Internal Architecture of Intel 8085 Micro Processor (Study Notes)

Intel 8085 Micro Processor 

• Intel 8085 is an Eight bit (8 bit) NMOS (n-type Metal Oxide Semiconductor type) Microprocessor.
• It has 40 Pin on chip.
• This package is made on a single LSI (Large Scale Integrated) chip.
• The Intel 8085 requires +5V Dc supply for its operation.
• Its clock speed is about 3MHz.
• The clock cycle is of 320ns.
• This microprocessor has 80 basic instruction and 246 Op-Codes (Operation Codes).

Internal Architecture (Figure):

Internal Architecture (In Detail):

1.     Arithmetic & Logic Unit (ALU)

·        ALU handles all kind of Arithmetical and Logical Operation.

·        It performs-

o Addition and Subtraction 

o Logic-AND, Logic-OR, Logic-XOR and Logic-NOT operation

o Increment and Decrement Operations

o Left Shift, Right Shift, Rotate Left, Rotate Right operation

o Clear

·        ALU receives input from Accumulator and Temperary Register, and sends output to Internal Bus to store the value in different register location.

·        The output helps in change of status flags and is sent to Accumulator.

2.     Timing and Controlling Unit

·        It is a section of CPU.

·        Data flow between CPU and Peripheral is controlled by it.

·        The entire operations of Microprocessor and peripheral connected to it is controlled by Timing and Controlling Unit.

·        It is seen that control unit of CPU acts as the brain of a computer system.

3.     Registers:

·        Registers are used as a temporary storage system by microprocessor.

·        It is also used for Manipulation of Data and instructions.

·        Data saves/stores in the registers till they are sent to the memory or i/o devices  

·        Intel 8085icroprocessor has 6 type of registers. They are –

o  Accumulator – 1 (8 bit)

ü It is associated with ALU.

ü It is also known as ‘A’ or ‘Acc’.

ü It is used to hold one of operand of Arithmetic or Logical operation.

ü It serves as one input of ALU.

ü The final result of an Arithmetic or Logical operation is stored in Accumulator.

o Temporary Register (8 bit)

ü It is associated with the ALU.

ü It holds data during Arithmetic or Logical operation.

ü It is not accessible to user.

ü It is used by microprocessor and not accessible to programmer.

o General Purpose Registers (8 bit)

ü 8085 microprocessor has six general purpose registers. They are B, C, D, E, H & L.

ü To store sixteen-bit (16 bit) data, the combination of two register is done like B-C, D-E and H-L.

ü The H-L pair is also used to act as a memory pointer to hold a 16-bit address.

o Program Counter (16 bit)

ü It is used to hold the memory address of the next instruction to be executed.

o Stack Pointer (16 bit)

ü It is a stacking system to store data of Accumulator, Flags, Program Counter and General-Purpose Registers.

ü It is a sequence of memory location being det by programmer.

ü Stack works in L-I-F-O formats. L-I-F-O means Last IN First Out.

ü The pointer which address the stack is known as stack pointer.

o Instruction Register

ü Instruction Register holds the op-code of the instruction.

4.     Status Flag

·        8085 Microprocessor has five status flags. These are-

o Carry Flag (CS) – [CP = 1 (Have Carry), CP = 0 (No Carry)]

o Parity Flag (P)

o Auxiliary Carry Flag (AC) – [AC = 1 (Have Auxiliary Carry), AC = 0 (No Auxiliary Carry)]

o Zero Flag (Z) – [Z = 1 (Result = 0), Z = 0 (Result is not Zero)]

o Sign Flag (S) – [S = 0 (+Ve), S =1 (-Ve)]

5.     Program Status Word (PSW)

6.     Data or Address Bus

·        8085 Microprocessor has an 8-bit data bus and 16-bit address bus.

·        Pin A₈ to A₁₅ is used for transmitting 8 most significant Bits (MSB) of sixteen address.

·        The eight least significant bit (LSB) of the address are transmitted by address/data bus (AD₀ – AD₇)