Microprocessor Design/Virtual Memory
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Virtual Memory is a computer concept where the main memory is broken up into a series of individual pages. Those pages can be moved in memory as a unit, or they can even be moved to secondary storage to make room in main memory for new data. In essence, virtual memory allows a computer to use more RAM then it has available.
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[edit] Implementation
Virtual memory can be implemented both in hardware and (to a lesser degree) in software, although many modern implementations have both hard and soft components. We discuss virtual memory here because many modern PC and server processors have virtual memory capabilities built in.
Pageing systems are designed to be transparent, that is the the programs running on the microprocessor do not need to be explicitly aware of the paging mechanism to operate correctly.
Many processor systems give pages certain qualifiers to specify what kinds of data can be stored in the page. For instance, many new processors specify whether a page contains instructions or data. Data pages may not be executed as instructions.
[edit] Memory Accessing
Memory addresses correspond to a particular page, and an offset within that page. If a page is 212 bytes in a 32-bit computer, then the first 22 bits of the memory address are the page address, and the lower 12 bits are the offset of the data inside that page. The top 22 bits in this case will be separated from the address, and they will be replaced with the current physical address of that page. If the page does not exist in main memory, the processor (or the paging software) will retrieve the page from secondary storage, which can cause a significant delay.
[edit] Pages
A page is a basic unit of memory, typically several kilobytes or larger. A page may be moved in memory to different locations, or if it is not being used frequently it can be moved to secondary storage instead. The area in the secondary storage is typically known as the page file, or as a "scratchpad" or something similar.
[edit] Page Table
The addresses of the various pages are stored in a paging table. The paging table itself can be stored in a memory unit inside the processor, or it can reside in a dedicated area of main memory.
[edit] Page Faults
A page fault occurs when the processor cannot find a page in the page table.
[edit] Translation Look-Aside Buffer
The translation look-aside buffer (TLB) is a small structure, similar to a cache, that stores the addresses of the most recently used pages. Looking up a page in the TLB is much faster then searching for the page in the page table. When the processor cannot find a particular page in the TLB, it is known as a "TLB Miss". When the TLB misses, the processor looks for the page in the page table. If the page is not in the table either, there is a page fault.
Notice that even though the TLB is similar to the cache mechanism, the two are not related.
