A bidirectional communications link, termed a customization window, comprises a series of routines including functions, procedures and status flags implemented in software, interfaced between a machine control logic (MCL) operating to control the auxiliary functions of a computerized numerical control (CNC) system and a numerical control (NC) operating to control the multi-axis motions of the system whereby the MCL, implemented in software, can access the NC, also implemented in software, and control the system but only under conditions dictated by the NC and enforced by the routines of the customization window so that, for example, the integrity of the NC software will not be compromised by any of the MCL software which is of a user programmable type. Additionally, machine setup data and the availability of optional system control features are routed from the NC to the MCL through the customization window. Also the customization window conveys the status of the various operational conditions of the NC to the MCL.