OVERVlEW OF MICROCOMPUTER STRUCTURE AND OPERATION

Memory

The memory section usually consists of a mixture of RAM (Random Access Memory) and ROM (Read Only Memory). It may also have magnetic floppy disks, magnetic hard disks, or optical disks (CDs, DVDs). Memory has two purposes. The first purpose is to store the binary codes for the sequences of instructions you want the computer to carry out. When you write a computer program, what you are really doing is writing a sequential list of instructions for the computer. The second purpose of the memory is to store the binary-coded data with which the computer is going to be working. This data might be the inventory records of a supermarket, for example.

Input/Output

The input/output or I/O section allows the computer to take in data from the outside world or send data to the outside world. Peripherals such as keyboards, video display terminals, printers, and modems are connected to the I/O section. These allow the user and the computer to communicate with each other. The actual physical devices used to interface the computer buses to external systems are often called ports. Ports in a computer function Just as shipping ports do for a count. An input port allows data from a keyboard, an A/D converter, or some other source to be read into the computer under control of the CPU. An output port is used to send data from the computer to some peripheral, such as a video display terminal, a printer, or a D/A converter. Physically, the simplest type of input or output port is just a set of parallel D flip-flops. If they are being used as an input port, the D inputs are connected to the external device, and the Q outputs are connected to the data bus which runs to the CPU. Data will then be transferred through the latches when they are enabled by a control signal from the CPU. In a system where they are being used as an output port, the D inputs of the latches are connected to the data bus, and the Q outputs are connected to some external device. Data sent out on the data bus by the CPU will be transferred to the external device when the latches are enabled by a control signal from the CPU.

Data Bus

The data bus consists of 8, 16, or 32 parallel signal lines. As indicated by the double-ended arrows on the data bus line in Figure 1, the data bus lines are bidirectional. This means that the CPU can read data in from memory or from a port on these lines, or it can send data out to memory or to a port on these lines. Many devices in a system will have their outputs connected to the data bus, but only one device at a time will have its outputs enabled. Any device connected on the data bus must have three-state outputs so that its outputs can be disabled when it is not being used to put data on the bus.

Address Bus

The address bus consists of 16, 20, 24, or 32 parallel signal lines. On these lines the CPU sends out the address of the memory location that is to be written to or read from. The number of memory locations that the CPU can address is determined by the number of address lines. If the CPU has N address lines, then it can directly address 2N memory locations. For example, a CPU with 16 address lines can address 216 or 65,536 memory locations, a CPU with 20 address lines can address 220 or 1,048,576 locations, and a CPU with 24 address lines can address 224 or 16,777,216 locations. When the CPU reads data from or writes data to a port, it sends the port address out on the address bus.

Control Bus

The control bus consists of 4 to 10 parallel signal lines. The CPU sends out signals on the control bus to enable the outputs of addressed memory devices or port devices. Typical control bus signals are Memory Read, Memory Write, I/O Read, and l/O Write. To read a byte of data from a memory location, for example, the CPU sends out the memory address of the desired byte on the address bus and then sends out a Memory Read signal on the control bus. The Memory Read signal enables the addressed memory device to output a data word onto the data bus. The data word from memory travels along the data bus to the CPU.

Central Processing Unit

The central processing unit or CPU controls the operation of the computer. In a computer the CPU is a microprocessor. The CPU fetches binary-coded instructions from memory, decodes the instructions into a series of simple actions, and carries out these actions in a sequence of steps. The CPU also contains an address counter or instruction pointer register, which holds the address of the next instruction or data item to be fetched from memory; general-purpose registers, which are used for temporary storage of binary data; and circuitry, which generates the control bus signals.

Hardware, Software, and Firmware

When working around computers, you hear the terms hardware, software, and firmware almost constantly. Hardware is the name given to the physical devices and circuitry of the computer. Software refers to the programs written for the computer. Firmware is the term given to programs stored in ROMs or in other devices which permanently keep their stored information.