The 8085 is one of Intel’s earliest microprocessors. It has a 40 pin IC and is an 8-bit microprocessor. This means that the microprocessor has an 8-bit data bus, which indicates that the microprocessor is capable of handling 8 bits of data. The 8085 can move 8-bits of data in a bidirectional direction. This processor has one of those hallmark architectures that a student can easily grasp.

Features/ Properties of 8085 Pin

  • It is an 8-bit microprocessor
  • Manufactured with N-MOS technology
  • 40 pin IC package
  • It has 16-bit address bus and thus has 216 = 64 KB addressing capability.
  • Operate with 3 MHz single-phase clock
  • +5 V single power supply

The logic pin layout and signal groups of the 8085nmicroprocessor are shown in Figure.

All the signals are classified into six groups:

  1. Address bus
  2. Data bus
  3. Control & status signals
  4. Power supply and frequency signals
  5. Externally initiated signals
  6. Serial I/O signals


                                           Fig. 6 8085 microprocessor pin layout and signal groups

Address and Data Buses:

  • A8 – A15 (output): Most significant eight bits of memory addresses and the eight bits of the I/O addresses.
  • AD0 – AD7 (input/output): Lower significant bits of memory addresses and the eight bits of the I/O addresses during first clock cycle. Behaves as data bus during third and fourth clock cycle.

Control & Status Signals:

  • ALE: Address latch enable
  • RD : Read control signal.
  • WR : Write control signal.
  • IO/M, S1 and S0: Status Power Supply & Clock Frequency:
  • Vcc: +5 V power supply
  • Vss: Ground reference
  • X1, X2: A crystal having frequency of 6 MHz is connected at these two pins
  • CLK: Clock output

Externally Initiated and Interrupt Signals:

  • RESET IN: When the signal on this pin is low, the PC is set to 0 and the processor is reset.
  • RESET OUT: This signal indicates that the processor is being reset. The signal can be used to reset other devices.
  • READY: When this signal is low, the processor waits for an integral number of clock cycles until it goes high.
  • HOLD: This signal indicates that a peripheral like DMA (direct memory access) controller is requesting the use of address and data bus.
  • HLDA: This signal acknowledges the HOLD request.
  • INTR: Interrupt request is a general-purpose interrupt.
  • INTA: This is used to acknowledge an interrupt.
  • RST 5, RST 6.5, RST 5,5 – restart interrupt: These are vectored interrupts and have highest priority than INTR interrupt.
  • TRAP: This is a non-maskable interrupt and has the highest priority.

Serial I/O Signals:

  • SID: Serial input signal. Bit on this line is loaded to D7 bit of register A using RIM instruction.
  • SOD: Serial output signal. Output SOD is set or reset by using SIM instruction.