Tagged Pointers are a strategy to represent objects and values of a runtime system. They are applicable under the consideration that aligning memory addresses with CPU word size is desirable.
Tagging means reserving a couple bits (usually 1-2) for metadata. Metadata can be things like:
- Is this a pointer?
- Is this a primitive type (int, float)?
- Is this a special object (true, null)?
Consequently, the effective number of bits is reduced. For example in a 32-bit system, a tagged pointer representation of an int would be limited to 31 bits.