kifer(a)cs.sunysb.edu (Michael Kifer) writes: There's the old code and there's the new code. In the old code, it was done in the s&m files -- look at "DATA_SEG_BITS" defines. It's explained in the "Internals" manual: [...] Also, note that there is an implicit assumption here that all pointers are low enough that the top bits are all zero and can just be chopped off. On standard machines that allocate memory from the bottom up (and give each process its own address space), this works fine. Some machines, however, put the data space somewhere else in memory (e.g. beginning at 0x80000000). Those machines cope by defining `DATA_SEG_BITS' in the corresponding `m/' or `s/' file to the proper mask. Then, pointers retrieved from Lisp objects are automatically OR'ed with this value prior to being used. In the new code, which you get using `--with-minimal-tagbits', we silently assume that two least significant bits are always available for tagging.