This microprocessor exhibits some unique characteristics and this is the reason it still holds popularity among the microprocessors. Basically, was the first commercially successful microprocessor by Intel. As some of the architectural drawbacks associated with was also eliminated by The size of the data bus of is 8 bits while that of the address bus is Therefore, can address 64 KB i. Also, as it can perform 8-bit operation thus the size of ALU is also 8-bit.

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Word: 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. Memory Word: The number of bits that can be stored in a register or memory element is called a memory word. Bus: A bus is a group of wires lines that carry similar information. System Bus: 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. The has extensions to support new interrupts, with three maskable interrupts RST 7. Three control signals are available on chip: i RD : it is a active low signal. Which indicate that the selected IO or Memory device is to be read and data is available on the data bus. If it is high then IO operation and If it is low then Memory operation.

Internal Architecture of Microprocessor: The architecture of consists of three main sections, ALU Arithmetic and Logical Unit , timing and control unit and Registers shown in the following figure. 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 controls data flow between CPU and peripherals including memory. Address Bus: 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 the address bus to the memory.

Address bus is unidirectional, i. Data Bus: Bus is bidirectional. 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 microprocessor uses these flags to test data conditions. The conditions set or reset of the flags are tested through the software instructions. Program Counter PC : 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. Stack Pointer SP : The stack pointer is also a 16 bit register used as a memory pointer. It points to a memory location in read-write memory, called the stack.

Latest instruction sent here from memory prior to execution. Decoder then takes instruction and decodes or interprets the instruction. Decoded instruction then passed to next stage. Register Selector: This block controls the use of the register stack. It can be used to store additional data during a program.

Memory Write: Writes data or instruction into memory. 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 opcode , and the second is the data to be operated on called the operand.

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

Two-Byte Instructions: In a two-byte instruction, the first byte specifies the operation code and the second byte specifies the operand. Source operand is a data byte immediately following the opcode. Three-Byte Instructions: 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. Load the immediate data to the destination provided. Example: MOV A, [] Indirect Addressing: 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. Example: MOV A, [[]] 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: Data transfer instructions: 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. Arithmetic instructions: These instructions are used to perform arithmetic operations such as addition, subtraction, increment or decrement of the content of a register or memory.

Machine Control Instructions: These instructions control machine functions such as Halt, Interrupt, or do nothing. Example Write assembly program for multiplying two 8 bit numbers. MVI A,00 ; Load immediate data into accumulator.

Although PIO is not necessarily slower than DMA, it does consume 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 to 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 on 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. 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.

The processor completes the execution of the current machine cycle; floats high impedance state the address, the data, and the control lines; and sends the Hold Acknowledge HLDA signal.

The DMA controller takes control of the buses and transfers data directly between source and destination, thus bypassing the microprocessor. At the end of data transfer, the controller terminates the request by sending a low signal to the HOLD pin, and the microprocessor regains control of the buses.


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