The unused instruction encodings each fall into

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Unformatted text preview: word and signed byte instructions described in Section 5.11 on page 128. These are likely to be used to support additional data transfer instructions, should these be required in the future. Figure 5.21 Data transfer instruction extension space. Memory faults 143 Unused Coprocessor instructions The following instruction format is similar to the coprocessor data transfer instruction described in Section 5.18 on page 138, and is likely to be used to support any additional coprocessor instructions that may be required: Figure 5.22 Coprocessor instruction extension space. Undefined instruction space The largest area of undefined instructions looks like the word and unsigned byte data transfer instruction described in Section 5.10 on page 125. However the future options on this space are being kept completely open. Figure 5.23 Undefined instruction space. Behaviour Of unused instructions All current ARM processors will take the undefined instruction trap if an attempt is made to execute an instruction that matches the encoding shown in Figure 5.23, in the undefined instruction space. The latest ARM processors should take the undefined instruction trap if any unused opcode is executed, but previous versions (including ARM6 and ARM?) will behave unpredictably. Therefore these instructions should be avoided! 5.22 Memory faults ARM processors allow the memory system (or, more usually, the memory management unit) to fault on any memory access. What this means is that instead of returning the requested value from memory, the memory system returns a signal that indicates that the memory access has failed to complete correctly. The processor will then enter an exception handler and the system software will attempt to recover from the problem. The most common sources of a memory fault in a general-purpose machine are: 144 The ARM Instruction Set Page absent The addressed memory location has been paged out to disk. In a virtual memory system infrequently used pages are held on disk. An attempt to access instructions or data on such a page...
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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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