>>> that lurk.
>>>
>>> The address space is flat.  The CPU ensures that.  There are no 
>>> segment registers on Alpha, Itanium, and x86 (in its 64-bit hardware 
>>> mode).
>> 
>> The CPU is certainly capable of this, yes. I've never stated that the 
>> Alpha and Itanium processors were not 64-bit. They are.
>> 
>> But until we've expunged all discussions of P0, P1, and P2, OpenVMS has 
>> segmented addressing.
>
> Obviously that depends on how you define segmented memory.
>
> But using Wikipedia definition:
>
> https://en.wikipedia.org/wiki/Memory_segmentation
>
><quote>
> In a system using segmentation, computer memory addresses consist of a 
> segment id and an offset within the segment. A hardware memory 
> management unit (MMU) is responsible for translating the segment and 
> offset into a physical address, and for performing checks to make sure 
> the translation can be done and that the reference to that segment and 
> offset is permitted.
></quote>
>
> then no.
>

I would say yes because the segmented definition is wider than that.

For example, I would consider Harvard architecture CPUs to have what is
effectively segmented memory due to how different instructions are used
to access different memory types, although I've never really thought of
it as segmented memory until now.

I mention that because the above definition doesn't even begin to address
Harvard architectures, let alone those architectures which have multiple
independent memory spaces with their own addressing (for example AVR).

In Stephen's case, what you are allowed to do with the various parts
of the user accessible address space at VMS API level depends on which
part of the address space you are trying to use.

I would call that segmented memory.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Walking destinations on a map are further away than they appear.