put primordial binaries and their evolution into FP calculation as Gao (1991) did. However, the standard one-dimensional FP calculation, which assumes the isotropic distribution, is not appropriate to follow the binary population, since most binaries are formed in the core and their orbits are nearly radial. Thus, we have to solve the FP equation at least in three dimensions ($E$, $J$ and the binary binding energy $E_b$). It would require prohibitively large computer power. In addition, reliable two-dimensional FP calculation has become possible only very recently (Takahashi 1995). One could also use the Monte-Carlo approach, but its result must be compared with $N$-body simulation anyway.

We demonstrated that the $N$-body simulation with the number of particles close to real globular clusters is now possible, thanks to the extremely powerful special-purpose computer and its full-time availability. We are now able to use the direct $N$-body simulation to study various aspects of evolution of globular clusters.

Our 32k-particle calculation took about three months of CPU time on 1/4 of GRAPE-4. If we tried to do a similar calculation on a Cray T90 vector supercomputer, it would have taken several years of CPU time. It is simply impossible to do on present-day supercomputers. 