1. A computer system comprising: a system bus having connected thereto at least a central processing unit (CPU), a coprocessor for receiving coprocessing commands from said CPU over said system bus, and at least one subcircuit for providing data to said CPU over said system bus, said system bus including system bus data signals and system bus address signals; a primary bus controller connecting said system bus to a transfer bus, said transfer bus having a storage device connected thereto, said transfer bus including transfer bus data signals and transfer bus address signals; and a secondary transfer bus controller for transferring data between said coprocessor and said storage device, independently of said system bus, simultaneously while said CPU is receiving data from said subcircuit over said system bus, said secondary transfer bus controller connected to said storage device by said transfer bus and connected to said coprocessor by a secondary transfer bus, said secondary transfer bus including secondary transfer bus data signals and secondary transfer bus address signals.