The Intel (“eighty-eighty-five”) is an 8-bit microprocessor produced by Intel and introduced in It is a software-binary compatible with the more-famous. Microprocessor: It is a programmable electronics chip (Integrated Circuit ( IC)). A single IC has computing and decision making capabilities similar to. In addition to the microprocessor, a personal computer has a keyboard for The AS/ uses the PowerPC microprocessor with its reduced instruction set.

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Internal Architecture of Microprocessor: The architecture of consists of three main sections, ALU Arithmetic and Logical Unittiming and control unit and Registers shown in the following figure. A bit is a single binary digit. A word refers to the basic data size or bit size that can be processed by the arithmetic and logic unit of the processor.

A bit binary number is called a word in a bit processor. The number of bits that can be stored in a register or memory element is called a memory word. A bus is a group of wires lines that carry similar information. The system bus is a group of wires used for communication between the microprocessor and peripherals.

A single IC has computing and decision making capabilities similar to central processing unit of a computer. Lower order address bus is multiplexed with data bus to minimize the chip size. It can run at a maximum frequency of 3 MHz.

Intel 8085

The has extensions to support new interrupts, with three maskable interrupts RST 7. Three control signals are available on chip: Which indicate that the selected IO or Memory device is to be read and data is available on the data bus. Three status signals are available on chip: If it is high then IO operation and If it is low then Memory operation.

The ALU performs the microproocessor numerical and logical operations. The ALU performs the following arithmetic and logical operations. ALU includes the accumulator, the temporary register, the arithmetic and logic circuits and flags. It always stores result of operations in Accumulator. It generates timing and control signals, which micrpprocessor necessary for mmicroprocessor execution of instructions.

It controls data flow between CPU and peripherals including memory.

Microprocessor Pin Diagram Explained

Microprocessor communicates with memory and other devices input and output using three buses: The Address bus consists of 16 wires. 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 micrroprocessor address bus to the memory.

Address bus is unidirectional, i. Size of the data bus determines what arithmetic can be done.

Data bus also carries instructions from memory to the microprocessor. The control bus carries control signals partly unidirectional, partly bidirectional. Control signals are things like read or write.

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 ALU includes five flip-flops. The microprocessor uses these flags to test data conditions.

The conditions set or reset of the flags are tested through the software instructions. This 16 bit register deals with sequencing the execution of instruction. The microprocessor uses this register to sequence the execution of the instructions.

The function of the program counter is to point to the memory address from which the next byte is to be fetched. The stack pointer is also a imcroprocessor bit register used as a memory pointer. It points to a memory location in read-write memory, called the stack. Microporcessor store for the current instructions of a program.

Latest instruction sent micriprocessor from memory prior to execution. Decoder then takes instruction and decodes or interprets the instruction. Decoded instruction then passed to next stage. Holds address, microproessor from PC of next program instruction.

This block controls the use of the register stack. It can be used to store additional data during a program. Reads data or instruction from memory. Writes data or instruction into memory.

Microprocessor Architecture

Accepts data from input device. Sends data to output device. The Instruction Format: An instruction is a command to the microprocessor to perform a given task on a specified data. Each instruction has two parts, one is task to be performed, called the operation code opcodeand the second is the data to be operated on called the operand.

The instruction set is classified according to word size. A 1-byte instruction includes the opcode and operand in the same byte. Operands are internal registers and are coded into the instruction.

In a two-byte instruction, the first byte specifies the operation code and the second byte specifies the operand. Source operand is a data mictoprocessor immediately following the opcode. In a three byte instruction, the first byte specifies the opcode and the following two bytes specify the bit address. Note that, the second byte is the low-order address and the third byte is the high-order address.

The Addressing Modes: The various formats for specifying operands are called the addressing modes. Forthey are Immediate Addressing: Load the immediate data to the destination provided. Data is provided through the registers.

Used to accept data from outside devices to store in the accumulator or send the data stored in the microprocfssor to the outside device.

Effective address is calculated by the processor and the contents of the address is used to form a second address. The second address is where the data is stored. MOV A, [[]] Implicit addressing: In this addressing mode the data itself specifies the data to be operated upon.

Each instruction is represented by 8 bit binary value. Instruction set can be categorised int0 5 types: These instructions are used to transfer data from one register to another register, from memory to register or register to memory. When an instruction of data transfer group is executed, data is transferred from the source to the destination without altering the contents of the source. These instructions are used to perform arithmetic operations such as addition, subtraction, increment mocroprocessor decrement of the content of a register or memory.


8085 Microprocessor Study notes for Electronics and Communication

These instructions control machine functions such as Halt, Interrupt, or do nothing. Write assembly program for multiplying two 8 bit numbers.

MVI A,00 ; Load immediate data into accumulator. It is fairly easy to implement, but requires the processor to constantly read or write a single memory word 8-bits, bits or bits, depending on the device interface until the data transfer is complete. Although PIO is not necessarily slower than DMA, it does microprocsesor more processor cycles and can be detrimental in a multi-processing environment. The system can now continue by selecting another process for execution, thereby utilizing the CPU cycles typically lost when using PIO.

The DMA controller will inform the system when its current operation has been completed by issuing an interrupt signal. Although the data is still transferred 1 memory unit at a time from the device, the transfer to main memory now circumvents the CPU because the DMA controller can directly access the memory unit.

Steps involved in the mode of DMA transfer are as follows. Device wishing microprocessro perform DMA asserts the processors bus request signal.

Processor completes the current bus cycle and then asserts the bus grant signal to the device. The device then asserts the bus grant ack signal. The processor senses in the change in the state of bus grant ack signal and starts listening to the data and address bus for DMA activity. The DMA device performs the transfer from the source to destination address. During these transfers, the processor monitors the addresses microprocessro the bus and checks if any location modified during DMA operations is cached in the processor.

If the processor detects a cached address on the bus, it can take one of the two actions: Processor invalidates the internal cache entry for the address involved in DMA write operation Processor updates the internal cache when a DMA write is detected Once the DMA operations have been completed, the device releases the bus by asserting the bus release signal.

Processor acknowledges the bus release and resumes its bus cycles from the point it left off. This is an active high input signal to the from another master requesting the use of the address and data buses. After receiving the HOLD request, the Microprocessor relinquishes the buses in the following machine cycle.


All buses are tri-stated and a Hold Acknowledge signal is sent out. This is an active high output signal indicating that the MPU is relinquishing the micriprocessor of the buses.