Foundation Express Packages

The VHDL operator overloading mechanism of VHDL determines the correct version from the function call’s argument types.

The CONV_INTEGER functions convert an argument of type INTEGER, UNSIGNED, SIGNED, or STD_ULOGIC to an INTEGER return value. The CONV_UNSIGNED and CONV_SIGNED functions convert an argument of type INTEGER, UNSIGNED, SIGNED, or STD_ULOGIC to an UNSIGNED or SIGNED return value whose bit width is SIZE.

The CONV_INTEGER functions have a limitation on the size of operands. VHDL defines INTEGER values as between -2147483647 and 2147483647. This range corresponds to a 31-bit UNSIGNED value or a 32-bit SIGNED value. You cannot convert an argument outside this range to an INTEGER.

The CONV_UNSIGNED and CONV_SIGNED functions each require two operands. The first operand is the value converted. The second operand is an INTEGER that specifies the expected size of the converted result. For example, the following function call returns a 10-bit UNSIGNED value representing the value in sig.

ten_unsigned_bits := CONV_UNSIGNED(sig, 10);

If the value passed to CONV_UNSIGNED or CONV_SIGNED is smaller than the expected bit-width (such as representing the value 2 in a 24-bit number), the value is bit-extended appropriately. Foundation Express places zeros in the more significant (left) bits for an UNSIGNED return value and uses sign extension for a SIGNED return value.

You can use the conversion functions to extend a number’s bit-width even if conversion is not required. An example follows.

CONV_SIGNED(SIGNED’("110"), 8) % "11111110"

An UNSIGNED or SIGNED return value is truncated when its bitwidth is too small to hold the ARG value. An example follows.

CONV_SIGNED(UNSIGNED’("1101010"), 3) % "010"

Arithmetic Functions

The std_logic_arith package provides arithmetic functions for use with combinations of Xilinx’s UNSIGNED and SIGNED data types and the predefined types STD_ULOGIC and INTEGER. These functions produce adders and subtracters.

VHDL Reference Guide

There are two sets of arithmetic functions; binary functions with two arguments, such as A+B or A*B, and unary functions with one argument, such as -A. The declarations for these functions are shown in the following examples.

Example 10-1: Binary Arithmetic Functions

function "+"(L: SIGNED; R: UNSIGNED) return STD_LOGIC_VECTOR; function "+"(L: UNSIGNED; R: INTEGER) return STD_LOGIC_VECTOR; function "+"(L: INTEGER; R: UNSIGNED) return STD_LOGIC_VECTOR; function "+"(L: SIGNED; R: INTEGER) return STD_LOGIC_VECTOR; function "+"(L: INTEGER; R: SIGNED) return STD_LOGIC_VECTOR;

Foundation Express Packages

function "-"(L: UNSIGNED; R: UNSIGNED) return STD_LOGIC_VECTOR; function "-"(L: SIGNED; R: SIGNED) return STD_LOGIC_VECTOR; function "-"(L: UNSIGNED; R: SIGNED) return STD_LOGIC_VECTOR; function "-"(L: SIGNED; R: UNSIGNED) return STD_LOGIC_VECTOR; function "-"(L: UNSIGNED; R: INTEGER) return STD_LOGIC_VECTOR; function "-"(L: INTEGER; R: UNSIGNED) return STD_LOGIC_VECTOR; function "-"(L: SIGNED; R: INTEGER) return STD_LOGIC_VECTOR; function "-"(L: INTEGER; R: SIGNED) return STD_LOGIC_VECTOR; function "-"(L: UNSIGNED; R: STD_ULOGIC) return STD_LOGIC_VECTOR; function "-"(L: STD_ULOGIC; R: UNSIGNED) return STD_LOGIC_VECTOR; function "-"(L: SIGNED; R: STD_ULOGIC) return STD_LOGIC_VECTOR; function "-"(L: STD_ULOGIC; R: SIGNED) return STD_LOGIC_VECTOR;

Example 10-2: Unary Arithmetic Functions

These unary arithmetic functions in the previous two examples determine the width of their return values as follows.

•When only one UNSIGNED or SIGNED argument is present, the width of the return value is the same as that argument’s.

•When both arguments are either UNSIGNED or SIGNED, the width of the return value is the larger of the two argument widths. An exception is when an UNSIGNED number is added to or subtracted from a SIGNED number of the same size or smaller, the return value is a SIGNED number one bit wider than the UNSIGNED argument. This size guarantees that the return value is large enough to hold any (positive) value of the UNSIGNED argument.

The number of bits returned by + and - is illustrated in the following table.

signal U4: UNSIGNED (3 downto 0); signal U8: UNSIGNED (7 downto 0);