Cirrus Logic CS485 Manual de usuario descargar pdf (Pagina 58)

CS485G Spring 2015 58

page table to ﬁnd a victim that has not been recently accessed,

hoping that it won’t be accessed soon.

(d) If the victim is dirty, it must be written to disk. If it is clean,

there is already a good copy on disk.

9. Programs exhibit locality of reference, which is why swapping in a

page works for a long time, and why processes tend not to need all

their virtual pages in memory most of the time. The kernel tries to

make sure that at least the working set of virtual pages are kept in

physical memory.

10. Multi-level page tables

(a) A table of length 2

is unwieldy. The situation is even worse on

an x86 64.

(b) Therefore, the table is built in multiple levels: 2 levels on the

x86, 3 levels on the x86 64.

level), 10 bits (second level), 12 bits (offset). The top-level bits

index an entry in a top-level table of length 2

= 1024 entries,

which is in a single physical page pointed to by a hardware reg-

ister.

(d) The top-level entry addresses the second-level page table, also

of length 2

= 1024 entries, ﬁtting in a single page.

(e) There can be a page fault to access that second-level page table.

56 Free-space allocation

1. malloc() and friends maintain data structures that implement the

heap.

2. When they run out of memory, they can use the sbrk() system call

to request more data space.

3. Allocation is usually aligned to 8-byte boundaries.

4. There are several data structures that malloc() could use.

(a) Boundary tag method

i. Every allocated region has a few bytes before and after, called

tags, that indicate the length of the region.

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Cirrus Logic CS485 Manual de usuario Pagina 58