ARM.SoC.Architecture

The arm multiple register transfer instructions

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Unformatted text preview: y addresses and the base register points to the first empty location below the stack. Block copy addressing Although the stack view of multiple register transfer instructions is useful, there are occasions when a different view is easier to understand. For example, when these instructions are used to copy a block of data from one place in memory to another a mechanistic view of the addressing process is more useful. Therefore the ARM assembler supports two different views of the addressing mechanism, both of which map onto the same basic instructions, and which can be used interchangeably. The block copy view is based on whether the data is to be stored above or below the address held in the base register and whether the address incrementing or decrementing begins before or after storing the first value. The mapping between the two views depends on whether the operation is a load or a store, and is detailed in Table 3.1 on page 62. The block copy views are illustrated in Figure 3.2 on page 62, which shows how each variant stores three registers into memory and how the base register is modified if auto-indexing is enabled. The base register value before the instruction is r9, and after the auto-indexing it is r9'. To illustrate the use of these instructions, here are two instructions which copy eight words from the location r0 points to to the location r1 points to: LDMIA r0!, {r2-r9} STMIA r1, {r2-r9} After executing these instructions r0 has increased by 32 since the '!' causes it to auto-index across eight words, whereas r1 is unchanged. If r2 to r9 contained useful values, we could preserve them across this operation by pushing them onto a stack: STMFD LDMIA STMIA LDMFD r13!, r0!, r1, r13!, {r2-r9} {r2-r9} {r2-r9} {r2-r9} save regs onto stack ; restore stack from Here the 'FD' postfix on the first and last instructions signifies the full descending stack address mode as described earlier. Note that auto-indexing is almost always specified for stack operations in order to ensure that the...
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