Specifications

Language Structure
2-18 VHDL Reference Manual
When used with an array, the 'high attribute has a value of the
highest array index:
type my_array is array (0 to 99) of Boolean;
variable info : my_array;
info'high -- has a value of 99
There is a set of attributes that gives access to information about
signal waveforms. Most of these signal attributes are for simulation,
and have no other meaning. There is one signal attribute, however,
that is often used to describe clock logic. You can use 'event on
signals to specify edge sensitivity, usually in combination with a value
test to specify a rising or falling edge:
signal clock : Boolean;
if not clock and clock'event -- falling edge.
Note: An alternative to using the "clock and clock'event" method of
specifying a clock edge is to use the rising_edge() function provided
with the IEEE 1164 library.