1. In a programmable controller system which includes a processor that executes a control program stored in the processor's memory to operate a machine and a program terminal which is coupled to the processor's memory to enable a user to enter and edit the stored control program through a keyboard and to display the stored control program on a display device, the improvement therein comprising: means coupled to the program terminal keyboard for receiving manually entered data which designates a location in the stored control program; first input means coupled to the program terminal keyboard for receiving a comment instruction which is manually entered through said keyboard including means for producing a corresponding mnemonic and comment number which are written into the designated location in the stored control program; comment storage means coupled to the program terminal; second input means coupled to the program terminal keyboard for receiving comment data and an associated comment number including means for writing said comment data and associated comment number into said comment storage means; means coupled to the program terminal display device for reading stored control program instructions and displaying symbolic representations of the control program instructions which includes: (a) means for detecting the comment instruction mnemonic when read from the stored control program for display; (b) means responsive to the comment number stored with the detected comment instruction mnemonic for fetching the corresponding stored comment data from the comment storage means; and (c) means for applying the fetched comment data to the program terminal display device for displaying the same to the user; wherein the processor includes means for inhibiting the execution of the comment instructions stored in the control program.