Buses, microprocessors and microcontrollers

The parts of a computer communicate through sets of wires called buses:

• The data bus carries data between the CPU, memory and devices.
• The address bus carries memory addresses from the CPU, saying where to read or write.
• The control bus carries control signals, such as 'read' or 'write'.

Together, they make up the system bus.

The data bus is bidirectional (two-way): the CPU both reads data from memory and writes data to it. The address and control buses are unidirectional (one-way): addresses and commands go out from the CPU. During a read, the memory controller places the requested data on the data bus.

A microprocessor is a CPU on a single microchip, made of millions of tiny transistors, resistors and diodes. Microprocessors have developed through generations, from LSI chips in 1971 to today's SLSI chips with 64-bit, multicore designs (Table 1.2).

A microprocessor is described by four specifications:

• Word size: the number of bits it processes at a time (today 16 to 64).
• Memory size: how much RAM it can use; with 64-bit words, up to 16 EB.
• Clock speed: the number of pulses per second that time its work, now measured in gigahertz (GHz).
• Cores: the number of separate processing units on the chip: dual-core (2), quad-core (4), octa-core (8).

A microcontroller is a small computer on one chip: a CPU, a fixed amount of RAM, ROM and other peripherals (input-output parts), all together. It is small and cheap, and is embedded in devices that do one job: a washing machine, a microwave, a digital camera, a remote control.