[Info-vax] OpenVMS.Org quick pool

[Stephen Hoffman]

On 2012-08-23 18:05:57 +0000, Johnny Billquist said:

> No point. You cannot address more than 64K with a PDP-11, so even if 
> you had 1 MB, you could not address it.

You're referring to the 16-bit addressing design of the PDP-11, most likely.

Later PDP-11 systems support 22-bit memory addressing.

And shuffling mapping tables also permitted accessing more than 16-bits 
of memory (though not all at once), using windowing.

The same basic window-mapping hackery was performed as far back as 
Apple II systems and very likely before; it was fairly typical on any 
system with memory addressing constraints, and most everybody was using 
architecture-specific variations of the same basic techniques.  With 
VMS, you can use non-file-backed sections, and a closely-related 
scatter-gather mapping was also a common technique for dealing with 
memory references on VMS drivers.  Or you can mess more directly with 
the page tables.




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