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Dale - Computer Science Illuminated 218

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7.3 Machine Language 191 (Refer back to Chapter 2 for more information on hexadecimal digits.) The word length in Pep/7 is 2 bytes, or 16 bits. Thus the information that flows into and out of the ALU (arithmetic/logic unit) is 16 bits in length. Recall from Chapter 5 that a register is a small area of storage in the arithmetic/logic unit of the CPU used to hold special data and intermediate values. Pep/7 has seven registers, four of which we focus on at this point: The program counter (PC), which contains the address of the next instruction to be executed The instruction register (IR), which contains a copy of the instruction being executed The index register (X register) The accumulator (A register) The index register and the accumulator are used to hold data and the
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Unformatted text preview: results of operations; these are the special storage registers referred in Chapter 5 in the discussion of the ALU. We realize that this is a lot of detailed information, but don’t despair! Remember that our goal is to give you a feel for what is actually happening at the lowest level of computer processing. By necessity, that processing keeps track of many details. Figure 7.1 shows a diagram of Pep/7’s CPU and memory. Notice that the addresses in memory are in red. This color is to emphasize that the addresses Figure 7.1 Pep/7’s architecture A register (accumulator) Pep/7's CPU X register Program counter (CP) Instruction register (IR) 0000 Pep/7's Memory 0001 0002 0FFE . . 0FFF...
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