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Unformatted text preview: ARM710TMMU The ARM710T memory management unit implements the ARM memory management architecture described in Section 11.6 on page 302 using the system control coprocessor described in Section 11.5 on page 298. The translation look-aside buffer (TLB) is a 64-entry associative cache of recently used translations which accelerates the translation process by removing the need for the 2-stage table look-up in a high proportion of accesses. The write buffer holds four addresses and eight data words. The memory management unit defines which addresses are bufferable. Each address may be associated with any number of the data words, so the write buffer may hold one word (or byte) of data to write to one address and seven words to write to another address, or two blocks of four words to write to different addresses, and so on. The data words associated with a particular address are written to sequential memory locations starting at that address. (Clearly multiple data words associated with one address are generated principally by store multiple register instructions, the only other potential source being a data transfer from an external coprocessor.) The mapping is illustrated in Figure 12.3 on page 322, which shows the first address mapping six data words and the second and third addresses mapping one data word each. The fourth address is currently unused. The write buffer becomes full either when all four addresses are used or when all eight data words are full. The processor can write into the buffer at the fast (cache) clock speed and continue executing instructions from the cache while the write buffer stores the data to memory at the memory clock rate. The processor is therefore fully decoupled from the memory speed so long as the instructions and data it needs are in the cache and the write buffer is not full when it comes to perform a write operation. The write buffer gives a performance benefit of around 15% for a modest hardware cost. The principal drawback of the write buffer is that it is not possible to recover from external memory faults caused by buffered writes since the processor state is not recoverable. The processor can still support virtual memory since translation faults are detected in the on-chip MMU, so the exception is raised before the data gets to the...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.

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