Updating segmentation schemes

Overlays are not a method of paging RAM to disk but merely of minimizing the program's use of RAM.

Subsequent architectures used memory segmentation, and individual program segments became the units exchanged between disk and RAM.

Satisfying page faults may require freeing pages that will soon have to be re-read from disk.

"Thrashing" is also used in contexts other than virtual memory systems; for example, to describe cache issues in computing or silly window syndrome in networking. A single MOVL crossing a page boundary could have a source operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and a destination operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and the source and destination could both cross page boundaries.

A program from a memory mapped file begins execution with none of its pages in RAM.

As the program commits page faults, the operating system copies the needed pages from the file or swap partition containing the page data into RAM.

To decrease excessive paging and resolve thrashing problems, a user can increase the number of pages available per program, either by running fewer programs concurrently or increasing the amount of RAM in the computer.

If the evicted page frame was dynamically allocated by a program to hold data, or if a program modified it since it was read into RAM (in other words, if it has become "dirty"), it must be written out to disk before being freed.

If a program later references the evicted page, another page fault occurs and the page must be read back into RAM.

When a process tries to reference a page not currently present in RAM, the processor treats this invalid memory reference as a page fault and transfers control from the program to the operating system.

The operating system must: When all page frames are in use, the operating system must select a page frame to reuse for the page the program now needs.

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