Data Types

Access Types

Foundation Express does not support access (pointer) types, because no equivalent hardware construct exists.

File Types

Foundation Express does not support file (disk file) types, such as a hardware file type RAM or ROM.

Express Data Types

The std_logic_arith package provides arithmetic operations and numeric comparisons on array data types. The package also defines two major data types; UNSIGNED and SIGNED. These data types, unlike the predefined INTEGER type, provide access to the individual bits (wires) of a numeric value. For more information, see

“std_logic_arith Package” section of the “Foundation Express Packages” chapter.

Subtypes

A subtype is a subset of a previously defined type or subtype. A subtype definition can appear anywhere a type definition is allowed.

Using subtypes is a powerful way to use VHDL type checking to ensure valid assignments and meaningful handling of data. Subtypes inherit all operators and subprograms defined for their parent (base) types.

You can also use subtypes for resolved signals to associate a resolution function with the signal type. (See the “Subtype Declarations” section in the “Design Descriptions” chapter for more information.)

In the example of the Foundation Express STANDARD Package (in the “Predefined VHDL Data Types” section of this chapter), NATURAL and POSITIVE are subtypes of INTEGER, and they can be used with any INTEGER function. These subtypes can be added, multiplied, compared, and assigned to each other, as long as the values are within the appropriate subtype’s range. All INTEGER types and subtypes are actually subtypes of an anonymous predefined numeric type.

VHDL Reference Guide

The following example shows some valid and invalid assignments between NATURAL and POSITIVE values.

For example, the type BIT_VECTOR is defined as follows.

type BIT_VECTOR is array(NATURAL range <>) of BIT;

If your design uses only 16-bit vectors, you can define a subtype MY_VECTOR as the following.

subtype MY_VECTOR is BIT_VECTOR(0 to 15);

The following example shows that all functions and attributes that operate on BIT_VECTOR also operate on MY_VECTOR.

type BIT_VECTOR is array(NATURAL range <>) of BIT; subtype MY_VECTOR is BIT_VECTOR(0 to 15);

. . .

VEC1(4) <= S_BIT;

VEC2 <= "0000111100001111";

. . .

RIGHT_INDEX := VEC1’high;