VHDL Reference Guide

signal S4: SIGNED (3 downto 0); signal S8: SIGNED (7 downto 0);

Table 10-2 Number of Bits Returned by + and -

In some circumstances, you might need to obtain a carry-out bit from the + or - operation. To do this, extend the larger operand by one bit. The high bit of the return value is the carry-out bit, as illustrated in the example below.

process

variable a, b, sum: UNSIGNED (7 downto 0); variable temp: UNSIGNED (8 downto 0); variable carry: BIT;

begin

temp := CONV_UNSIGNED(a,9) + b; sum := temp(7 downto 0);

carry := temp(8); end process;

Comparison Functions

The std_logic_arith package provides functions to compare UNSIGNED and SIGNED data types with each other and with the predefined type INTEGER. Foundation Express compares the numeric values of the arguments, returning a BOOLEAN value. For example, the following expression evaluates TRUE.

UNSIGNED’("001") > SIGNED’("111")

The std_logic_arith comparison functions are similar to the built-in VHDL comparison functions. The only difference is that the std_logic_arith functions accommodate signed numbers and varying bit-widths. The predefined VHDL comparison functions perform bit-wise comparisons and so do not have the correct semantics for comparing numeric values. (See the “Relational Operators” section of the “Expressions” chapter.)

Foundation Express Packages

These functions produce comparators. The function declarations are listed in two groups, ordering functions (<, <=, >, and >=) and equality functions (= and /=) in the following examples.

Example 10-3: Ordering Functions

function "" functions">">"(L: UNSIGNED; R: UNSIGNED) return Boolean;

function ="" functions">">="(L: UNSIGNED; R: UNSIGNED) return Boolean;

Example 10-4: Equality Functions