CS 3733 Operating Systems

Reading: Tanenbaum Section 3.3

Paging

• Method of choice on most systems.
• Programs do not have to be loaded contiguously.
• Physical memory is divided into fixed-size blocks called frames.
• The logical memory of the programs is broken into fixed-size blocks called pages.
• A logical address generated by the CPU is divided into a page number (p) and a page offset (d).
• The page number is used as an index into the page table which contains the frame number of the actual page in memory.
• The page offset gives the position within the page. If the page size is 2_n, the offset will be n bits long.
• When a process enters the system, the required number of pages is allocated from the free list of pages.
• There is no external fragmentation.
• There is a small amount of internal fragmentation because frames are allocated as units.

Paging Implementation

• The page table is usually kept in main memory, so the address translation will take two memory references rather than just one. This effectively doubles the memory access time.
• To make paging more efficient, part of the page table is kept in a fast memory which is content or key addressable (translation lookaside buffer).
• The percentage of times that a page number is found in the associative memory is called the hit ratio.

Example
If the associative memory access time is 50 ns, the main memory access time is 250 ns and the hit ratio is 80%, calculate the effective memory access time.

Virtual Memory

• Each page table entry has a bit indicating whether the page actually occupies a page frame.
• Initially when the process is allocated resources, the page table is set up with all (or most) of the entries marked invalid.
• When a particular page is referenced, the hardware checks to see if the page operation is permitted (e.g. writable) and if not, the program aborts.
• When a particular page is referenced, the hardware checks to see if the page table entry is valid. If it is, everything is as before.
• If an invalid page is referenced, a page fault trap will occur and the following must be done:
  • Remove the process from the CPU. When restarted, the instruction which caused the page fault will be reexecuted.
    (Be careful to undo operations such as MOV -(R2), (R3)+.)
  • Find a free frame from the free frame list and update this list.
  • Schedule a disk operation to read page into frame.
  • Update the page table.
• If the process never attempts to access a particular page of memory, it is never brought in.
• PSECTS in VAX assembly language are defined on page boundaries. You can put restrictions such as noexc or nowrt on pages and keep the protection information in the page table.