Paging
Idea
• Break the program’s address space into
equal sized pages
• Break the main memory into equal sized
frames
• Page size=frame size=2^k words where k is
a positive integer
Effective address
• View the effective address is page number
and offset within that page
• Splitting this way is easy: the last k bits will
form the offset and the other bits will form
the page number
Address translation
• Maintain a page map table (one per process)
• For each address, find the page number p
• Use p as an index and find the
corresponding frame number f
• Concatenate f and offset.
Fragmentation
• There is no external fragmentation
• There is internal fragmentation: The last
page of a process may not be fully utilized
and the part that is not fully used is wasted
Storing the page map table
• In registers
– expensive and fast
• In memory
– slow and inexpensive
• Hybrid scheme (use cache)
– some rows of the table are in registers
– rest are in main memory
– if DAT can be done using register contents, you
have a hit and well and good; otherwise,
Choice of page size
• Too small?
– Too many pages for a process; page map table
is too large
• Too large?
– Waste memory due to internal fragmentation
• How to decide:
Choice of page size
• Let g be page size; s be size of average
process; m be size of one row of page map
table
• memory wasted (internal frag+page map
table) =g/2+s*m/g
• minimize this (g = sqrt(2*s*m))