23 Mar
2000
23 Mar
'00
11:48 p.m.
--- Megan <mbg(a)world.std.com> wrote:
I thought the physical/virtual memory distinction in this case was more
of a PDP-11 processor concept than specifically RT-11.
I think he's talking about the opposite of
what you mean. You mean using
RAM as a disk, right? I believe he means using disk for RAM. Which is
something I personally try to avoid like the plague!
Actually, it gets more confusing with RT-11, since the term 'virtual
memory' refers to the address space a program can access... it has
nothing to do with disk space... The way to think of it is -- physical memory is the
memory the machine
has while virtual memory is what a program can see at any one time.
And with 16 bits worth of address space, that maximum is 32kw (64kb)
at any one time... of course, you can remap portions of your address
space to elsewhere in physical memory, but you are always limited to
accessing 64kb at one time (128kb -- 64kb I-space and 64kb D-space if
you have a processor with separated I and D spaces)
It used to bend my brain around backwards until I worked with RT-11 for
a while. The problem is that the PDP-11 has mini-computer attributes,
but is only 16-bit. Let's try a cross-platform comparison... Machine A:
VAX running BSD. Machine B: PDP-11 running BSD. (Easy so far ;-) The
PDP-11 has 2Mb of physical memory; the VAX has 2Mb of physical memory.
Now comes the tricky part: the *virtual* address space for a program on
the VAX is 4Gb, 32 bits. Since well-behaved PDP-11 programs don't twiddle
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
nature of the beasts, the VAX has much more virtual address space than
physical memory, but the PDP-11 is the other way around.
It's just like Megan said, but I've added what might be a more familar
model for comparison. It made me woozy at first, too.
-ethan
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