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Alias declarations

An alias can be used to assign a name to an already declared object. This can be used as a shortcut or to reference part of an object as if it was a whole object. The syntax for the alias declaration is:

alias-declaration : ALIAS simple-name ":" subtype IS name ";"

The simple name will be an alias for name. The subtype specifies the type of the object denoted by the alias. Any time the alias is referenced it is equivalent to substituting that reference for the name part.

A typical example of how an alias might be used is to alias different fields represented by a vector. For example, an instruction code of a processor contains different fields, but they are stored together in one vector. The following example shows how an alias can be used to distinguish the fields of an IEEE floating point number stored in a vector.

  signal float : bit_vector (31 downto 0);
  alias sign : bit is float(31);
  alias exponent : bit_vector (7 downto 0) is float(30 downto 23);
  alias mantissa : bit_vector (22 downto 0) is float(22 downto 0);