Architecture of 8085 Microprocessor

Architecture of 8085 Microprocessor

Register Structure:

• General Purpose Registers: There are six 8-bit general purpose registers B, C, D, E, H and L. They can be used as separate 8-bit registers or as 16-bit register pairs BC, DE and HL.

• Temporary Registers: 

Temporary Data Register: The ALU has two inputs. One input is provided by accumulator and other is provided from temporary data register.

W and Z Registers: These registers are used to hold 8-bit data during execution of some instructions.

• Special Purpose Registers: 

Register A: It is known as accumulator. It is an 8-bit register. It is used in arithmetic load and store operations as well as in input/output operations. It also stores the result of ALU.

Flag Register: It is an 8-bit register. Five bits carry significant information in form of flags:

Flag Register

1. S (Sign flag): After the execution of ALU operation, if D7 bit of the result is 1, sign flag is set. Otherwise it is reset.
2. Z (Zero flag): If the result of ALU operation is zero, zero flag sets. Otherwise it resets.
3. AC (Auxiliary carry flag): If there is a carry from D3 to D4 bit, AC flag sets. Otherwise it resets.
4. P (Parity flag): If the result of ALU operation has even number of 1s, the parity flag is set. If the result has odd number of 1s, the parity flag is reset.
5. CY (Carry flag): If there is overflow out of bit 7, then the carry flag sets. otherwise it resets.

Instruction Register: The processor fetches the opcode of instruction from memory and then it stores this opcode in instruction register.

• Sixteen Bit Registers:

Program Counter: The processor fetches the instructions from memory and executes them sequentially. Program counter is a special purpose register which stores the address of next instruction to be executed. So it acts like a pointer.

Stack Pointer: This 16-bit register is used to hold the address of the most recent stack entry.

Arithmetic and Logic Unit (ALU): ALU performs arithmetic and logic operations on 8-bit variables like addition, subtraction, complement, AND, OR, EX-OR, rotate and clear.

Instruction Decoder: The processor stores the fetched opcode from memory into the instruction register and then it is sent to instruction decoder. The instruction decoder decodes the opcode and then gives timing and control signals.

Address Buffer: It is an 8-bit unidirectional buffer. It drives external high order address bus. It also tristates higher order buses under certain conditions.

Incrementer/Decrementer Address Latch: It is a 16-bit register. It is used to increment or decrement the contents of PC or SP.

Address/Data Buffer: It is an 8-bit bidirectional buffer. It drives multiplexed address/data bus. It also tristates AD0 - AD7 under certain conditions.

Interrupt Control: The processor executes the instructions in sequence. But sometimes it is necessary to execute a special routine in a program. But after its execution, the program control must be transferred back to the main program. This function is done through interrupt control.

Serial I/O Control: In case of long distance data transmission, it is necessary to transmit data bit by bit to reduce the cost of cabling. For this serial communication, there are two lines SOD and SID. SOD is used to send data serially and SID is used to receive data serially.

Timing and Control Circuit: This circuitry is responsible for all the operations. All the operations are synchronized with the help of clock signal.