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Unformatted text preview: arting at an even byte address. The ARM programmer's model 41 Figure 2.3 ARM memory organization. (This is the standard, 'little-endian', memory organization used by the ARM. ARM can also be configured to work with a 'big-endian' memory organization; we will return to this issue in Chapter 5.) Load-store architecture In common with most RISC processors, ARM employs a load-store architecture. This means that the instruction set will only process (add, subtract, and so on) values which are in registers (or specified directly within the instruction itself), and will always place the results of such processing into a register. The only operations which apply to memory state are ones which copy memory values into registers (load instructions) or copy register values into memory (store instructions). CISC processors typically allow a value from memory to be added to a value in a register, and sometimes allow a value in a register to be added to a value in memory. ARM does not support such 'memory-to-memory' operations. Therefore all ARM instructions fall into one of the following three categories: 1. Data processing instructions. These use and change only register values. For example, an instruction can add two registers and place the result in a register. 2. Data transfer instructions. These copy memory values into registers (load instructions) or copy register values into memory (store instructions). An addi tional form, useful only in systems code, exchanges a memory value with a reg ister value. 3. Control flow instructions. Normal instruction execution uses instructions stored at consecutive memory addresses. Control flow instructions cause execution to switch to a different address, either permanently (branch instructions) or saving a return address to resume the original sequence (branch and link instructions) or trapping into system code (supervisor calls). 42 The ARM Architecture Supervisor mode The ARM processor supports a protected supervisor mode. The protecti...
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- Spring '09