Chapter 3 Memory Management: virtual memory

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address revolution

the process of changing the address of an instruction or data item to the address in main memory at which it is to be loaded or relocated

associative memory

the name given to several register, allocated to each active process, whose contents associate several of the process segments and page numbers with their main memory addresses

cache memory

a small, fast memory used to hold selected data and to provide faster access than would otherwiese be possible

clock cycle

the elapsed time between two ticks of the computer's system clock

clock page replacement policy

a variation of the LRU policy that removes from main memory the pages that show the least amount of activity during recent clock cycles

demand paging

a memory allocation scheme that loads a program's page into memory at the time it is needed for processing

displacement

in a paged or segmented memory allocation environment, the difference between a page's relative address and the actual machine language address. Also called offset.

FIFO anomaly

an unusual circumstance through which adding more page frames causes an increase in page interrupts when using a FIFO page replacement policy. Also known as Beady's anomaly

first-in first-out (FIFO) policy

a page replacement policy that removes from main memory the pages that were brought in first

job Table (JT)

a table in main memory that contains two values for each active job: the size of the job and the memory locaiton where its page map table is stored

least recently used (LRU) policy

a page-replacement policy that removes from main memory the pages that show the least amount of recent activity

locality of reference

behavior observed in many executing programs in which memory locations recently referenced, and those near them, are likely to be referenced in the near future

memory map table (MMT)

a table in main memory that contains as many entries as there are page frames and lists the location and free/busy status for each one

page

a fixed-size section of a user's job that corresponds in size to page frames in main memory

page fault

a type of hardware interrupt caused by a reference to a page not residing in memory. The effect is to move a page of main memory and into secondary storage so another page can be moved into memory

page fault handler

the part of the memory manager that determines if there are empty page frames in memory so that the requested page can be immediately copied from secondary storage, or determines which page must be swapped out if all page frames are busy. Also known as page interrupt handler.

page frame

an individual section of main memory of uniform size into which a single page may be loaded without causing external fragmentation

page map table (PMT)

a table in main memory with the vital information for each page including the page number and its corresponding page frame memory address

page replacement policy

an algorithm used by virtual memory systems to decide which page or segment to remove from main memory when a page frame is needed and memory is full

page swapping

the process of moving a page out of main memory and into secondary storage so another page can be moved into memory in its place

paged memory allocation

a memory allocation scheme based on the concept of dividing a user's job into section of equal size ot allow for noncontiguous program storage during execution

reentrant code

code that can be used by two or more processes at the same time; each shares the same copy of the executable code but has separate data areas

sector

a division in a disk's tack, sometimes called a "block." The tracks are divided into sectors during the formatting process

segment

a variable-size section of a user's job that contains a logical grouping of code

segment map table (SMT)

a table in main memory with the vital information for each segment including the segment number and its corresponding memory address

segmented/demand paged memory allocation

a memory allocaiton scheme based on the concept of dividing a user's job into logical groupings of code and loading them into memory as needed to minimize fragmentation

segmented memory allocation

a memory allocation scheme based on the concept of dividing a user's job into logical groupings of code to allow for noncontiguous program storage during execution

subroutine

also called a "subprogram," a segment of a program that can perform a specific function. Subroutines can reduce programming time when a specific function is required at more than one point in a program

thrashing

a phenomenon in a virtual memory system where an excessive amount of page swapping back and forth between main memory and secondary storage results in higher overhead and little useful work

virtual memory

a technique that allows programs to be executed even though they are not stored entirely in memory

working set

a collection of pages to be kept in main memory for each active process in a virtual memory environment

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