Apollo Guidance Computer (AGC)

The operation of the AGC was through the Digital Display Keyboard (DSKY). Operation of the DSKY involved punching various sequences of letters and codes.  The interface to the computer was extremely advanced at a time when most modern computers were using punch cards for programming.

The layout of the DSKY included a series of program lights (on the left), an LED display (on the right), and a keyboard (underneath).  The LED display shows the current program running, an activity light (how busy is the computer), and three registers for displaying / entering information.

The designers of the DSKY developed a method of communicating with the AGC by issuing commands based on Nouns and Verbs.  Nouns were codes meant to be acted upon.  Verbs were codes telling what to do.  The sequence of issuing a command began with pressing “Verb” (do something), and the number of the function (what to do).  Typically this could be followed by pressing “Noun” (display something) and a number (what to display). 

For example entering: Verb > 37 > Enter meant a request to change the current program.  The next number would be the program to start.  So, using the syntax of NASA the command would look like, V37E68E. This means type in the sequence by using the keys in the order: Verb>”37”>Enter>”68”>Enter.

Another example is to enter Minimum Impulse Mode.  You can try this in EL3D while in orbital flight.  This is an RCS thruster mode that just fires a quick 14ms burst when the controller is moved.  It is used for fine alignment when doing star sightings in space.  To command the AGC to utilize this mode the astronaut will type V76E.  76 means “Enter minimum impulse mode”.  There is nothing to display so it is not necessary to enter any other information (Noun, Program, etc.)  To return from that mode the astronaut will type V77E.  77 means “Exit minimum impulse mode”.

The AGC ‘knows’ what to do for a particular flight phase by running a program referenced by a number. Programs are referred to as PXX where XX is a program name.  Some programs start automatically at the conclusion of the previous program (P66 after P64) and others need to be started manually (P12 – ascent program).  Still others may start when pushing a certain button (P71 abort when you hit the abort button).  Available programs for EL3D LM operations are:

P00      Idle loop

P12      Ascent

P20      Rendezvous

P70      DPS Abort

P71      APS Abort

P63      Breaking phase

P64      Pre-Landing phase

P66      Terminal Phase

P68      Landed

Anytime during your EL3D flight you can hit F2 for help on the DSKY commands available for the current program.  Let’s look at the example for P66 after pressing F2:

The first section describes the function of the program

Purpose: To control the LM in final phase of landing.  With Mode Control in Auto forward and lateral velocities are nulled.  When in Manual the inertial attitude is controlled by the astronaut.  Throttle is calculated and Rate of Descent (ROD) is controlled by toggling the ROD switch. P66 is started automatically by Landing Auto Modes Monitor routine when targeted conditions for P64 are met or the ROD switch has been activated after P63.

This section shows the various displays available on the DSKY registers depending on the Verb/Noun combination.  Whenever a display shows Flashing then the program is expecting a confirmation request.  In the case of the following program pressing PRO (Proceed) indicates the finish of P66 following landing.

Displays and Actions                     

      Flashing    Forward Velocity             XXXX.X ft/s

      V06N60      Altitude Rate                XXXX.X ft/s

                  Computed Altitude            XXXXX  ft


Supplemental information about other switches and functions that affect this program are shown below.

Use ROD switch as desired.

To manually null forward and lateral velocities: Mode Control: PGNS > Attitude Hold.

Manual throttle available


Typing V16N92E (Verb>16>Noun>92>Enter) brings up an alternate display.  This display might be useful if you are using manual throttle.


       Flashing    Percent Full Thrust         00XXX ft/s

       V16N92      Altitude Rate               XXXX.X ft/s

                   Computed Altitude           XXXXX  ft

The KEY REL entry below indicates that the Key Release light is on.  When this light comes on it means the astronaut is doing something with the DSKY that is not part of the primary display or function of the current program.  By pressing the Key Rel key the program resumes the ‘normal’ display and function.  In the example below the program (P66) really wants to display V06N60 parameters.  When V16N92E was entered in the previous section it was showing a display that was not the preferred one.   


       Flashing    Forward Velocity            XXXX.X ft/s

       V06N60      Altitude Rate               XXXX.X ft/s

                   Computed Altitude           XXXXX  ft

       Engine Stop



And finally the next program that should be run.  In this case it is started manually by typing V37E68E

Next Program is Landing Confirmation Program P68: