The Present Bit

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 ...