24 Mar
2000
24 Mar
'00
2:23 a.m.
memory management bits, the *virtual* address space
for a program on the
PDP-11 is 64K (or perhaps 128K if split I&D). Because of the physical
No the addressable space is 65k, the logical addressable space is 64k
without virtualization and 4mb with(for q22). Virtual address space is
something programs believes exists and is an artificail construct.
VAXes that space is 4gb as a contigious space where pdp11s is only 64k as
contigious space. however if done to the max there is no limit for the
PDP-11 case save for physical disk. The real part is virtual space is not
always fast and has high overhead to manage. The desireable aprt is even
with a vax it's was rare back when to have 16 or 32 MB yet want to process
a model whose matrix could easily be several hundres of megabytes.
Virtualization allows the program to ignore physical boundries at the
expense of speed. This is why systems like 11/70s (and big novas) were so
hot in the late 70s. They could support the virtualization using software
and MMU.
It's' a cool concept and when applied to something like Z280 with 16mb
address space it's quite powerful.
Allison