[f-cpu] TLB design

[Michael Riepe <michael@stud.uni-hannover.de>]

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)
	- dirty bit (indicating that the page has been written to)
	- used bit (indicating that the page has been accessed)

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.

A TLB lookup is performed by XORing the address and ASI fields with the
actual values; if the result is 0, the entry matches. If we are going
to use multiple page sizes, the result must be ANDed with a mask that
depends on the page size.

The question is how we choose the page sizes. With 4K/32K/256K/2M pages
and 8 entries per size we can map only ~18M of memory at the same time,
and we will have a hard time if we want to map arbitrary segments (with
sizes that are not a power of two). Additionally, the CPU will spend a
lot of time inside the TLB miss handler. Note that there are applications
that easily take megabytes of code space and hundreds or even thousands
of megabytes of data -- that's the kind of program a 64-bit CPU is most
useful for, because it has a large address space.

It's probably better to provide all page sizes that are powers of two,
from 4K up to at least 4G. It may also be a good idea to implement a
`best match' strategy: if multiple TLB entries match a virtual address,
the most specific one (that is, the one with the smallest page size)
is chosen. That way you can put small `windows' inside a bigger page.
The total number of TLB entries should not be smaller than 256.

-- 
 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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