Instruction Modification

the automatic transformation of the instructions of a program while it is being executed by a digital computer; a programming procedure in the organization of group operations, the ordering of data allocation in the memory cells, and so on. Instruction modification makes possible the performance of a large number of calculations with programs having a relatively small number of instructions: the same instructions are transformed and used for various purposes, such as processing data from various locations in the computer’s memory or composing new instructions. Instruction modifications can be made either by software or hardware—for example, using the control elements on the operator’s panel. Any part of an instruction may be changed: depending on the nature of the change, a distinction is made among address substitution, changing the operation code, and changing the indicators.

The address part of an instruction is most frequently altered: instruction addresses are changed in the cells of the internal memory or in the computer’s control unit. In the second case the instructions in the memory cells remain unchanged, but the addresses are increased or decreased in the control unit just before an instruction is executed. During the compilation of a program the instructions are set up by means of utility programs from codes that are prepared in advance or are computed during the running of the program. In such cases, when it is necessary to give an instruction its original form (such a need may occur during address substitution in memory cells), the instruction modification is called restoration of the code. Instruction modification facilitates programming, reduces the “length” of a program, effects a saving in the number of memory devices, and increases the output of the computer.