Levels of the Memory Hierarchy

Page Table and Process state° Each program, on compilation, is given its ownpage table° Page Table itself resides in the main memory° Address of the start of the page table is in the pagetable register° Page table Register, together with other registers,specifies the state of that program – referred to asthe process state• If a program needs to relinquish CPU for another program (preemption),the process state needs to be stored first by theOperating System• Restoring the program execution involves Operating System reloadingthe state° Program Counter (PC):• One of the other registers• holds the address of next instruction to be executed.PDE1C_33Page Table – Continued° Indexed by virtual page number• That is, the page table has an entry for each virtual page.• How many page table entries the virtual memory of figurein #1C_32 will require?° So, no index is needed as in fully associative (FA) cache° But … there is a subtle difference with FA cache• For a given memory address, we extract the tag andsearch the entire cache (see #1C_22, 23)• The outcome is either an entry for the reference addressis found (hit) or not found° With Page table, an entry for a virtual page is always found(see above) – this does not mean it is a hit° Hit or miss (page fault) is decided by the V-bit in the entry° V bit = 1 means that the entry has the physical address wherethe desired virtual page resides (hit)° V bit = 0 means the entry points to the disk address where thedesired page can be fetched from (page fault)PDE1C_34