If another process were ready to use the memory allocated to the blocked process, the OS simply
copied the entire contents of the blocked process??™s memory, as well as the CPU register contents, to a temporary
???scratch??? or ???swap??? area on the disk.
CHAP. 6] OPERATING SYSTEMS 117
Since the process state at the time of the swap included the values of the registers and the values of the
variables, the OS had to save the image of the executing process. Later, when the I/O completed, the OS could
decide to bring the swapped process back into memory, and it was essential that the interrupted image be
brought back. The OS could not bring in an original copy of the program from the original executable file,
because the original executable file would not have the current values for the registers or variables.
Most general-purpose operating systems today continue to use swapping as part of a more sophisticated
virtual memory management capability.
VIRTUAL MEMORY
The concept of virtual memory further separated the concepts of logical address and physical
address. The logical address is the address as known to the program and the programmer; the physical address
is the actual address known to the computer memory hardware.
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