GNU Emacs Lisp manipulates many different types of data. The actual data are stored in a heap and the only access that programs have to it is through pointers. Pointers are thirty-two bits wide in most implementations. Depending on the operating system and type of machine for which you compile Emacs, twenty-four to twenty-six bits are used to address the object, and the remaining six to eight bits are used for a tag that identifies the object's type.
Because Lisp objects are represented as tagged pointers, it is always
possible to determine the Lisp data type of any object. The C data type
Lisp_Object can hold any Lisp object of any data type. Ordinary
variables have type Lisp_Object, which means they can hold any
type of Lisp value; you can determine the actual data type only at run
time. The same is true for function arguments; if you want a function
to accept only a certain type of argument, you must check the type
explicitly using a suitable predicate (see section Type Predicates).
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