Abstract: A computer system has a system bus with several sockets or slots therein which receive memory modules. Each module can be used to implement at least one of a variety of functions. The functions include operating system programs, application programs, and random access memories (RAM). Each slot has a module select line coupled to it at the same location. A module select circuit is provided which enables the module select lines to select the appropriate module or modules. Modules implementing the same type of function, or different functions, can be plugged into the slots in any order. Each module implementing the same type of function is allocated the same address block and starts at the same address. In the cases of operating system modules and application program modules, the module which is enabled by the module select line acts as the program memory for the processor so that the program is instantaneously in memory from the time the module is selected.