Comment 2 for bug 1119626

don't know if this helps:

     The AS3 VM uses 32-bit Atom formats to store various types. 3-bits of the Atom describe the type data, leaving 29-bits for actual data. In some cases, like null/undefined/void/etc, we use 3-bits plus a few more bits to define the type.

    For signed integers, we have 28-bits to work with plus a sign bit. For unsigned integers we have 29-bits (or maybe 28 with the upper bit always zero?). Once your number gets larger than 28/29 bits, we dynamically switch it to a Number Atom type, which contains a 3-bit type value with a 29-bit ptr. All ptrs are 8-BYTE aligned so we get away with using 29-bits there.

    All three formats are basically interchangeable. When storing a number in an Atom, we see if it’s an integer and fits within the 28/29 bits. If so, we store it as a int/uint value – otherwise we store it as a Number value.