Re: [9fans] Virtual memory & paging

[geoff@collyer.net]

Even if your physical memory were as large as your virtual address
space, an obvious advantage to mapping addresses is to permit programs
linked to start at a fixed address (usually the second page of the
address space) to be run without first running a loading pass to
relocate all the addresses in the entire program.

In addition, memory management units normally provide varying forms of
protection for pages (e.g., instructions are usually made read- or
execute-only) and checking for out-of-bounds (unmapped) addresses.  On
many machines, notably the PC, a read from an address with no
corresponding memory (i.e., out-of-bounds) yields garbage; with memory
management on, many such references can be caught.  The MMU can thus
be used to ensure that a program doesn't access data in other programs
(including the kernel) without their consent.

Another potential use of mapping is to avoid copying data by remapping
pages.

I would turn the question around: when would you want a one-to-one
mapping of virtual to physical addresses?  There are a few such cases:
if memory serves, the old VAX 750 Plan 9 file server ran with the MMU
off because it runs no user-mode processes and it was found that
running with the MMU off made the machine run much faster.