When DMA is used with a block device, the data are transferred directly between the device and main
memory, without requiring assistance from the operating system and the CPU. The operating system starts the
transfer by specifying an address at which to start and the count of bytes to transfer. The CPU is then free to
continue computing while the data are moved in or out. This is a further improvement in system efficiency, and
today DMA is almost universally used for disk and other block transfers.
SUMMARY
Modern computers implement the von Neumann architecture, or stored program computer design. Program
instructions and data are both stored in main memory. The components of the computer design are the CPU
(including the control unit and the ALU), memory, and input/output.
Computers operate in base-2 arithmetic. Any number base can be used for computation and, just as humans
find 10 fingers a convenient basis for computation, machine builders find 2-state (on??“off) devices easy to build and
convenient for computation. We discussed the simple math facts for binary math, and showed how subtraction is
accomplished using 2??™s-complement addition. We also discussed the concept of the computer word, and the
implications of computer word sizes of different numbers of bits.
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