VHDL allows the user to define own data types.
1 user-defined integer types
-- This is indeed the pre-defined type integer type integer is range -2147483647 to +2147483647; -- indeed the pre-defined type natural type natural is range 0 to +2147483647; -- user-defined subset of integers type my_integer is range -32 to 32; -- user-defined subset of integers or naturals type student_grade is range 0 to 100;
2 user-defined enumerated types
-- This is indeed the pre-defined type BIT type bit is (‘0‘,‘1‘); -- user-defined subset of std_logic type my_logic is (‘0‘,‘1‘,‘Z‘); -- indeed the pre-defined type of BIT_VECTOR -- range <> is used to indicate that the range is unconstrained -- NATURAL range <> indicate the range must fall within the NATURAL range type BIT_VECTOR is array (NATURAL range <>) of BIT; -- an enumerated data type, typical of finte state machines type state is (idle, forward, backward, stop); -- another enumerated data type type color is (red, green, blue, white)
The encoding of enumerated types is done sequentially and automatically. For example, for the type color above, two bits are necessary (there are four states), being ‘‘00’’ assigned to the ?rst state (red), ‘‘01’’ to the second (green), ‘‘10’’ to the next (blue), and ?nally ‘‘11’’ to the last state (white).
原文：http://www.cnblogs.com/mengdie/p/4596565.html