BLOCK DIAGRAM OF

INTEL 8085

Daggubati. SirishaVamsi
Asst.Prof-CSED-MVSR

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 Introduction to 8085
Introduced in 1977.

It is 8-bit MP.

It is a 40 pin dual-in-line chip.

It uses a single +5V supply for

its operations.
Its clock speed is about 3MHz.

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 Block Diagram of 8085

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 Three Units of 8085
Processing Unit

Instruction Unit

Storage and Interface Unit

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 Processing Unit
Arithmetic and Logic Unit

Accumulator

Status Flags

Temporary Register

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 Instruction Unit
Instruction Register

Instruction Decoder

Timing and Control Unit

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 Storage and Interface Unit
General Purpose Registers

Stack Pointer

Program Counter

Increment/Decrement Register

Address Latch

Address/Data Latch

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 Three Other Units
Interrupt Controller

Serial I/O Controller

Power Supply

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 Accumulator
 It the main register of microprocessor.

 It is also called register ‘A’.

 It is an 8-bit register.

 It is used in the arithmetic and logic operations.

 It always contains one of the operands on which

arithmetic/logic has to be performed.
 After the arithmetic/logic operation, the contents of
accumulator are replaced by the result.

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 Arithmetic & Logic Unit (ALU)
It performs various arithmetic and logic operations.

The data is available in accumulator and

temporary/general purpose registers.
Arithmetic Operations:

Addition, Subtraction, Increment, Decrement etc.

Logic Operations:

AND, OR, X-OR, Complement etc.

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 Temporary Register
It is an 8-bit register.

It is used to store temporary 8-bit operand from

general purpose register.
It is also used to store intermediate results.

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 Status Flags
Status Flags are set of flip-flops which are used to
check the status of Accumulator after the operation is
performed.

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 Status Flags
S = Sign Flag
Z = Zero Flag
AC = Auxiliary Carry Flag
P = Parity Flag
CY = Carry Flag

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 Status Flags
Sign Flag (S):

It tells the sign of result stored in Accumulator after the

operation is performed.

If result is –ve, sign flag is set (1).

If result is +ve, sign flag is reset (0).

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 Status Flags
Zero Flag (Z):

It tells whether the result stored in Accumulator is zero

or not after the operation is performed.

If result is zero, zero flag is set (1).

If result is not zero, zero flag is reset (0).

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 Status Flags
Auxiliary Carry Flag (AC):

It is used in BCD operations.

When there is carry in BCD addition, we add 0110 (6) to

the result.

If there is carry in BCD addition, auxiliary carry is set

(1).

If there is no carry, auxiliary carry is reset (0).

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 Status Flags
Parity Flag (P):

It tells the parity of data stored in Accumulator.

If parity is even, parity flag is set (1).

If parity is odd, parity flag is reset (0).

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 Program Status Word (PSW)
The contents of Accumulator and Status Flags clubbed
together is known as Program Status Word (PSW).
It is a 16-bit word.

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 Instruction Register
It is used to hold the current instruction which the
microprocessor is about to execute.
It is an 8-bit register.

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 Instruction Decoder
It interprets the instruction stored in instruction
register.
It generates various machine cycles depending upon
the instruction.
The machine cycles are then given to the Timing and
Control Unit.

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 Timing and Control Unit
It controls all the operations of microprocessor and
peripheral devices.
Depending upon the machine cycles received from
Instruction Decoder, it generates 12 control signals:
S0 and S1 (Status Signals).

ALE (Address Latch Enable).

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 Timing and Control Unit
RD (Read, active low).

WR (Write, active low).

IO/M (Input-Output/Memory).

READY

RESET IN

RESET OUT

CLK OUT

HOLD and HLDA

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 General Purpose Registers
 There are 6 general purpose registers, namely B, C, D, E, H, L.

 Each of the them is 8-bit register.

 They are used to hold data and results.

 To hold 16-bit data, combination of two 8-bit registers can be used.

 This combination is known as Register Pair.

 The valid register pairs are:

 B – C, D – E, H – L.

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 Program Counter
It is used to hold the address of next instruction to be
executed.
It is a 16-bit register.

The microprocessor increments the value of Program

Counter after the execution of the current instruction,
so that, it always points to the next instruction.

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 Stack Pointer
It holds the address of top most item in the stack.

It is also 16-bit register.

Any portion of memory can be used as stack.

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 Increment/Decrement Register
This register is used to increment or decrement the
value of Stack Pointer.
During PUSH operation, the value of Stack Pointer is
incremented.
During POP operation, the value of Stack Pointer is
decremented.

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 Address Latch
It is group of 8 buffers.

The upper-byte of 16-bit address is stored in this latch.

And then it is made available to the peripheral devices.

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 Address/Data Latch
 The lower-byte of address and 8-bit of data are
multiplexed.
 It holds either lower-byte of address or 8-bits of data.

 This is decided by ALE (Address Latch Enable) signal.

 If ALE = 1 then

Address/Data Latch contains lower-byte of address.

 If ALE = 0 then

It contains 8-bit data.

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 Serial I/O Controller
It is used to convert serial data into parallel and
parallel data into serial.
Microprocessor works with 8-bit parallel data.

Serial I/O devices works with serial transfer of data.

Therefore, this unit is the interface between

microprocessor and serial I/O devices.

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 Interrupt Controller
It is used to handle the interrupts.

There are 5 interrupt signals in 8085:

TRAP

RST 7.5

RST 6.5

RST 5.5

INTR

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 Interrupt Controller
Interrupt controller receives these interrupts according
to their priority and applies them to the
microprocessor.
There is one outgoing signal INTA which is called
Interrupt Acknowledge.

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 Power Supply
This unit provides +5V power supply to the
microprocessor.
The microprocessor needs +5V power supply for its
operation.

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