Bus Structure

A microprocessor unit performs basically four operations: memory read, memory write, I/O read, I/O write. These operations are part of communication between MPU & peripheral devices. 

A communication includes identifying peripheral or memory location, transfer of data & control functions. These are carried out using address bus, data bus and control bus respectively. All the buses together are called the system bus. 

In case of 8085 MPU we have

• 8 unidirectional address pins
• 8 bi-directional multiplexed address/ data pins
• 11 control output pins
• 11 control input pins 

Data bus:

The data bus provides a path for data flow between the system modules. It consists of a number of separate lines, generally 8, 16, 32 or 64. The number of lines is referred to as width of the data bus. A single line can only carry one bit at a time, the number of lines determine how many bits can be transmitted at a time. The width also determines the overall system performance. 

An 8 bit data bus would require twice the time required by 16 bit data bus to transmit 16 bit data 

    8085 – 8 bit data bus

    8086 – 16 bit data bus

Address bus:

The address bus is used to designate the source and destination of data in data bus. In a computer system, each peripheral or memory location is identified by a binary number called an address. 

The width of the address bus determines the maximum possible memory capacity of the system. The address bus is also used to address I/O ports. Usually higher order bits are used to select particular modules and lower order bit select a memory location or I/O port within a module. 

    8085 – 16 bit address bus 

Thus, maximum amount of memory locations it can address 216 = 65, 536 or 64 Kb

Control bus:

These are group of lines used to control the data and address bus. Since this bus is shared by all the component of the microcomputer system; there must be some control mechanism to distinguish between data and address. The timing signals indicate the validity of data and address information; while command signal specify operations to be performed. Some of the control signals are:

• Memory write
• Memory Read
• I/O write
• I/O read
• ALE
• Interrupt Request
• Interrupt Acknowledge