880 nanosecond particle in cell mover for the CDC 7600

A very fast computational method of moving particles for one dimensional electrostatic plasma simulations using integer arithmetic is described. The cloud in cell method forms the basis of this technique. This paper is in the form of a compass compilable subroutine with comments and examples describing methods of using an entirely integer representation to gain up to an order of magnitude increase in speed over equivalent floating point Fortran coding. Integer arithmetic has several advantages over floating point arithmetic for one dimensional particle movers. The adds are much faster, and the binary numerical description allows the implementation of very simple boundary conditions if the simulation region extends from zero to a power of two. Furthermore, integer arithmetic makes very efficient use of each memory bit since there is no floating point exponent. Consequently, it becomes feasible and fast in long word length machines to pack the velocity and position of a particular particle into the same word, thus saving a factor of two in computer storage and/or 10. These advantages are combined with the fact that integer adds and logicals complete in only two computer cycles allowing nearly complete optimization of register and instruction interleaving. The measured timings on a CDC 7600 are 880 nanoseconds and 935 nanoseconds per particle for the periodic and general boundary condition sections respectively. Methods for adapting the technique to other computers are discussed