VHDL Data Types:

Scalar objects can hold only one data value at a time. A simple example is the integer data type. Variables and signals of type integer can only be assigned integers within a simulator-specific (although the VHDL standard imposes a minimum) range.

In the above example, the first two variable assignments are valid since they assign integers to variables of type integer. The last variable assignment is illegal because it attempts to assign a real number value to a variable of type integer.