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8085 Architecture


The functional block diagram of architecture of 8085 Microprocessor is very important as it gives the complete details about a Microprocessor. Fig. shows the Block diagram of a Microprocessor.


8085 Bus Structure:
Address Bus:
  • The address bus is a group of 16 lines generally identified as A0 to A15.
  • The address bus is unidirectional: bits flow in one direction-from the MPU to peripheral devices.
  • The MPU uses the address bus to perform the first function: identifying a peripheral or a memory location.

Data Bus:
  • A data bus is a group of eight lines used for data flow.
  • These lines are bi-directional – data flow in both directions between the MPU and memory and peripheral devices.
  • The MPU uses the data bus to perform the second function: transferring binary information.
  • The eight data lines enable the MPU to manipulate 8-bit data ranging from 00 to FF (28 = 256 numbers).
  • The largest number that can appear on the data bus is 11111111.
Control Bus:
  • The control bus carries synchronization signals and providing timing signals.
  • The MPU generates specific control signals for every operation it performs. These signals are used to identify a device type with which the MPU wants to communicate.
Registers of 8085:
  • The 8085 have six general-purpose registers to store 8-bit data during program execution.
  • These registers are identified as B, C, D, E, H, and L.
  • They can be combined as register pairs-BC, DE, and HL-to perform some 16-bit operations.

Accumulator (A):
  • The accumulator is an 8-bit register that is part of the arithmetic/logic unit (ALU).
  • 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 that are set or reset according to the result of an operation.
  • The microprocessor uses the flags for testing the data conditions.
  • They are Zero (Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags. The most commonly used flags are Sign, Zero, and Carry.
The bit position for the flags in flag register is,

1.Sign Flag (S):

       After execution of an arithmetic and logical operation, if D7 of the result is 1, the sign flag is set. Otherwise it is reset.

       D7 is reserved for indicating the sign; the remaining is the magnitude of the number.

       If D7 is 1, the number will be viewed as a negative number. If D7 is 0, the number will be viewed as a positive number.


2.Zero Flag (z):

       If the result of arithmetic and logical operation is zero, then zero flags is set otherwise it is reset.


3.Auxiliary Carry Flag (AC):

       If D3 generates any carry when doing any arithmetic and logical operation, this flag is set.

       Otherwise it is reset.


4.Parity Flag (P):

       If the result of arithmetic and logical operation contains even number of 1’s then this flag will be set and if it is an odd number of 1’s it will be reset.


5.Carry Flag (CY):

       If any arithmetic and logical operation result any carry then carry flag is set otherwise it is reset.

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.
Interrupt Control Unit:
  • It receives hardware interrupt signals and sends an acknowledgment for receiving the interrupt signal.

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.


Stack Pointer (Sp):


  • 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).