🚀 go-pugleaf

In article <7092@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>> ... Does virtual memory imply demand paging or just address translation?
>
>"Virtual memory", strictly speaking, refers to memory that acts like it's
>always there but sometimes isn't.  That is, demand paging or some analogous
>scheme (paging is not the only memory-management technique possible, and
>demand paging is not the only page-fetching policy possible).  Address
>translation, by itself, is not virtual memory.

Hmmmmm.....  I think I want to take issue with this.  Although what Henry
says is valid, his point is too narrow.  Two immediate counter-examples are
the SDS-940 (later the XDS-940) and the PDP-11, both of which are virtual-
memory machines, and yet essentially just only do address translation.  (I
know, I know, later versions of the PDP-11 can restart instructions; the
point remains.)

Perhaps more to the point would be to say that the memory image seen by the
program is independent of the real memory provided by the machine.  That is,
the operating system can assign arbitrary portions of real memory to the
program and this mapping can change over time.

In this context, some amusing definitions:
   Real:  You've got it, and you know you've got it.
   Transparent:  You've got it, but you don't know you've got it.
   Virtual:  You don't have it, but you don't know it.
-- 
-- Greg Noel, NCR Rancho Bernardo    Greg@ncr-sd.UUCP or Greg@nosc.ARPA