Protected Virtual Address Mode (PVAM)
80286 is the first processor to support the concepts of virtual memory and memory management. The virtual memory does not exist physically it still appears to be available within the system. The concept of VM is implemented using Physical memory that the CPU can directly access and secondary memory that is used as a storage for data and program, which are stored in secondary memory initially.
The Segment of the program or data required for actual execution at that instant is fetched from the secondary memory into physical memory. After the execution of this fetched segment, the next segment required for further execution is again fetched from the secondary memory, while the results of the executed segment are stored back into the secondary memory for further references. This continues till the complete program is executed
During the execution the partial results of the previously executed portions are again fetched into the physical memory, if required for further execution. The procedure of fetching the chosen program segments or data from the secondary storage into physical memory is called swapping. The procedure of storing back the partial results or data back on the secondary storage is called unswapping. The virtual memory is allotted per task.
The 80286 is able to address 1 G byte (230 bytes) of virtual memory per task. The complete virtual memory is mapped on to the 16Mbyte physical memory. If a program larger than 16Mbyte is stored on the hard disk and is to be executed, if it is fetched in terms of data or program segments of less than 16Mbyte in size into the program memory by swapping sequentially as per sequence of execution.
Whenever the portion of a program is required for execution by the CPU, it is fetched from the secondary memory and placed in the physical memory is called swapping in of the program. A portion of the program or important partial results required for further execution, may be saved back on secondary storage to make the PM free for further execution of another required portion of the program is called swapping out of the executable program.
80286 uses the 16-bit content of a segment register as a selector to address a descriptor stored in the physical memory. The descriptor is a block of contiguous memory locations containing information of a segment, like segment base address, segment limit, segment type, privilege level, segment availability in physical memory, descriptor type and segment use another task.
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