8085 System Bus

Typical system uses a number of busses, collection of wires, which transmit binary numbers, one bit per wire. A typical microprocessor communicates with memory and other devices (input and output) using three busses: Address Bus, Data Bus and Control Bus.

Address Bus
One wire for each bit, therefore 16 bits = 16 wires. Binary number carried alerts memory to ‘open’ the designated box. Data (binary) can then be put in or taken out.The Address Bus consists of 16 wires, therefore 16 bits. Its "width" is 16 bits. A 16 bit binary number allows 216 different numbers, or 32000 different numbers, ie 0000000000000000 up to 1111111111111111. Because memory consists of boxes,
each with a unique address, the size of the address bus determines the size of memory, which can be used. To communicate with memory the microprocessor sends an address on the address bus, eg 0000000000000011 (3 in decimal), to the memory. The memory the selects box number 3 for reading or writing data. Address bus is unidirectional, ie numbers only sent from microprocessor to memory, not other way.

Question?: If you have a memory chip of size 256 kilobytes (256 x 1024 x 8 bits), how many wires does the address bus need, in order to be able to specify an address in this memory? Note: the memory is organized in groups of 8 bits per location, therefore, how many locations must you be able to specify?

Data Bus
Data Bus: carries ‘data’, in binary form, between μP and other external units, such as memory. Typical size is 8 or 16 bits. Size determined by size of boxes in memory and μP size helps determine performance of μP. The Data Bus typically consists of 8 wires. Therefore, 28 combinations of binary digits. Data bus used to transmit "data", ie information, results of arithmetic, etc, between memory and the microprocessor.Bus is bi-directional. Size of the data bus determines what arithmetic can be done. If only 8 bits wide then largest number is 11111111 (255 in decimal). Therefore, larger number have to be broken down into chunks of 255. This slows microprocessor. Data Bus also carries instructions from memory to the microprocessor. Size of the bus therefore limits the number of possible instructions to 256, each specified by a
separate number.

Control Bus
Control Bus are various lines which have specific functions for coordinating and controlling uP operations. Eg: Read/NotWrite line, single binary digit. Control whether memory is being ‘written to’ (data stored in mem) or ‘read from’ (data taken out of mem) 1 = Read, 0 = Write. May also include clock line(s) for timing/synchronising, ‘interrupts’, ‘reset’ etc. Typically μP has 10 control lines. Cannot function correctly without these vital control signals.
The Control Bus carries control signals partly unidirectional, partly bi-directional. Control signals are things like "read or write". This tells memory that we are either reading from a location, specified on the address bus, or writing to a location specified. Various other signals to control and coordinate the operation of the system.
Modern day microprocessors, like 80386, 80486 have much larger busses. Typically 16 or 32 bit busses, which allow larger number of instructions, more memory location, and faster arithmetic. Microcontrollers organized along same lines, except: because microcontrollers have memory etc inside the chip, the busses may all be internal. In the microprocessor the three busses are external to the chip (except for the internal data bus). In case of external busses, the chip connects to the busses via buffers, which are simply an electronic connection between external bus and the internal data bus.

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