The Present Bit
Explore the role of the present bit in virtual memory systems to understand how an operating system handles swapping pages to and from disk. Learn the hardware and OS steps involved in managing page faults and maintaining efficient memory access during swapping operations.
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Now that we have some space on the disk, we need to add some machinery higher up in the system in order to support swapping pages to and from the disk. Let us assume, for simplicity, that we have a system with a hardware-managed TLB.
Extracting the present bit
Recall first what happens on a memory reference. The running process generates virtual memory references (for instruction fetches, or data accesses), and, in this case, the hardware translates them into physical addresses before fetching the desired data from memory.
Remember that the hardware first extracts the VPN from the virtual address, checks the TLB for a match (a TLB hit), and if a hit, produces the resulting physical address and fetches it from ...