The three fundamental units, which comprise a CPU, have now been discussed in general terms. So too has a microprocessor, because a microprocessor can be defined as the central processing part of a computer contained within an IC (Integrated Circuit). Figure 15 illustrates how a microprocessor can be used as part of a microcomputer.
The microprocessor is small, lightweight, and relatively cheap when compared to any CPU. But it is also relatively slow, capable of processing only hundreds of instructions per second, compared to a large CPU which can process thousands of instructions per second, or a very fast CPU which can process millions of instructions per second (mips). However, many computing applications can tolerate the relative speed disadvantage of the microprocessor hence, its popularity. Microprocessors are typically available in 4, 8 and 16-bit word lengths.
The preceding paragraphs defined a microprocessor as a CPU within an IC. This is true of all microprocessors; however, many go beyond this 'minimum' definition. Microprocessors for machine control (lathes, robots, petrol pumps, etc) often incorporate ADC and DAC on the same chip, plus a small amount ROM and RAM.
Some microprocessors incorporate all the elements of a total computing system: I/O, ROM, RAM and CPU. Manufacturers designate these as single chip microcomputers. Obviously, their computing power is somewhat limited, because there is a limited amount of space available in just one IC.