pin daigram of 8085 microprocessor, addressing modes, flag register

                           

                 
                              PIN DIAGRAM OF 8085

8085 microprocessor consist 40 pins which are explain below :-

                                                                  

                                

HIGHER ORDER ADDRESS BUS :- The 8085 has eight higher order address lines A_15—A_{8, A}  Which are unidirectional and used as the high order address bus.

MULTIPLEX ADDRESS OR DATA BUS:-  the signal line  AD₇—AD₀  are bidirectional &   used for dual purpose. They are use  as the  low order address bus as well as data bus.in executing an instruction during the earlier part of the cycle these lines are used as lower order address bus , during the letter  part of the cycle these lines are used as data bus.

     

CONTROL AND STATUS SIGNAL:- This group of signal include two control signal   (RD & WR) and three status signal {I.O/M(bar) , S₁ & S_O}  to identify the nature of the operation and one special signal ALE(address latch enable) to indicate   the beginning of operation these signal are as follows.

 

      ALE:-   ALE stands for Address Latch Enable. It is the 3oth pin of 8085 which is used to enable or disable the address bus. the address bus will be enabled during the 1st clock cycle as the ALE pin goes high   during the first half cycle .

       READ(bar):- the read control signal (active low) this signal indicates the selected I/O or memory device Is to be read and data are available on the data bus.

                                    

  

WRITE(bar):- this write signal  (active low) this signal indicates the selected I/O or Memory device

is to be write.

IO/M(bar):-  this is a status signals used to diffrentiate b/w I/O and memory operation . when it is high, it indicates as I/O operations, when it is low it indicates memory operations.

S0--S1:-  these status signals similar to IO/M can identify various operation but they are used in small system.


                                        FLAG  REGISTER

It is a 8 bit register & it has five flags (flip flop) that are set or reset according to the result of the operation perform by ALU.
flags generally reflect data condition in the accumulator because the result is stored in accumulator in most operations.

S

z

AC

P

--

CR

                 

S- sign:-  After the execution of an arithmetic  or logic operation if bit D3 of the result usually in accumulator is 1, the flag is set. this flag is used with signed number in a given byte.

Z- Zero flag :-  zero flag is set if the ALU operation results in zero & the flag is re-set if the result is not zero. this flag is modified by the results in the accumulator as well as in the other register.

AC- auxiliary carry flag:-   in a arithmetic operation, when a carry is generated  in digit D3 and passed on the flag D4, the AC flag is set. the flag is used to only internally for BCD (binary code decimal) operations & is not available for the programmer to change the sequence of  a program with a jump instruction.

Parity flag:- after an arithmetic or logical operation if  the result has even number of one's (1), the flag is set. if it has odd number of one's the flag is reset.

Carry flag:- if an arithmetic operation results in a carry, the flag is set. other wise it is reset. the carry flag is also serves as a borrow flag for substraction.


          ADDRESSING MODE OF 8085 

different  ways of specifying the operand in the instruction are called addressing mode. 8085 supports 5 types  of addressing mode in the instruction. 

1. register addresing mode:-   in this addressing mode operand is available  in the registers only.

      Example:- mov A , B 

      Example:- ADD R

 2. immediate addressing mode:- in this addressing mode operand is available in the instruction itself. 

Example:- MVI  C , 65 H

Example:- LXI H A001 H

 3. direct addressing mode:-  in this addressing mode address of the operand is available in the instruction directly.

Example:- LDA 2000 H

Example:- STA  2001 H 

4. Indirect addressing mode:- in this addressing mode address of the operand is given in the instruction indirectly.

Example:- MOV A , M

5. Implied addressing mode:- in this addressing mode operand is available in the opcode.

Example:-  C= xx

                   D= xx    ( add c+d and result will be save in H register).