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Unformatted text preview: nd coprocessor instructions. The condition field occupies the top four bits of the 32-bit instruction field: Figure 5.2 The ARM condition code field. Each of the 16 values of the condition field causes the instruction to be executed or skipped according to the values of the N, Z, C and V flags in the CPSR. The conditions are given in Table 5.3 on page 113. Every ARM instruction mnemonic may be extended by appending the two letters defined in this table, though the 'always' condition (AL) may be omitted since it is the default condition that is assumed if no other condition is specified. The 'never' condition The 'never' condition (NV) should not be used - there are plenty of other ways to write no-ops (instructions that have no effect on the processor state) in ARM code. The reason to avoid the 'never' condition is that ARM Limited have indicated that they may use this area of the instruction space for other purposes in the future (and have done so in architecture v5T), so although current ARMs may behave as expected, there is no guarantee that future variants will behave the same way. Where alternative mnemonics are shown in the same row in the condition table this indicates that there is more than one way to interpret the condition field. For instance, in row 3 the same condition field value can be invoked by the mnemonic extension CS or HS. Both cause the instruction to be executed only if the C bit in the CPSR is set. The alternatives are available because the same test is used in different circumstances. If you have previously added two unsigned integers and want to test whether there was a carry-out from the addition, you should use CS. If you have compared two unsigned integers and want to test whether the first was higher or the same as the second, use HS. The alternative mnemonic removes the need for the programmer to remember that an unsigned comparison sets the carry on higher or the same. The observant reader will note that the conditions are in pairs where the second condition is the inverse of the first, so for any condition the opposite condition is also available (wit...
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- Spring '09