--- Megan <mbg_at_world.std.com> wrote:
>
> >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...
I thought the physical/virtual memory distinction in this case was more
of a PDP-11 processor concept than specifically RT-11.
> 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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Received on Fri Mar 24 2000 - 07:48:16 GMT