The Architecture of 8085 Microprocessor

The functional block diagram or the internal architecture of the 8085 microprocessor is shown in the figure.

The functional blocks of an 8085 microprocessor are,

• Registers.
  • General Purpose Registers.
  • Special Purpose Registers.
  • Flag Registers.
• Arithmetic Logic Unit.
• Instruction Register and Decoder.
• Timing and Control Unit.
• Address Buffer.
• Address/Data Buffer.
• Incrementer/Decrementer Address Latch.
• Interrupt Control.
• Serial I/O Control.

Registers

Registers are used by the microprocessor for temporary storage and manipulation of data and instructions.

General-purpose Registers

• 6 general purpose registers – B, C, D, E, H and L.
• Stores 8-bit data.
• Can be used by the programmer.
• Can be combined as register pairs- BC, DE and HL – to store 16-bit data.

Special Purpose Registers

Accumulator(AC)

• 8-bit register.
• Part of ALU.
• Stores result of an operation.

Instruction Register(IR)

• 8-bit register.
• Hold the operation code of the instruction which is being decoded and executed.
• Not programmable.

Program Counter(PC)

• 16-bit register.
• Stores the address of the next instruction.

Stack Pointer(SP)

• 16-bit register.
• Points to a memory location in R/W memory called the stack.
• Beginning of the stack is defined by loading the address into SP.

Temporary Register and W and Z Registers

• 8-bit register(s).
• Used by microprocessor to store data temporarily.
• Internally used for execution of instructions.
• Cannot be accessed by the programmer.

Flag Registers

• Flag registers are special purpose registers that indicate status conditions.
• Flags are set or reset after an operation to indicate the data condition of the result.

Arithmetic and Logic Unit

• Performs the computing functions.
• It includes accumulator, temporary register, the arithmetic and logic circuits and five flags.

Instruction Register and Decoder

• The instructions that is fetched from memory is loaded in the instruction register.
• The decoder decodes the instruction and establishes the sequence of events to follow.

Address Buffer

• 8-bit unidirectional bus.
• Drive external high-order address bus. (A₁₅ – A₈)

Address/ Data Buffer

• 8-bit bidirectional bus.
• Drive multiplexed address/data bus. (AD₇-A₀)

Incrementer/ Decrementer Address Latch

• 16-bit register.
• Used to increment or decrement the contents of PC or SP.

Interrupt Control

• Controls interrupts

Serial I/O Control

• Provides control signals for serial communication.
• SID- Serial Input Data.
• SOD- Serial Output Data.

Timing and Control Unit

• Synchronizes all the microoperations with the clock and generate control signals necessary for communication between microprocessor and peripherals.