[Info-vax] VAX VMS going forward

[Simon Clubley]

On 2020-07-24, Stephen Hoffman <seaohveh at hoffmanlabs.invalid> wrote:
> On 2020-07-24 18:51:20 +0000, John Reagan said:
>
>> 
>> I've been trying to follow this whole conversation but I might have 
>> missed something.
>> 
>> I don't get this whole hybrid thing.  VMS has a 64-bit flat virtual 
>> address layout.
>> 
>> It is just that only parts of it can be accessed with a 32-bit pointer. 
>>  The rest requires a 64-bit pointer.  Due to things like descriptors 
>> and itemlists which have a builtin assumption about only holding 32-bit 
>> pointers, we go out of our way to make sure that anything that you can 
>> take the address of resides in 32-bit space.   On Alpha, Itanium very 
>> little S2 is used and no P2 without explicit code or linker qualifiers 
>> to do so.  x86 will get code in P2 (and trampolines in 32-bit space so 
>> you can still take their address).
>
> Tried building a 64-bit app without having to contend with 32-bit 
> giblets including the 32-bit APIs, the 64-bit APIs, and the either-bit 
> APIs, and the switch qualifiers to get everything to 64-bit, and the 
> stuff that isn't yet 64-bit?
>

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.

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.

> Every time I've tried creating a 64-bit app, I get a sad and revert to 
> 32-bit with hunks of data up in P2 and/or S2.  At one point?haven't 
> verified this limit lately?code wasn't permitted in some of the 
> segments.
>

I hope you are not having to copy the same data between P2 and P0 to use it.

If you are, that would effectively be no better than the old MS-DOS extended
memory APIs (not the earlier expanded memory APIs) where you had use special
techniques to access the extended memory.

See https://en.wikipedia.org/wiki/Extended_memory for details.

Simon.

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