A machine tool data system for receiving and storing abbreviated instructions representing blocks of machine tool control data and including pattern type instructions such as copy, mirror image and tool change each involving a series of such blocks, the computer responding to editing instructions such as insert or delete as to specified lines of stored instructions or computing the blocks represented by the stored instructions for transmission to an output device such as a tape punch, the computer generating the blocks of a pattern type series at the rate of utilization of said blocks by the output device. A machine tool may be connected on line and controlled by the computer simultaneously with the editing or tape preparation operations thereof, and the machine tool may be controlled manually from a data terminal on line with the computer when the terminal is not in use for tape preparation.