In virtual mode, the processor translates an effective address into a physical address using the process shown in the following figure. With 64-bit MicroBlaze and with the Physical Address Extension (PAE) the physical address can be extended up to 64 bits. Virtual mode can be enabled by setting the VM bit in the MSR.
Each address shown in the previous figure contains a page-number field and an offset field. The page number represents the portion of the address translated by the MMU. The offset represents the byte offset into a page and is not translated by the MMU. The virtual address consists of an additional field, called the process ID (PID), which is taken from the PID register (see Process Identifier Register). The combination of PID and effective page number (EPN) is referred to as the virtual page number (VPN). The value n is determined by the page size, as shown in (Table 1).
System software maintains a page-translation table that contains entries used to translate each virtual page into a physical page. The page size defined by a page translation entry determines the size of the page number and offset fields. For example, with 32-bit MicroBlaze, when a 4 kB page size is used, the page-number field is 20 bits and the offset field is 12 bits. The VPN in this case is 28 bits.
Then the most frequently used page translations are stored in the translation look-aside buffer (TLB). When translating a virtual address, the MMU examines the page-translation entries for a matching VPN (PID and EPN). Rather than examining all entries in the table, only entries contained in the processor TLB are examined. When a page-translation entry is found with a matching VPN, the corresponding physical-page number is read from the entry and combined with the offset to form the physical address. This physical address is used by the processor to reference memory.
System software can use the PID to uniquely identify software processes (tasks, subroutines, threads) running on the processor. Independently compiled processes can operate in effective-address regions that overlap each other. This overlap must be resolved by system software if multitasking is supported. Assigning a PID to each process enables system software to resolve the overlap by relocating each process into a unique region of virtual-address space. The virtual-address space mappings enable independent translation of each process into the physical-address space.