Computer Organization: Address Bus

Address Bus

• The address bus is 16 bits wide, allowing it to specify 65,536 different addresses.
• The low-order bits on the address bus select a particular location within a selected memory IC.
• The address bus is used by the microprocessor to select a particular location or IC to be active.

Data Bus

• The data bus is bidirectional, allowing the microprocessor to send or receive 8 bits of data to or from a selected address.
• The data bus is used for data transfer between the microprocessor and memory or I/O devices.

Control Bus

• The control bus carries control signals that are tapped into by other ICs to determine the type of operation being performed.
• The control signals indicate whether the operation is a read, write, I/O, memory access, or other operation.

Address Decoder

• The address decoder is usually an octal decoder that ensures only one IC is active at a time to avoid bus conflicts.
• The address decoder receives information from the microprocessor via the control and address buses to determine which IC to activate.

Memory

• ROM (Read-Only Memory) contains initialization instructions and subroutines that can be called by the microprocessor.
• The monitor program or operating system is stored in ROM and is permanently stored.
• RAM (Random Access Memory) is volatile, meaning it loses its contents when power is turned off, and is used for temporary data storage.

Input Port

• The input port provides data to the microprocessor via the data bus.
• It is an octal buffer with three-stated outputs and receives input from devices like a keyboard or toggle switches.
• The input port dumps its information to the data bus when it receives a chip enable from the address decoder and a read command from the control bus.

Output Port

• The output port provides a way for the microprocessor to talk to the outside world.
• It can send data to output devices like printers or LEDs.
• An octal D flip-flop is used as the interface, and the microprocessor must set up the data bus and address of the output port to load the D flip-flop.