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    INTRODUCTION TO MICROPROCESSOR BASED SYSTEM The microprocessor is a semiconductor device (Integrated Circuit) manufactured by the VLSI (VeryLarge Scale Integration) technique. It includes the ALU, register arrays and control circuit on asingle chip. To perform a function or useful task we have to form a system  by using microprocessor as a CPU and interfacing memory, input and output devices to it. A system designed using amicroprocessor as its CPU is called a microcomputer. The Microprocessor based system (single board microcomputer) consists of microprocessor asCPU, semiconductor memories like EPROM and RAM, input device, output device and interfacingdevices. The memories, input device, output device and interfacing devices are called peripherals.The popular input devices are keyboard and floppy disk and the output devices are printer,LED/LCD displays, CRT monitor, etc. The above block diagram shows the organization of a microprocessor based system. In this system,the microprocessor is the master and all other peripherals are slaves. The master controls all the peripherals and initiates all operations. The work done by the processor can be classified into the following three groups. 1.Work done internal to the processor 2.Work done external to the processor 3.Operations initiated by the slaves or peripherals.  The work done internal to the processors are addition, subtraction, logical operations, data transfer operations, etc. The work done external to the processor are reading/writing the memory andreading/writing the J/O devices or the  peripherals. If the peripheral requires the attention of themaster then it can interrupt the master and initiate an operation. The microprocessor is the master, which controls all the activities of the system. To  perform aspecific job or task, the microprocessor has to execute a program stored in memory. The programconsists of a set of instructions. It issues address and control signals and fetches the instruction anddata from memory. The instruction is executed one by one internal to the processor and based onthe result it takes appropriate action. BUSES: The buses are group of lines that carries data, address or control signals.    The CPU Bus has multiplexed lines, i.e., same line is used to carry different signals .The CPU interface is provided to demultiplex the multiplexed lines, to generate chip selectsignals and additional control signals.    The system bus has separate lines for each signal .All the slaves in the system are connected to the same system bus. At any time instantcommunication takes place between the master and one of the slaves. All the slaves have tri-state logic and hence normally remain in high impedance state. Only when the slave isselected it comes to the normal logic. PERIPHERAL DEVICES:    The EPROM memory is used to store permanent programs and data.    The RAM memory is used to store temporary programs and data.    The input device is used to enter the program, data and to operate the system.    The output device is used for examining the results. Since the speed of I/O devices does not match with the speed of microprocessor, an interfacedevice is provided between system bus and I/O devices. Generally I/O devices are slowdevices. Advantages of Microprocessor based system 1. Computational/processing speed is high. 2. Intelligence has been brought to systems. 3. Automation of industrial processes and office administration.  4. Since the devices are programmable, there is flexibility to alter the system by changing the softwarealone. 5. Less number of components, compact in size and cost less. Also it is more reliable. 6. Operation and maintenance are easier. Disadvantages of Microprocessor based System 1. It has limitations on the size of data. 2. The applications are limited by the physical address space. 3. The analog signals cannot be processed directly and digitizing the analog signals introduces errors. 4. The speed of execution is slow and so real time applications are not possible. 5. Most of the microprocessors do not support floating point operations. Internal Architecture of 8085 Microprocessor   Control Unit Generates signals within uP to carry out the instruction, which has been decoded. In reality causes certain connections between blocks of the uP to be opened or closed, so that data goes where it is required, and so that ALU operations occur. Arithmetic Logic Unit The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’, ‘AND’, ‘OR’, etc. Uses data from memory and from Accumul ator to perform arithmetic. Always stores result of operation in Accumulator. Registers The 8085/8080A-programming model includes six registers, one accumulator, and one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack pointer and the  program counter. They are described briefly as follows. The 8085/8080A has six general-purpose registers to store 8-bit data; these are identified as B,C,D,E,H, and L as shown in the figure. They can be combined as register pairs - BC, DE, and HL - to perform some 16-bit operations. The  programmer can use these registers to store or copy data into the registers by using data copy instructions. Accumulator The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator. The accumulator is also identified as register A. Flags The ALU includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; they are listed in the Table and their  bit positions in the flag register are shown in the Figure below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data conditions. For example, after an addition of two numbers, if the sum in the accumulator id larger than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY)  –   is set to one. When an arithmetic operation results in zero, the flip-flop called the Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag register, adjacent to the accumulator. However, it is not used as a register; five bit positions out of eight are used to store the outputs of the five flip-flops. The flags are stored in the 8-bit register so that the programmer can examine these flags (data conditions) by accessing the register through an instruction. These flags have critical importance in the decision-making process of the microprocessor. The conditions (set or reset) of the flags are tested through the software instructions. For example, the instruction JC (Jump on Carry) is implemented to change the sequence of a program when CY flag is set. The thorough understanding of flag is essential in writing assembly language  programs.
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