We know a computer is a machine that processes data (stored in main memory) into information, under control of a stored program. We also know that, internally, a computer is a binary machine; thus the data and the program instruictions must be stored in binary form. Characters are represented in (71) . Numbers are stored as binary numbers, with each bit’s positional value significant. A computer’s main memory is divided into bytes, words or both (depending on the system), and each of these basic storage units is assigned an (72) . Using this address, the processor can read or write selected bytes or words.
The processor consists of a clock, an instruction control unit, an arithmetic and logic unit, and registers. Once a program is stored in main memory, the processor can begin to execute it. During (73) , the instruction control unit fetches an instruction from main memory; during (74) , the arithmetic and logic unit executes it. Precisely timed electronic pulses generated by the clock drive this basic (75)
（75）处填（）。
A．clock pulse
B．instruction
C．memory access
D．machine cycle