Pc generation the behaviour of r15 as seen by the


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Unformatted text preview: instruction, so the correct r15 value is obtained without additional hardware. 4.3 ARM instruction execution The execution of an ARM instruction can best be understood by reference to the datapath organization as presented in Figure 4.1 on page 76. We will use an annotated version of this diagram, omitting the control logic section, and highlighting the active buses to show the movement of operands around the various units in the processor. We start with a simple data processing instruction. Data processing instructions A data processing instruction requires two operands, one of which is always a register and the other is either a second register or an immediate value. The second operand is passed through the barrel shifter where it is subject to a general shift operation, then it is combined with the first operand in the ALU using a general ALU operation. Finally, the result from the ALU is written back into the destination register (and the condition code register may be updated). All these operations take place in a single clock cycle as shown in Figure 4.5 on page 83. Note also how the PC value in the address register is incremented and copied back into both the address register and r15 in the register bank, and the next instruction but one is loaded into the bottom of the instruction pipeline (i. pipe). The immediate value, when required, is extracted from the current instruction at the top of the instruction pipeline. For data processing instructions only the bottom eight bits (bits [7:0]) of the instruction are used in the immediate value. A data transfer (load or store) instruction computes a memory address in a manner very similar to the way a data processing instruction computes its result. A register is used as the base address, to which is added (or from which is subtracted) an offset which again may be another register or an immediate value. This time, however, a 12-bit immediate value is used without a shift operation rather than a shifted 8-bit value. The address is sent to the address register, and in a second cycle the data transfer takes place. Rat...
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