[Info-vax] VAX VMS going forward

[Simon Clubley]

On 2020-07-27, Arne Vajhøj <arne at vajhoej.dk> wrote:
> On 7/27/2020 8:47 AM, Simon Clubley wrote:
>> 
>> John, this is why the current setup is not a flat 64-bit implementation
>> but is a hybrid of 32-bit and 64-bit modes.
>
> It is not a 32 mode and a 64 bit mode.
>
> The CPU always use 64 bit addresses.
>
> A program is not running in any mode.
>
> It is API's that can use either 64 bit addresses or 32 bit "compressed"
> addresses (by only passing the lower part and assuming high bits
> are all 0).
>
> In many ways that is way harder to fix than 32 and 64 bit mode.
>

Only on VMS. This is (mostly) not a significant problem on other
operating systems because the vast majority of the work is done for
you by the compiler and linker in those environments after you tell
the compiler whether you want your code to run with 32-bit or 64-bit
addresses and other features.

>> The current 64-bit memory model on VMS reminds me somewhat of the old
>> segmented memory models that used to exist under MS-DOS before the flat
>> 32-bit address space came into use on the x86.
>
> Not close.
>
> That was a CPU feature/problem.
>
> This is an API feature/problem.
>

True. But it gives the same feeling about having to manage where you
write the data and how.

Simon.

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