The input register is either set to zero or remains unchanged (depending upon the selected register).  codice_31 (Output): The contents of the Accumulator A are placed in output register specified by address. First design ideas of the Abort Guidance System did not include the use of the computer but rather a sequencer without any navigation capability. This would be adequate to put the Lunar Module into lunar orbit where the crew would wait for rescue by the Apollo CSM. Later design included a digital computer to provide some autonomy. The AGS software was written in LEMAP assembly language that uses 27 instructions described above and a set of pseudo-operations used by the assembler. The main computation cycle was 2 seconds long. This 2-second cycle was divided into 100 segments; each of these segments had a duration of 20 ms. These segments were used for computations that needed to be recalculated every 20 ms (like IMU signal processing, update of PGNCS downlink data, direction cosines update, etc.). There was also a set of computations that had to be performed every 40 ms (engine commands, external signal sampling, attitude control, etc.). Other computations were performed every 2 seconds and these equations were divided into smaller groups so they could be recalculated during the remaining (i.e. unused) time of 20 ms segments (e.g. radar data processing, calculation of orbital parameters, computation of rendezvous sequence, calibration of IMU sensors, etc.) The software for AGS was reviewed many times to find program errors and to reduce the size of the software. There are some known versions of the software that were used for uncrewed and crewed tests. The AGS User interface unit was named DEDA ("Data Entry and Display Assembly"). Its function was entry and readout of data from the AGS. Some of the system's functionality was built into DEDA unlike the DSKY used by AGC. DEDA had the following elements:   There are few actual descriptions of the use of the AGS, as a landing abort was never needed during the Apollo missions. There were, however, four cases in which the AGS was used. Its first use was for testing of the Lunar Module descent stage in Earth orbital flight during the Apollo 9 mission. It was used again in the Apollo 10 mission, following separation of the Lunar Module descent stage prior to the APS burn. An incorrect switch setting leaving AGS in "Auto" rather than "Attitude Hold" mode led to a prompt and pronounced deviation in attitude moments before staging.