The 8085 is an 8-bit microprocessor that operates on a single +5V power supply. It has features like 16 address lines allowing access to 64KB of memory, 8 I/O address lines, and ability to operate at up to 3MHz clock frequency. The architecture of the 8085 includes registers like accumulator, flag, and program counter registers, an ALU, and functional blocks for instruction decoding, addressing, interrupts, and I/O. It has a 40-pin package with connections for power, data/address bus, control signals, interrupts, serial I/O, DMA and reset.

2. Features:-
• It is an 8-bit microprocessor i.e. it can accept, process
or provide 8-bit data simultaneously.
• It operates on a single +5V power supply connected at
Vcc, power supply ground is connected to Vss.
• It can operate with a 3 MHz clock frequency.
• It has 16 address lines, hence it can access 64 Kbytes
of memory.
• It provides 8 bit I/O addresses to access 256 I/O
ports.

3. • In 8085, the lower 8-bit address bus(A0-A7) and
data bus(D0-D7) are multiplexed. Due to this
external latch is required to separate these.
• It supports 74 instructions.
• It has 8-bit accumulator, flag register, instruction
register, six 8-bit general purpose registers
(B,C,D,E,H and L) and two 16-bit registers (SP
and PC).

4. • It provides five hardware interrupts:
Trap, RST 7.5, RST 6.5,RST 5.5 and INTR.
• It has serial I/O control which allows serial
communication.
• It provides control signals (IO/M, RD, WR)
• The 8085 has an ability to share system bus with
Direct Memory Access (DMA) controller.

5. Architecture of 8085
It consists of various functional blocks as listed below:
• Registers
• ALU
• Instruction decoder and machine cycle encoder
• Address buffer and Data Buffer
• Interrupt control
• Serial I/O control
• Timing and control circuitry

7. Register Structure
• 8085 includes six 8-bit
registers (B, C, D, E, H and
L)
• One accumulator
• One flag register
• Two 16-bit registers (SP and
PC)
• Two temporary registers W
and Z

8. • General Purpose Register: B, C, D, E, H and L are
8-bit general purpose register can be as a separate 8-
bit registers or as 16-bit register pairs, BC, DE and
HL.
• Temporary Register: The ALU has two inputs, one
input is supplied by the accumulator and other from
temporary data register. These are internally used for
execution of most of the arithmetic and logical
instructions.

9. • Special Purpose Register:-
Accumulator: It is an 8-bit register. It is
extensively used in arithmetic, logic, load and store
operations, as well as in input/output operations.
Most of the times the result of arithmetic and
logical operations is stored in the accumulator.

10. • Flag Register: It is an 8-bit register, in which five of the bits
carry significant information in the form of flags.
S (Sign Flag): After the execution of arithmetic or logical
operation, if bit D7 of the result is 1, the sign flag is set. In a
given byte if D7 us 1, it is viewed as a negative number, else it
is viewed as a positive number.

11. Z (Zero Flag): The zero flag is set if the result of
operation in ALU is zero and flag resets if result is
non zero.
AC (Auxiliary Carry Flag): This flag is set if
there is an overflow out of bit 3, i.e. carry from
lower nibble to higher nibble(D3 to D4). This flag
is used for BCD operations.

12. P(Parity Flag): Parity is defined by the number
of ones present in the accumulator. After an
operation if the result has an even number of ones,
the flag is set, if the number is odd, flag is reset.
CY(Carry Flag): This flag is set if there is an
overflow out of bit 7. The carry flag also serves as a
borrow flag for subtraction.

13. • Instruction Register: The CPU stores the opcode in a
register called the instruction register.
• Program Counter: It stores the address of the next
instruction to be fetched.
• Stack Pointer: It is the reserved area in the RAM
where temporary information may be stored. It is
used to store the address of the most recent stack entry.

14. Arithmetic Logic Unit
The 8085’s ALU performs arithmetic and logical
functions on eight variables. The arithmetic unit
performs bitwise fundamental arithmetic operations
such as addition and subtraction. The logic unit
performs logical operations such as complement,
AND, OR, EX-OR, rotate and clear. The ALU
also looks after the branching decisions.

15. PIN
CONFIGURATION:-

16. The signals of 8085 can be classified into seven groups
according to there functions:-
• Power Supply and Frequency signals
• Data bus and address bus
• Control bus
• Interrupt signals
• Serial I/O signals
• DMAsignals
• Reset Signals

17. Power Supply and Frequency Signals:
• Vcc: It requires a single +5V power supply.
• Vss: Ground reference.
• X1 and X2: A tuned circuit LC, RC or crystal is
connected to these two pins for generating clock cycles.
• CLK OUT: This signal is used as a system clock for
other devices. Its frequency is half the oscillator
frequency.

19. Data Bus and Address Bus:-
• AD0 to AD7: The 8-bit data bus(D0–D7) is
multiplexed with the lower half of the 16 bit
address bus(A0-A7).
• A8 to A15: The upper half of the 16-bit address
bus appears on the address lines A8 to A15.
These lines are most significant 8 bits of the 16-
bit address lines.

21. Control and Status Signals:-
• ALE(Address Latch enable): The latching of lower half
of an address bus is done using an external latch and ALE
signals.
• RD and WR: These signals are basically used to control
the direction of the data flow between processor and
memory or I/O devices. A low on RD indicates that the
data must be read from the selected memory location or
I/O port via data bus. A low on WR indicates that the
data must be written into the selected memory location or
I/O port via data bus.

22. • IO/M, S0 and S1: IO/M indicates whether
I/o operation or memory operation is being
carried out. S1 and S0 indicates the type of
machine cycle in progress.
• READY: It is used by the microprocessor to
sense whether a peripheral is ready or not for
data transfer.

24. Interrupt Signals:-
The 8085 has five hardware interrupt signals: RST
5.5, RST 6.6, RST 7.5, TRAP and INTR. The
microprocessor recognizes request on these lines at the
end of the current instruction execution.
The INTA(interrupt acknowledge) signal is used to
indicate that the processor has acknowledged an
INTR interrupt.

26. Serial I/O Signals
• SID (Serial I/P Data): This input signal is
used to accept serial data bit by bit from the
external device.
• SOD (Serial O/P Data): This is an output
signal which enables the transmission of serial
data bit to the external device.

27. DMA Signal
• HOLD: This signal indicates that another
master is requesting for the use of address bus,
data bus and control bus.
• HLDA: This activate high signal is used to
acknowledge HOLD request.

28. Reset Signals
• RESET IN: A low on the pin, Sets the PC to zero
and Resets the interrupt enable and HLDA flip-flops.
• RESET OUT: This active high signal indicates
that processor is being reset. This signal is
synchronized to the processor clock and it can be used
to reset other devices connected to the system.

30. •