virt.cpp depends on defining the compiler
directive IN_RAM. If it is defined, then
VIRTRAM.CPP will be included for the in-ram memory system. The
virtual memory version will be used by including
VIRTDISK.CPP if IN_RAM is not defined. Both of these
files must be in the include path definition of your project, or
BC cannot find them. The matrix operation file,
VIRTOP.CPP works the same way for both memory models.