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286 CHAPTER 6 / NONARITHMETIC COMBINATIONAL LOGIC DEVICES

begin
return((and_table)(l,r));
end "and";

architecture example of and_operation is
signal Y,A,B: std_logic_1164;
begin
Y <= A and B;
end example;

Here, std_logic_1164 refers to an IEEE standard logic package within which UX01 is a
subtype for 4-valued logic systems. Thus, the operation "and" takes on the new meaning
"and_table" contained in the standard package. Also, 1 and r (in line 1) are two of a class of
value kind attributes that return the leftmost element index (1) or rightmost element index
(r) of a given type or subtype.
Several operators, or constructs as they are sometimes called, have been used in the
examples given previously. Although it is true that the VHDL language contains a large
number of these constructs, only a few are necessary for most logic synthesis purposes. The
most important of these are given in Fig. 6.44. Included are assignment, relational, logical,

Assignment Operators:
<= Signal assignment := Variable assignment

Relational Operators:
= Equality /= Inequality > Greater than
< Less than <= Less than or equal >= Greater than or equal

Logical Operators:
and AND or OR nand NAND nor NOR
xor XOR xnor EQV not Logical negation
Arithmetic Operators:
+ Addition - Subtraction * Multiplication / Division
Shift Operators:
sll Shift left logical srl Shift right logical
rol Rotate left logical ror Rotate right logical
Miscellaneous Operators:
** Exponentiation abs Absolute value & Concatenation

FIGURE 6.44
Partial listing of important VHDL operators supported by the IEEE 1076-1993 standard.