which contained a hybrid explicit guidance system for the first time. A new class of program was required by the Joint Strategic Targeting Planning Staff to select targets for the multiple warhead system. The Missile Application Programs were developed for these functions. The next major update to the operational software was made under the Guidance Replacement Program. Autonetics (later acquired by The Boeing Co.) developed the necessary software for the new flight computer. Functional description This section was excerpted from the original document, "Minuteman" D-37C Digital Computer System Depot Overhaul. Autonetics, Division of North American Rockwell, Inc. Anaheim, California. FET-D-120-D37/4. Control Unit The control unit interprets and processes all machine functions and consists of a location counter, the instruction register, and the phase register. Location Counter - The location counter determines the channel from which the next instruction is to be obtained. Instruction Register - The instruction register holds the instruction to be executed by the computer. This instruction defines the type of operation to be performed such as add, subtract, etc.; specifies the location address of the operand when necessary and indicates the sector address of the next instruction. Phase Register - The phase register consists of three flip-flops which may be set to one of eight possible states to indicate the phase of flight. It also serves as a selector switch to determine which group of voltage inputs are to be sampled and as an index register for a modify-flagged instruction. The state of the phase register is available as the stage reference outputs. Arithmetic Unit The arithmetic unit consists of three registers: the accumulator (A), lower accumulator (L), and the number register (N). Only the A and L registers are addressable. Accumulator (A-register) - The accumulator serves as the main register of the computer and holds the results of all arithmetic operations. This register serves as an output register for telemetry and character outputs. Lower Accumulator (L-register) - This register is used for certain arithmetic, input, logical operations or for rapid access storage. Number Register (N-register) - This register is used by the logic of the computer during multiplication and division and is not addressable. Input Unit The discrete input lines generally serve as communication lines from external equipment. There are three sets of "on - off" type signals: One set samples 24 input signals. One set samples 19 external input signals and 5 flip-flops from within