Internal architecture of 8085

Arithmetic and Logic Unit (ALU):

• It is used to perform the arithmetic operations like addition, subtraction, multiplication, division, increment and decrement and logical operations like AND, OR and EX-OR.

• It receives the data from accumulator and registers.

• According to the result it set or reset the flags.

Program Counter (PC):

• This 16-bit register sequencing the execution of instructions.

• It is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a 16-bit register.

• The function of the program counter is to point to the memory address of the next instruction to be executed.
• When an opcode is being fetched, the program counter is incremented by one to point to the next memory location.
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• Stack Pointer (Sp):
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  • The stack pointer is also a 16-bit register used as a memory pointer.
  
  
  • It points to a memory location in R/W memory, called the stack.
  
  
• The beginning of the stack is defined by loading a 16-bit address in the stack pointer (register).

       Temporary Register: 

                     It is used to hold the data during the arithmetic and logical operations.

      Instruction Register: 

                  When an instruction is fetched from the memory, it is loaded in the instruction register.

        Instruction Decoder: 

                    It gets the instruction from the instruction register and decodes the instruction.        It identifies the instruction to be performed.

Serial I/O Control: 

                    It has two control signals named SID and SOD for serial data transmission.

Timing and Control unit:

• It has three control signals ALE, RD (Active low) and WR (Active low) and three status signals IO/M(Active low), S0 and S1.

• ALE is used for provide control signal to synchronize the components of microprocessor and timing for instruction to perform the operation.

• RD (Active low) and WR (Active low) are used to indicate whether the operation is reading the data from memory or writing the data into memory respectively.

• IO/M(Active low) is used to indicate whether the operation is belongs to the memory or peripherals.
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Interrupt Control Unit:

• It receives hardware interrupt signals and sends an acknowledgement for receiving the interrupt signal.