188.8.131.52. Virtual Addressing
The MMU contains a hardware translation lookaside buffer (TLB). The operating system is responsible for creating and maintaining a page table (or equivalent data structures) in memory. The hardware TLB acts as a software managed cache for the page table. The MMU does not perform any operations on the page table, such as hardware table walks. Therefore the operating system is free to implement its page table in any appropriate manner.
There is a 20 bit virtual page number (VPN) and a 12 bit page offset.
|Virtual Page Number|
|Virtual Page Number||Page Offset|
As input, the TLB takes a VPN plus a process identifier (to guarantee uniqueness). As output, the TLB provides the corresponding physical frame number (PFN).
Distinct processes can use the same virtual address space. The process identifier, concatenated with the virtual address, distinguishes identical virtual addresses in separate processes. To determine the physical address, the Nios II MMU translates a VPN to a PFN and then concatenates the PFN with the page offset. The bits in the page offset are not translated.
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