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Dushyanth Kundanati
Dushyanth Kundanati
MICROPROCESSORS by DUSHYANTH KUNDANATI UNIT-I Overview of 8085 1.1 Introduction 1.2 Draw and explain the functional block diagram of 8085 1.3 List the features of 8085 microprocessor 1.4 Explain multiplexing of Address and Data Bus 1.5 List different registers in 8085 and state their function. 1.6 Draw the pin out diagram of 8085 1.7 Explain the terms operation code, operand and illustrate these terms by writing an instruction. Understand the hex code for the same 1.8 Define fetch cycle, execution cycle and instruction cycle 1.9 Give the syntax and function of STA, LDA, IN, OUT instructions. 1.10 Draw the timing diagrams of the above instructions, and understand thoroughly in terms of clock cycles Introduction 1.1 A microprocessor can be simpl y defined as follows: • The microprocessor is a programmable device that takes in numbers, performs on them arithmetic or logical operations according to the program stored in memory (PROM) and then produces other numbers as a result.  The word microprocessor can be split into “micro” and “processor” : • The word “micro” means the level of scale used to fabricate an integrated chip (IC) i.e., a micro meter scale. • The word “processor” means the device that processes data esp., digital data (1’s and 0’s). • To process mean to manipulate i.e., to do some ar ithmetic and logical operations. ECE Page 1
MICROPROCESSORS by DUSHYANTH KUNDANATI Figure 1.1: Block diagram of a micro -computer Now, let me give you a brief history of microprocessors.  Latel y in the year 1960s processors were built using discrete components to perform few specific operations.  It is a part of the micro -computer that is defined in the figure 1.1 shown above.  A micro-computer is an entit y or unit in which a microprocessor is fabricated on a single chip IC -integrated chip.  These devices used to be too large in physical size and very slo w to operate. The processor time line is given in the figure 1. 2 for knowledge.. The functional block diagram o f 8085 1.2 The figure 1. 3 shows the functional block diagram of 8085 microprocessor. Let us look into the details of each block. ECE Page 2
MICROPROCESSORS by DUSHYANTH KUNDANATI Figure 1.3: Functional Block diagram or ARCHIT ECTURE of 8085 micr oprocessor  Accumulator • It is an 8-bit register used to perform arithmetic, logical, I/O & LOAD/STORE operations. • It is connected to internal data bus & ALU.  Arithmetic and logic unit • As the name suggests, it performs arithmetic and logical operations like Addition, Subtraction, AND, OR, etc. on 8 -bit data.  General purpose register • There are 6 general purpose registers in 8085 processor, i.e. B, C, D, E, H & L. • Each register can hold 8 -bit data. • These registers can work in pair to hold 16 -bit data and their pairing combination is like B-C, D-E & H-L.  Program counter ECE Page 3
MICROPROCESSORS by DUSHYANTH KUNDANATI • It is a 16 -bit register used to store the memory address location of the next instruction to be executed. • Microprocessor increments the program whe never an instruction is being executed, so that the program counter points to the memory address of the next instruction that is going to be executed.  Stack pointer • It is also a 16-bit register works like stack, which is always incremented / decremented by 2 during push & pop operations.  Temporary register • It is an 8 -bit register, which holds the temporary data of arithmetic and logical operations.  Flag register • It is an 8-bit register having five 1 -bit flip-flops, which holds either 0 or 1 depending upon the result stored in the accumulator. These are the set of 5 flip -flops: Sign (S), Zero (Z), Auxiliary Carry (AC), Parit y (P) and Carry (C) Its bit position is shown in the following diagram: Description of flag bits is given under “Registers” section.  Instruction register and decoder • It is an 8 -bit register. When an instruction is fetched from memory then it is stored in the instruction registers . • Instruction decoder decodes the information present in the Instruction register.  Timing and control unit • It provides timing and control signal to the microprocessor to perform operations. Following are the timing and control signals, which control external and internal circuits: ECE Page 4

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