signal_name'pos
signal_name'val
signal_name'length
signal_name'event

T'POS(X)    = is the integer position of X in the discrete type T.
T'VAL(X)    = is the value of discrete type T at integer position X.
A'LENGTH    = is the integer value of the number of elements in array A.
A'LENGTH(N) = is the number of elements of dimension N of array A.
S'EVENT     = is true if signal S has had an event this simulation cycle.

VHDL Predefined Attributes
Attributes in VHDL
VHDL Reference Guide – Attributes
User-defined Attributes