1. A computer system, comprising: an external interface controller, operatively connected to an external interface bus; a control processor, connected to execute a first separate respective stream of instructions; a data transfer processor, which is operable concurrently with said control processor to execute a separate respective stream of instructions, and is connected to control said external interface controller; a numeric processor, which is operable concurrently and asynchronously with said control processor and data transfer processor, and which executes a respective instruction sequence under the overall control of said control processor; and a shared clock, connected to clock said control processor and said data transfer processor, said clock having a variable duration which is dependent on the instructions being executed by said control processor and also on the instructions being executed by said data transfer processor.