It is assumed that instruction words are stored in

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Unformatted text preview: It holds the address of the active memory location. It is loaded from the program control register when an instruction is read from memory. 2. Memory Buffer Register (MBR). It holds the contents of the memory word read from, or written in, memory. An instruction word placed in this register is transferred to the instruction register. A data word placed in this register is accessible for operation with the accumulator register or for transfer to-the I/O register. A word to be stored in a memory location must first be transferred to the MBR, from where it is written in memory. 3. Program Control Register (PC). It holds the address of the next instruction to be executed. This register goes through a step-by-step counting sequence and causes the computer to read successive instructions previously stored in memory. It is assumed that instruction words are stored in consecutive memory locations and read and executed in sequence unless a branch instruction is encountered. A branch instruction is an operation that calls for a transfer to a non-consecutive instruction. The address part of a branch instruction is transferred to the PC register to become the address of the next instruction. To read an instruction, the contents of the PC register are transferred to the MAR and a memory read cycle is initiated. The instruction placed in the MBR is then transferred to the instruction register. 4. Accumulator Register (A). This register holds the initial data to be operated upon, the intermediate results, and also the final results of processing operations. It is used during the execution of most instructions. The results of arithmetic operations are returned to the accumulator register for transfer to main memory through the memory buffer register. In many computers there are more than one accumulator registers. 5. Instruction Register (I). It holds the current instruction that is being executed. As soon as the instruction is stored in this register, the operation part and the address part of the instruction (see Chapter 12) are separated. The address part of the instruction is sent to the MAR while its operation part is sent to the control section where it is decoded and interpreted_and ultimately command signals are generated to carry out the task specified by the instruction....
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This document was uploaded on 04/07/2014.

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