The function count-gen-num-allocation
returns the amount of allocated data in generation gen-num. If include-lower-generations is non-nil, the returned value allocation also includes the data in the younger generations.
count-gen-num-allocation
is implemented only in 64-bit LispWorks. It is not relevant to the Memory Management API in 32-bit implementations, where you can use room-values instead.
On the Mobile GC, the gen-num argument can be 0, 1, 2 or 3 (3 means permanent).
LispWorks User Guide and Reference Manual - 20 Sep 2017