Architecture of 8085 Microprocessor


Definition: 8085 is an 8-bit microprocessor as it operates on 8 bits at a time and is created with N-MOS technology. This microprocessor exhibits some unique characteristics and this is the reason it still holds popularity among the microprocessors.

The following are the functional blocks in the 8085 Microprocessor

1. Accumulator (A-register)

It is an 8-bit register. It is associated with ALU. The accumulator is also called A-register. During the arithmetic / logic operations, one of the operands is available in Accumulator. The result of the arithmetic / logic operations is also stored in the Accumulator.


2. Temporary (TEMP) register

It is an 8-bit register. It is also associated with ALU. This register is used to hold one of the data (from memory or general-purpose registers) during an arithmetic / logic operation.


3. Arithmetic and Logic Unit (ALU)

The Arithmetic and Logic Unit includes Accumulator, Temporary register, arithmetic and logic circuits and flag register. The ALU can perform arithmetic (such as addition and subtraction) and logic operations (such as AND, OR and EX-OR) on 8-bit data. It receives the data from accumulator and or TEMP register. The result is stored in the accumulator. The conditions of the result (such as carry, zero) are indicated in the flags.


4. Flag register

It is an 8-bit register. But only five bits are used. The flag positions in the flag register are shown below.



Flag register of 8085: The flags are affected by the arithmetic and logic operations in the ALU. The flag register is also known as Status register or Condition code register. There are five flags namely Sign (S) flag, Zero (Z) flag, Auxiliary Carry (AC) flag, Parity (P) flag and Carry (CY) flag.


i) Sign (S) flag: Sign flag is set (1) if the bit D7 of the result in the accumulator is 1, otherwise it is reset (0). This flag is set when the result is negative. This flag is used only for signed numbers.


ii) Zero (Z) flag: Zero flag is set (1) if the result in the accumulator is zero, otherwise it is reset (0).


iii) Auxiliary Carry (AC): Auxiliary Carry flag is set (1) if there is a carry from bit position D3of result in the accumulator, otherwise it is reset (0). This flag is used for BCD operations.


iv) Parity (P) flag: Parity flag is set (1) if the result in the accumulator has even number of 1s, otherwise it is reset (0).


v) Carry (CY) flag: Carry flag is set (1) if the result of an arithmetic operation results in a carry from bit position D7, otherwise it is reset (0). This flag is also used to indicate a borrow condition during subtraction operations.


5. Instruction register

When an instruction is fetched from memory, it is stored in the Instruction register. It is an 8-bit register. This resister cannot be used in the programs.


6. Instruction Decoder and Machine cycle encoding

This unit decodes the instruction stored in the Instruction register. It determines the nature of the instruction and establishes the sequence of events to be followed by the Timing and control unit.


7. General purpose registers

There are six 8-bit general purpose registers namely B, C, D, E, H and L registers. B and C registers are combined together as BC register pair for 16-bit operations. Similarly D and E registers can be used as DE resister pair and H and L as HL register pair. The HL register pair is also used as memory pointer (M-register) for storing 16-bit address in some instructions. There are two more 8-bit temporary registers W and Z. These registers are used to hold data during the execution of some instructions. W and Z registers cannot be used in programs.


8. Stack Pointer (SP)

Stack is a portion of memory (RAM) used as FILO (First in Last Out) buffer. This is mainly used during subroutine operations. Stack Pointer is a 16-bit register used as a memory pointer (16-bit address) for denoting the stack position in memory. The Stack pointer is decremented each time when data is loaded into the stack and incremented when data is retrieved from the stack. Stack pointer always points to the top of the stack memory.


9. Program Counter (PC)

The Program Counter (PC) is a 16-bit register. It is used to point the address of the next instruction to be fetched from the memory. When one instruction is fetched from memory, PC is automatically incremented to point out the next instruction.


10. Incrementer / Decrementer

This unit is used to increment or decrement the contents of the 16-bit registers.


11. Timing and Control unit

The internal clock generator is available in this unit. This unit has the micro programs for all the instructions to carry out the micro steps required in completing the instructions. This unit receives signals from the Instruction decoder and Machine cycle encoding unit and generates control signals according to the micro-program for the instruction.


12. Interrupt control

There are five hardware interrupts available in 8085 Microprocessor namely TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR for interfacing the peripherals with the microprocessor. These interrupts are handled by the Interrupt control unit. INT A signal is generated by the Interrupt control unit as an acknowledgement for an interrupting device. If two or more interrupts occur at the same time, service is given according to the priority basis.


13. Serial I/O control

Serial data is transmitted to the peripherals through SOD pin and received through the SID pin. The SOD and SID pins are handled by the Serial I/O control unit using the SIM and RIM instructions.


14. Address buffer and Address / Data buffer


The Address buffer is an 8-bit unidirectional buffer from which the higher order address bits A8 – A15 leaves the microprocessor to the memory and peripherals. The Address / Data buffer is an 8-bit bidirectional buffer used for sending the lower order address bits A0 – A7 and sending and receiving the data bits D0 – D7 to the memory and peripherals.

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