Architecture of 8085

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8085

ALU

The ALU performs the arithmetic and logicaloperations. The operations performed by ALUof 8085 are addition, subtraction, increment,decrement, logical AND, OR, EXCLUSIVE -OR,compare, complement and left / right shift .

Accumulator and temporary register • The accumulator and temporary register are

used to hold the data during an arithmetic /logical operation. After an operation theresult is stored in the accumulator and theflags are set or reset according to the result ofthe operation.

TIMING & CONTROL UNIT

The timing and control unit synchronizes all the

microprocessor operations with the clock and

generates the control signals necessary for

communication between the microprocessor

and peripherals.

INSTRUCTION REGISTER & DECODER

• When an instruction is fetched from memory it is placed in instruction register. Then it is decoded and encoded into various machine cycles.

STACK POINTER (SP)

• It is also a 16-bit register which is used as amemory pointer.

• It maintains the address of the last byte that isentered into stack.

PROGRAM COUNTER (PC)

• The program counter (PC) keeps track ofprogram execution.

• It is a special purpose register.

• A program counter stores the address of thenext instruction to be executed.

FLAG REGISTER

There are five flags in 8085, which are sign flag

(8), zero flag (Z), auxiliary carry flag (AC), parity

flag (P) and carry flag (CY). The bit positions

reserved for these flags in the flag register are

shown in figure below.

The Flags register• There is also a flag register whose bits are affected by

the arithmetic & logic operations.

• S-sign flag

• The sign flag is set if bit D7 of the accumulatoris set after an arithmetic or logic operation.

• Z-zero flag

• Set if the result of the ALU operation is 0.Otherwise is reset. This flag is affected byoperations on the accumulator as well as otherregisters. (DCR B).

• AC-Auxiliary Carry

• This flag is set when a carry is generated frombit D3 and passed to D4 . This flag is used onlyinternally for BCD operations.

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• P-Parity flag

• After an ALU operation, if the result has an even #of 1s, the p-flag is set. Otherwise it is cleared. So,the flag can be used to indicate even parity.

• CY-carry flag

• This flag is set when a carry is generated from bitD7 after an unsigned operation.

• OV-Overflow flag

• This flag is set when an overflow occurs after asigned operation.

Unspecified

7 6 5 4 3 2 1 0SF ZF X AF X PF X CF

General Purpose Registers• 8085 consists of 6 special types of registers called

General Purpose Registers. These GeneralPurpose Registers are used to hold data like anyother registers .

• The GPR in 8085 processors are B,C,D,E,H,L.Eachregisters can hold 8-bit data.

• They can work in pair such as B-C,D-E,H-L to store16-bit data.

Temporary Register

This register acts as a temporary memory during

the arithmetic and logical operations.

Unlike other registers, this temporary registers

can only be accessed by the microprocessor and

it is completely inaccessible to programmers.

Temporary register is an 8-bit register.

PSW

• The combination of these 8-bits is calledProgram Status Word(PSW).PSW and theaccumulator as a 16-bit unit for the stackoperation.