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Unformatted text preview: 6. Input/Output Register (I/O). This register is used to communicate with the input/output
devices. All input information such as instructions and data are transferred to this register
by an input device. Similarly, all output information to be transferred to an output device
is found in this register.
Figure 7.2 summarizes the functions of each of these registers.
SI. Name of Register
Memory Address (MAR)
Holds the address of the active memory location
Memory Buffer (MBR)
Holds information on its way to and from
Program Control (PC)
Holds the address of the next instruction to be
Accumulates results and data to be operated
Holds an instruction while it is being executed
Communicates with the I/O devices
Figure 7.2. Functions of various registers.
The execution of an instruction by the CPU during program execution normally involves
the following steps:
1. The control unit takes the address of the next program instruction to be executed
from the program control register and reads the instruction from the corresponding
memory address into the instruction register of the control unit.
2. The control unit then sends the operation part and the address part of the instruction
to the decoder and the memory address register respectively.
3. The decoder interprets the instruction and accordingly the control unit sends signals
to the appropriate unit that needs to be involved in carrying out the task specified in the
instruction. For example, if it is an arithmetic or logic operation, the signal is sent to the
ALU. In this case the control unit also ensures that the data corresponding to the address
part of the instruction is loaded in a suitable register in the ALU before the signal is sent
to the ALU. The ALU performs the necessary operation on the data and signals the
control unit as soon as it has finished.
4. As each instruction is executed, the address of the next instruction to be executed
automatically gets loaded into the program control register, and steps 1 to 4 get repeated.
The operations of the ALU and t...
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- Spring '14