Memory cont. #

Fragmentation #

The small 2B chunk will never be able to be used, and the sum of all of these small gaps in memory is wasted, called internal fragmentation.

Compaction #

• compaction is expensive to invoke

Swapping #

Segmentation #

One of the methods of implementing non-contiguous memory is segmentation.

• an address may be something like <3, 14>, which represents segment 3 with a 14 byte offset
• another table keeps track of the segment’s addresses and lengths

Paging #

• similar to segmentation, except that everything is divided into equal size blocks

Addressing #

VPN = “virtual page number”, PFN = “physical frame number”

Paging hardware #

• paging suffers internal fragmentation
• paging solves external fragmentation

• D is the dirty bit, which indicates “write needs to be updated on disk”
• A is access bit, indicates if the page has been recently accessed
• R/W is protection bit, indicates read or write permission
• P is the valid/invalid bit, the “present” bit

Paging problems #

• where do we store the page table itself? in memory

The TLB is the “translation lookaside buffer”. MMU is part of the CPU suite.

We want a high TLB hit rate.

Effective access time #

Structure of a page table #