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The Memory Manager, the Interface Manager, the User Manager, and the File Manager are the basis of all operating systems.
When the Processor Manager receives a command, it determines whether the program must be retrieved from storage or is already in memory, and then notifies the appropriate manager.
Since the mid-1970s rapid advances in computer technology have blurred the distinguishing characteristics of early machines.
Real-time systems are used in time-critical environments where reliability is key and data must be processed within a strict time limit.
A process requires space in main memory where it resides during its execution although, from time to time, it requires other resources such as data files or I/O devices
To overlay is to transfer segments of a program from main memory into secondary storage for execution, so that two or more segments take turns occupying the same memory locations.
The algorithm used to store jobs into memory requires a few more steps than the one used for a single-user system because the size of the job must be matched with the size of the partition to make sure it fits completely.
A null entry in the busy list occurs when a memory block between two other busy memory blocks is returned to the free list.
In the relocatable dynamic partitions scheme, the Memory Manager relocates programs to gather together all of the empty blocks and compact them to make one block of memory large enough to accommodate some or all of the jobs waiting to get in.
The bounds register is used to store the highest (or lowest, depending on the specific system) location in memory accessible by each program.
Paged memory allocation usually results in internal fragmentation, but never external fragmentation.
Each page of a job is actually stored in a page frame that can be located anywhere in available main memory.
Demand paging was the first widely used scheme that removed the restriction of having the entire job in memory from the beginning to the end of its processing.
A variation of the LRU page replacement algorithm is known as the clock page replacement policy because it is implemented with a circular queue and uses a pointer to step through the reference bits of the active pages, simulating a clockwise motion
The process of shifting bits to the right and resetting the leftmost bit to 1 when a page is referenced gives a history of each page's usage.
The use of virtual memory requires cooperation between the Memory Manager (which tracks each page or segment) and the processor hardware (which issues the interrupt and resolves the virtual address)
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