A VHDL design begins with an ENTITY block that describes the interface for the design. The interface defines the i/p and o/p logic signals of the ckt to be designed.

The ARCHITECTURE Block describes the internal operation of the design.

Fig below shows the VHDL program structure.

Entity entity-name is

[port (interface-signal-declaration;]

End [entity] [entity – name] ;

Architecture architecture-name of entity – name is

[declarations]

Begin

Architecture Body

End[architecture] [architecture name];

e.g. VHDL (format) example for NOR latch

Library ieee;

Use ieee_std_logic_1164.all;

Entity latch is

Port (s,r : m std_logic;

Q,rq:out std_logic);

End later;

Architecture flip flop of latch is

Begin

Q < = n more nq;

Nq < = s nor q;

End flip flop;