One of the central issues that a garbage collector has to figure out is how to distinguish between what is a pointer and an integer? Given any value, is it a real reference or just a really big number? It's difficult to tell. One method to tell if a value is a reference is to check if the pointer resides in an area of allocated memory. If so, we just assume that this value is really a pointer, but there is no real proof that it isn't an actual integer. Another method is to use some of the bits in all values as a type tag. This is what SquirrelFish does.

The lowest 2 bits in a 32 bit field are for the type tag. If the two least significant bits are 0, the next 30 bits refer to a pointer address. If the least significant bit is a 1, the next 31 bits are used as a signed integer. For example, consider the signed integer 4.

Normal binary would be:
0000 0100

SquirrelFish internal representation:
0000 1001

There are other tags in SquirrelFish as well. The implementation details as well as the other comments are in JSImmediate.h..

 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX                     00
[ high 30 bits: pointer address ]   [ low 2 bits -- always 0 ]
 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX                     1
[ high 31 bits: signed integer  ]   [ lowest bit -- always 1 ]
 000000000000000000000000000      01               10
[ boolean value ]              [ bool ]       [ tag 'other' ]
000000000000000000000000000      10               10
[ zero ]                    [ undefined ]     [ tag 'other' ]
000000000000000000000000000      00               10
[ zero ]                       [ zero ]       [ tag 'other' ]

* Tip from Aaron. Pointers are guaranteed to point to aligned word boundaries.