8085 Microprocessor

by E-Computer Concepts September 7, 2019 at 7:10 pm

A microprocessor is a semiconductor chip that implements the central processor of a computer. The microprocessor works as a brain of a computer. It consists of an arithmetic and logic unit (ALU) and a control unit.

The microprocessor are usually characterized by speed, word length, architecture and instruction set.

This central processing unit built into single chip is called as Microprocessor.

Features of 8085 Microprocessor

  • It is an 8 bit microprocessor i.e. it can accept or provide 8 bit data simultaneously.
  • It is a single chip, NMOS device implemented with 6200 transistors.
  • It requires a single + 5V power supply
  • It provides on chip clock generator, hence it does not require external clock generator, but it requires external tuned circuit like LC, RC or crystal.
  • The maximum clock frequency is 5 MHz. Normally the 8085 microprocessor operates at clock frequency of 3 MHz.
  • It has an 8 bit data bus and 16 bit address bus.
  • It provides 74 instructions with the following addressing modes : Register, Direct, Immediate, Indirect and Implied.
  • The data bus ( D0 – D7 ) is multiplexed with lower 8 bit address bus ( A0 – A7 ), hence it requires external hardware to separate data lines from address lines.
  • It provides 16 address lines, hence it can access 216 = 64 K bytes of memory, program as well as data memory.
  • It generates 8 bit I/O address, hence it can access 28 = 256 input ports and 256 output ports.
  • It performs the following arithmetic and logical operations : 8 bit binary addition, 16 bit binary addition with and without carry, 2 digit BCD addition, 8 bit binary subtraction with and without borrow, logical AND, OR, EX-OR, Complement and shift operations.
  • It provide 5 hardware interrupts : TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR.
  • It provides control signals (IO/M, RD, WR) to control the bus cycles hence external bus controller is not required.
  • It provides an eight bit accumulator, flag register, 6 general purpose registers (B, C, D, E, H, L), two special purpose sixteen bit registers (SP and PC)
  • It provides status for advanced control signal.
  • It can be used to implement three chip microcomputer using IC 8155 and IC 8355.
  • It provides two serial I/O lines viz SOD and SID, hence serial peripherals can be interfaced with 8285 directly.
  • It supports a mechanism that allows the 8085 to increase its interrupt handling capacity.

Pin Definitions of 8085

The 8085A is an 8 bit general purpose microprocessor having 40 pins and works on single power supply.

  1. Address Bus and Data Bus
  2. Status Signals and control Signals
  3. Interrupt Signals
  4. DMA request Signals
  5. Reset Signals
  6. Serial input output signals
  7. Clock Signals
  8. Power Supply Signals
Pin Diagram of 8085 | E-Computer Concepts

Address Bus and Data Bus

1) Address Bus ( A8 – A15 )

  • These are output, tristate signals used as higher order 8 bits of 16 bit address. These signal are unidirectional meaning that the address is given by 8085 to select a memory or an I/O device.

2) Multiplexed address/data bus (AD0 – AD7)

  • These are input/output, tristate signals having two set of signals. They are address and data. The lower order 8 bits, of 16 bit address is multiplexed or time shared with data bus. In an operation cycle during the earlier part it is used as lower order address and in later part it is used as data bus.

Status and Control Signals

1) Address Latch Enable (ALE)

  • This is an output signal, used to give information of AD0 – AD7 contents.
  • It is a positive going pulse generated during the first clock cycle of a machine cycle.
  • When pulse is high it indicates that the contents of AD0 – AD7 are address. When it is low it indicates that the contents are data.
  • The ALE signal is used to separate AD0 – AD7 ( i.e demultiplex) to A0 – A7 and D0 – D7.
  • To do this seperation a latch is connected to AD0 – AD7 lines and this latch is controlled by ALE signal.

2) Input output / Memory

  • This is an output status signal, used to give information of operation to be performed with memory or I/O device IO/M = 0 the microprocessor is performing a memory related operation.
  • IO/M = 1 the microprocessor is performing an I/O device related operation.

3) Status Signal ( S1 & S0 )

  • These are output status signals used to give information of operation performed by microprocessor. The S1 & S0 lines specifies four different conditions of 8085 machine cycles.
  • These 4 cycles are follows :
1.Opcode fetch (Instruction read from memory)11
2.Read (Data read from memory)01

When S0 and S1 is combined with IO/M we get status of all the machine cycles performed by 8085 as shown in Table

IO/M S1 S0 StatusControl Signal used
011Opcode FetchRD = 0
010Memory ReadRD = 0
001Memory WriteWR = 0
110I/O ReadRD = 0
101I/O WriteWR = 0
111Interrupt acknowledgeINTA = 0
Z00HaltRD, WR = Z & INTA =1
ZXXHold RD, WR = Z & INTA =1
ZXXReset RD, WR = Z & INTA =1

4) Read (RD)

  • This is an active low, output control signal used to read data from the selected memory locations or an I/O devices or port via the data bus.

5) Write (WR)

  • This is an active low, output control signal used to write data to the selected memory locations or an I/O devices or port via the data bus.


  • This is an active high input control signal. It is used by microprocessor to detect whether a peripheral is ready for the data transfer or not. If not the processor waits till the signal goes high.
  • The main function of this pin it to synchronize the microprocessor 8085 with slower peripherals.

Interrupt Signals


  • This is an active high, level and edge triggered, non-maskable, highest priority interrupt.
  • When TRAP line is active microprocessor performs internal restart automatically at address 0024 H.

2) Restart interrupt (RST 7.5, RST 6.5 & RST 5.5 )

  • These are active high, triggered maskable interrupt. They cause an internal restart to be automatically inserted.
  • The priorities of these are RST 7.5, RST 6.5, RST 5.5.
  • When RST 7.5, RST 6.5 or RST 5.5 is active microprocessor performs internal restart automatically at addresses 003C H, 0034 H, 002C H respectively.


  • INTR is an active high, level triggered general purpose interrupt.
  • It has the lowest priority.


  • INTA is used to indicate that the microprocessor has received an INTR interrupt.

DMA Request Signals


  • HOLD is an active high, input signal used by other controller to request microprocessor about use of address, data and control signals.
  • The microprocessor in response to HOLD generates a signal to acknowledge the requesting device by HLDA signal. When HLDA is active it indicates that microprocessor has received HOLD request and will relinquish the buses in next clock cycle. The other controller will use buses and upon completion of work will remove HOLD signal, because of this microprocessor will also make HLDA low. The microprocessor takes control of buses half clock cycle after HLDA goes low.

Reset Signals

1) Reset IN

  • This is an active low, input reset signal. When RESET IN = 0, it clears program counter i.e. 0000 and makes address, data and control lines tristated. After reset the status of internal register and flags are unpredictable.
  • The CPU is held in the reset condition as along as RESET IN is applied.
  • After reset the microprocessor starts executing instructions from 0000 H onwards.

2) Reset OUT

  • This is an active high output signal used to indicate that the microprocessor is reset. This signal is used as system reset, to reset other devices connected in system.

Serial I/O Signals

1) SID (Serial Input Data)

  • This is an active high, serial input port pin, used to accept serial 1 bit data under software control.
  • When a RIM instruction is executed the SID pin data is loaded in bit D7 of accumulator.

2) SOD (Serial Output Data)

  • This is an active high, serial output port pin, used to transfer serial 1 bit data under software control.
  • When a SIM instruction is executed the SOD pin is set or reset depending on D7 and D6 bits of accumulator.

Clock Signals

1) X1 X2

  • These are clock input signals, connected to crystal, LC or RC network. The X1 and X2 pins drive the internal clock generator circuit.
  • The frequency is divided by 2 and used as operating frequency. Generally the 6.014 MHz crystal is connected to X1 and X2, this is divided by 2. So the opearting frequency of 8085 is 3.07 MHz.


  • This is an output signal, used as a system clock. The internal operating frequency is available on CLK OUT pin.

Power Supply Signals


  • Power supply VCC – +5 V, VSS-Ground reference.

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