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Re: [f-cpu] TLB design
On Sun, Jul 14, 2002 at 08:12:19PM +0200, Cedric BAIL wrote:
> > I've been thinking about the TLB before; IMHO, we need at least the
> > following (assuming <n> bits for the page offset):
> >
> > - virtual address (64-<n> bits)
> > - physical address (64-<n> bits)
> > - address space identifier (ASI; 8 bits was suggested)
> > - supervisor access rights (RWX, 3 bits)
> > - user access rights (RWX, 3 bits)
> > - valid bit (indicating that the entry is valid)
> > - present bit (indicating that the page is in memory)
> What did you mean whith this bit ?
If the bit is set, the page is mapped to physical ram. If it is
cleared, a page fault occurs.
> > - dirty bit (indicating that the page has been written to)
> Same question.
It's used for memory management inside the OS. If a page is dirty, it
must be written back to permanent storage; if it's not, it can be
simply discarded to make room for something else.
> > - used bit (indicating that the page has been accessed)
> Why not a counter ? I currently don't see how the OS can make it's decision
> about how to remove a TLB entry if TLB is full.
A counter would be more expensive (and would take more bits).
The OS will have to scan (and reset) the `used' bits on a regular basis
(e.g. inside a (slow) timer interrupt) and record the entries that have
been used. The entry that has been used least recently can be replaced.
> > The latter can be used to implement an LRU algorithm.
>
> > That is, we need 128+2*<n>-18 bits. If an entry shall fit into 128 bits,
> > <n> must be at least 9 (i.e. a page must have at least 512 bytes).
> > With 4K pages, we will have 6 bits left. Some (or all) of them may be
> > used to indicate the page size.
> That's a really nice idea in fact. But what did this bit mean, are they
> number of Ko for a page, or predefined size page ?
Anything you like. I'd say: pagesize = 4K << sizebits; but that is my
personal opinion.
--
Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
"All I wanna do is have a little fun before I die"
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