We describe GRAPE-4, a special-purpose computer for gravitational N-body simulations. In gravitational N-body simulations, almost all computing time is spent for the calculation of interaction between particles. GRAPE-4 is a specialized hardware to calculate the interaction between particles. It is used with a general-purpose host computer that performs all calculations other than the force calculation. With this architecture, it is relatively easy to realize a massively parallel system. In 1991, we developed the GRAPE-3 system with the peak speed equivalent to 14.4 Gflops. It consists of 48 custom pipelined processors. In 1992 we started the development of GRAPE-4. The GRAPE-4 system will consist of 1920 custom pipeline chips. Each chip has the speed of 600 Mflops, when operated on 30 MHz clock. A prototype system with two custom LSIs has been completed July 1994, and the full system is now under manufacturing.

Introduction. In this paper, we describe the GRAPE-4 system, a special-purpose computer for astrophysical N-body simulations. Many astrophysical objects are well approximated as a gravitational N-body system, in which point mass particles interact only through gravity. The solar system, open clusters, globular clusters, galaxies, clusters of galaxies, and superclusters are all well expressed as gravitational N-body systems. The N-body simulation is the most powerful tool to investigate the behavior of these astrophysical objects. N-body simulations are, however, very expensive since the number of interactions is proportional to the square of the number of particles. In the following, we give a few examples of the difference between what is possible with present computers and the real systems. Planets are believed to be formed from smaller planetesimals. It is believed that there were about 1010 planetesimals, while the number of particles used in simulations is 103 or less.