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Unformatted text preview: d instruction (including absent coprocessor). These are mutually exclusive instruction encodings and therefore cannot occur simultaneously. Reset starts the processor from a known state and renders all other pending exceptions irrelevant. The most complex exception scenario is where an FIQ, an IRQ and a third exception (which is not Reset) happen simultaneously. FIQ has higher priority than IRQ and also masks it out, so the IRQ will be ignored until the FIQ handler explicitly enables IRQ or returns to the user code. If the third exception is a data abort, the processor will enter the data abort handler and then immediately enter the FIQ handler, since data abort entry does not mask FIQs out. The data abort is 'remembered' in the return path and will be processed when the FIQ handler returns. If the third exception is not a data abort, the FIQ handler will be entered immediately. When FIQ and IRQ have both completed, the program returns to the instruction which generated the third exception, and in all the remaining cases the exception will recur and be handled accordingly. Address exceptions The observant reader will have noticed that Table 5.2 on page 109 shows the use of all of the first eight word locations in memory as exception vector addresses apart from address 0x00000014. This location was used on earlier ARM processors which operated within a 26-bit address space to trap load or store addresses which fell outside the address space. These traps were referred to as 'address exceptions'. Since 32-bit ARMs are unable to generate addresses which fall outside their 32-bit address space, address exceptions have no role in the current architecture and the vector address at 0x00000014 is unused. 5.3 Conditional execution An unusual feature of the ARM instruction set is that every instruction (with the exception of certain v5T instructions) is conditionally executed. Conditional branches are a standard feature of most instruction sets, but ARM extends the 112 The ARM Instruction Set conditional execution to all of its instructions, including supervisor calls a...
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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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