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Unformatted text preview: of its memory address register. It then decrements the value
of the counter by one and increments the value stored in its memory address register by
7. The DMA controller then reads the next word from the I/O device and stores it in its
data register. It then interrupts the CPU, which yields a memory cycle and the memory
address and data buses to the DMA controller to enable it to transfer the word from its
data register to the appropriate memory location. The DMA controller then decrements
the counter value and increments the value stored in its memory address register. This
procedure is repeated till the counter value becomes zero.
This procedure, where a memory cycle is taken by the DMA controller from the CPU to
enable it to transfer data directly to memory is known as cycle stealing.
Note that the DMA architecture leads to more efficient CPU utilization because the I/O
request is largely processed by the DMA controller without direct participation of the
CPU. However, this advantage is gained by providing an extra hardware (the DMA
controller) and an additional bus (the device address bus).
Points to Remember
1. The CPU is the brain of a computer system. All major calculations and comparisons
performed by a computer are carried out inside its CPU. The CPU is also responsible for
activating and controlling the operations of other units of the computer system. Hence no other single component of a computer determines its overall performance as much as the
2. The two basic components of a CPU are the control unit and the arithmetic logic
The control unit of the CPU acts as a central nervous system for the other
components of the computer. It manages and coordinates the entire computer system
including the input and output units. It obtains instructions from the program stored in
the main memory, interprets the instructions, and issues signals that cause other units of
the system to execute them.
4. The Arithmetic Logic Unit (ALU) of the CPU is the place where the actual execution
of the instructions takes place during the data processing operation.
5. Every CPU has the built-in ability to execute a particular set of machine instructions,
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- Spring '14