1. In an on-line data processing system for controlling equipment, a method, by means of an improved executive program, of changing at least a segment of an on-line application program and of optimizing memory space utilization by assuring contiguous positions of identifiable program segments comprised of instructions located in the memory after modification of a segment size while without interruption continuing the control of the equipment in accordance with the instructions in the modified segment comprising the steps: (A) providing in the memory at least first, second and third portions, (1) said first portion retaining said improved executive program governing the operation of the data processing system, (2) said second portion having located therein a repetitively executable application program comprised of a plurality of individually identifiable segments disposed contiguously to one another, the instructions thereof being representative of logical states for controlling the equipment, and (3) said third portion serving as a temporary store for a desired replacement segment which is to replace a segment in said second portion; (B) inserting, under control of the executive program, a first branch instruction in the first memory instruction location of the segment to be replaced and a second branch instruction after the last instruction location of the replacement segment so that the application program is still repetitively executable by using the replacement segment while that segment is still in the third portion; (C) determining, under control of the executive program, the amount of memory space that the replacement segment will occupy in the second portion; (D) moving, under control of the executive program, all the application program segments in the memory following the segment to be replaced in said second portion in dependence upon the space requirements determined for said replacement segment; and (E) writing, under control of the executive program, said replacement segment into the second portion of the memory in the space created by the moving of all the segments.