VHDL program for half subtractor:

Continued: LIBRARY ieee;

USE ieee.std_logic_1164_all;

Entity HS IS

PORT (A, B : IN STD_LOGIC;

Diff,Borrow: OUT STD_Logic);

END HS;

ARCHITECTURE BEHAVIOUR OF HS IS

BEGIN DIFF < = A XOR B;

BORROW < = (NOT A) AND B;

END BEHAVIOUR;

• A shift register which can shift the data in only one direction called as unidirectional shift register.

• A shift reg. which can shift data in both the directions is called as Bi-directional shift register.

• Applying the same logic a shift register which can shift the data in both directions as well as load it parallel, then it is called as universal shift register.

• It is capable of performing following operations:

1] Parallel loading (PIPO).

2] Left shift.

3] Right shift.

• For serial left operation the i/p is applied to serial i/p which goes to AND gate in figure. Whereas for shift right operation, the serial i/p is applied to D i/p (of AND – 8)

Operation.

1] With M = 0 shift right operation:

If M = 0 then AND gate 1,3,5,7 and 9 are enabled whereas remaining 2,4,6,8 and 10 are disabled. Clock 1 is s/g provided as for all ff for right suit operation. Serial i/p is provided through AND gates and data will shift from A ff to D ff bit by bit.

2] With M = 1 shift left operation:

If M = 1 then the AND gates 2,4,6,8 and 10 are enabled whereas the remaining gates 1,3,5,7 and 9 are disabled. Clock s/g is provided through AND gate 10 to all flip flops.