Abstract: In a control unit having a read only memory, a control program is provided which has provision for all features with which the machine may be equipped and for a selected number of future features. The control program contains one or more mainline control loops which interrogate mainline and feature functions sequentially to determine whether a function requires service. If a function is present and requires service, the appropriate routine is executed whereafter the mainline control program proceeds to execute the next indicated control program instruction. If a function is present, but does not require service, a normal return or other procedure is executed to return to the control loop. When a feature is addressed which is not present, a feature return is executed forcing the control program back to the instruction following the last branch.