It's a bit more complex than this, but roughly each block must contain either all pointers/ints or no pointers. There's a tag at the beginning of each block in

It's a bit more complex than this, but roughly each block must contain either all pointers/ints or no pointers. There's a tag at the beginning of each block indicating what type of block it is (and some other stuff - the GC stores some state in the tag as well). If you have, for example, a record consisting of a pointer, an int, and a float, it'll be tagged as having all pointers/ints, and the float will actually be a pointer to a block tagged as data, that contains the float. This is the "float boxing" problem - floats cost an extra pointer chase in most cases. The 63-bit int is an optimization that avoids needing the extra indirection for ints. You can still have 64-bit ints, but they aren't the default, and they cost this extra pointer chase.