Re: [f-cpu] Do we really need virtual memory?

["Christophe Avoinne" <christophe.avoinne@laposte.net>]

There is a lot of examples where VM is necessary :

- No memory limit : a virtual memory helps OS and applications to works with
huge virtual memory whatever the physical memory capacity is.

- File mapping : a way not to be forced to load all the unnecessary parts of
a file on memory, a way to work on a file like a memory block. A lot of
databases used to work on very huge files though file mappings.

- Shared access memory mapping :  a way to let processes to share a memory
with the same or different access rights, with the same or different content
for each block of this memory. An example is the data you share when forking
a process, the data of the new process would be shared until it is modified
(Copy-On-Write mechanism).

Etc.

What we must have in mind is to know what F-CPU is purposed for :
work-station or embedded ? depending the target you would need VM.

F-CPU may not have VM (nothing prevents someone from getting rid of it when
not necessary), but we must consider it all the same from the begining to
avoid starting on a bad design.


----- Original Message -----
From: "Beat Steiner" <linux@iprolink.ch>
To: <f-cpu@seul.org>
Sent: Saturday, November 22, 2003 6:39 PM
Subject: [f-cpu] Do we really need virtual memory?


>
> The question might be stupid, but I ask it anyway:
> Do we need virtual memory? In a huge address space,
> every program could be physical memory aware.
>
> The only case I see now that VM makes sense is memory fragmentation:
> Example: I need to expand a memory block, but there is already one
allocated
> next to it. VM could map the new block from anywhere to the adjacent
> address...
> With VM, we save the work of moving around the memory block to be
enlarged.
>
> Any further examples welcome.
>
> Getting rid of VM would simplify the "TLB" (only permission checks) and
> significantly reduce its footprint.
>
> We may get rid of VM for the FC0 only and add it again later.



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