Components of Microprocessor
I/O (Input/Output)
I/O (input/output) communicates with the outside world. I/O includes two types of
devices: input and output; these I/O devices are known as peripherals.The input
devices such as keyboard, switches, and an analog to digital (A/D) converter
transfer binary information (data and instructions) from the outside world to the
microprocessor.
The output devices transfer data from the microprocessor to the
outside world. They include devices such as light emitting diodes (LEDs), the
cathode ray tube (CRT) or video screen, a printer, etc. uCs(PCs) are generally
equipped with output devices such as video screen(monitors) and a printer.
Memory
Memory stores such binary information as instructions and dataand provides that
information to the microprocessor whenever necessary. To execute programs, the
microprocessor reads instructions and data from the memory and performs the
computing operations in its ALU section. Results are either transferred to the output
section for display or stored in memory for later use.
Memory can be classified into two groups: Primary (system or main) memory and
Secondary (storage) memory.
Primary Memory
This is the memory the microprocessor uses in executing and storing programs.
This memory should be able to respond fast enough to keep up with the execution
speed of the microprocessor.Therefor it should be random access memory, meaning
that the microprocessor should be able to access information from any register with
the same speed(independent of its place in the chip). The size of the memory chip
is specified in terms of bits.Forexample,1K memory chip means it can store 1024
bits(not bytes).Examples of primary memory are R/WM (RAM) and ROM.
Secondary Memory
The other group is the storage memory, such as magnetic disks and tapes. This
memory is used to store programs and results after the completion of program execution. Information stored in these memories is nonvolatile, meaning
information remains intact even if the system is turned off. The microprocessor
cannot directly execute or process programs stored in these devices; programs need
to be copied into the primary memory first. Therefor the size of primary memory,
such as 512K or 8M (megabytes), determines how large a program the system can
process. The size of the storage memory is unlimited; when one disk or tape is full,
the next one can be used.
Cache Memory
Cache memory is a very high-speedsemiconductor memory which can speeds up
the CPU. It acts as a buffer between the CPU and the main memory. It is used to
hold those parts of data and program which are most frequently used by the CPU.
The parts of data and programs are transferred from the disk to cache memory by
the operating system, from where the CPU can access them.
PROCESSOR UNIT
The Microprocessor in many ways similar to the CPU, but includes all the logic
circuitry, including the control unit, on one chip. The microprocessor can be divided
into three segments for the sake of clarity: arithmetic/logic unit (ALU), register
array and control unit.
Arithmetic/Logic Unit: This is the area of the microprocessor where various
computing functions are performed on data. The ALU unit performs such arithmetic
operations as addition and subtraction, and such logic operations as AND, OR and
exclusive OR etc.
Register Unit: This area of microprocessor consists of various registers identified
by letters such as B, C, D, E, H, L. These registers are primarily used to store data
temporarily during the execution of a program and are accessible to the user through
instructions. These registers are also called general purpose registers. So, there are
all together six general purpose registers in 8085 Microprocessor.
Control Unit: The control unit provides the necessary timing and control signals
to all the operation in the microcomputer. It controls the flow of data between the
microprocessor and memory and peripherals.
Bus Structure of 8085
A bus in a microprocessor-based system is defined as a group of separate wires
which work together to perform a particular task. A microprocessor-based system,
or microcomputer, has three buses which combine to transfer information between
the microprocessor and other parts of the system, such as memory or input/output
devices. Typical tasks performed by these buses include selecting the source or
destination location address for a data transfer, actually moving the data from one
part of the system to another, and finally, controlling and synchronizing the
electronic devices involved in the data transfer process.
There are three buses in Microprocessor:
1. Address Bus
2. Data Bus
3. Control Bus
1. Address Bus: Generally, microprocessor has 16-bit address bus(Ao-A1s). The
bus over which the CPU sends out the address of the memory location is
known as Address Bus. The address bus carries the address of the memory
location to be written or to be read from. The address bus is unidirectional. It
means bit flowing occurs only in one direction, only from the microprocessor
to the peripheral devices.
We can find how much memory location is using by microprocessor by the formula
2N where N is the number of bits used for address lines.
Here 2'°= 65536 bits or 64KB. So, we can say 8085 can access 64KB of memory
locations.
Let us try this question “If a processor has 4GB of memory then how many address
lines are required to access this memory?Ans is 32 lines you need to find this by
your own.
2. Data Bus: 8085 microprocessor has 8-bit data buses. So, it can be used to
carry 8 bits of data starting from 00000000(00H) to 11111111(FFH). Here H
tells hexadecimal number. The data buses are bidirectional. These lines are
used for data flowing in both direction means data can be transferred or
received through these lines. The data bus also connects I/O ports and the
CPU. The largest number that can be appeared in the bus is 11111111. It has
8 parallel lines of data bus so it can access upto 28=256 data bus lines.
3. Control Bus: The control bus is used for sending control signals to the
memory and I/O devices. The CPU sends control signal on the control bus to
enable the outputs of addressed memory devices or I/O port devices.
Some of the control bus signals are as follows:
a) Memory read
b) Memory Write
c) VOread
d) TO Write
