[Info-vax] VAX VMS going forward

[gah4]

On Monday, July 27, 2020 at 10:52:36 AM UTC-7, Arne Vajhøj wrote:



(snip)

> I am a bit confused. 

> VMS does not have 32 and 64 bit mode. 

> There is hundreds or thousands of VMS API's and 
> compiler generated code that use 32 bit 
> compressed addresses. 

> But changing all that to use full 64 bit addresses 
> does not really matter for DCL. 

> And P1 addresses are not really a problem. Just let 
> the new images starting using addresses at 4GB. 

All this sounds amazingly close to what IBM did extending from OS/360
with 24 bit addressing, to XA/370 with 31 bit, and z/OS with 64 bit.

Much of OS/360 uses control blocks with 24 bit addresses, with other
data in the high bits.  Anything addressed that way goes "below the line",
addressable with 24 bits.  31 bit addresses can have the high bit set, so that
the system can distinguish them.  (But only in some contexts.)
You can branch between 24 bit and 31 bit code, and back, and the
system will figure it out.

z/OS extended to 64 bit addressing, with 31 bit and 24 bit still around.
The address space from 2GB to 4GB is not used, to avoid some problems
when 31 bit addresses have the high bit set.  That is a z/OS convention,
not enforced, for example, by z/VM or I suspect z/Linux.