Apollo Guidance Computer · operated by verb & noun.

The machine.

The Apollo Guidance Computer is a flight computer designed at the MIT Instrumentation Laboratory, built by Raytheon, and flown to the Moon eleven times. It is sixteen bits wide, ones-complement, with about two thousand words of erasable magnetic-core memory and roughly thirty-six thousand words of fixed core-rope read-only memory. It runs at a clock frequency of two-point-zero-four-eight megahertz. It weighs about thirty-two kilograms and consumes around seventy watts. There were two design generations: Block I flew on Apollo 1 and unmanned tests; Block II flew from Apollo 7 onward and is the version this emulator reconstructs.

All of that is interesting, but it is also not how you operate the machine. The AGC ran continuously throughout the lunar mission, executing tens of thousands of instructions per second from its core-rope, and the astronauts were not concerned with any of it. What the astronauts touched was the DSKY — the Display and Keyboard unit — a brushed-aluminum panel of indicator lamps, seven-segment electroluminescent digit displays, and nineteen square push-buttons. They keyed verbs (actions) and nouns (data references) and the DSKY answered.

The astronaut's manual was, in effect, a vocabulary of pairs.

This emulator reconstructs the DSKY behaviorally. It does not simulate the AGC's actual instruction set — the TC, CCS, AD, MASK, EXTEND opcodes are not present, and there is no Yul-assembler source for you to step through. What is faithful is the operator's experience: the verb-noun grammar, the eleven verbs and seven nouns that the historical panel exposed for routine flight operations, the major programs (P00 idle, P01 pre-launch, P11 earth-orbit insertion monitor, the lunar-descent triplet P63 → P64 → P66), the lamp behavior, the seven-segment electroluminescent green at five-hundred-thirty nanometers with the appropriate glow.

The headline feature of the emulator is a ninety-second scripted simulation of the Apollo 11 lunar-descent sequence, complete with the 1202 program alarm firing at T+12 seconds in the descent — at the moment the real one did, in the sequence the real one fired. Acknowledging the alarm with the RSET key continues the descent, exactly as Steve Bales did from Mission Control when he called "Go!" on the loop.

The DSKY.

The DSKY is twelve indicator lamps, six numeric displays, and nineteen keys. Its layout is a brushed-aluminum face plate with the lamps arrayed in two columns at the top-left, the program / verb / noun displays at the top-right, three signed five-digit register displays (R1, R2, R3) in the middle, and the keyboard at the bottom. Everything you do with the AGC, you do through this one panel.

The lamps

Twelve indicator lamps, color-coded by severity. Red indicates a hardware-class fault that requires acknowledgment. Amber indicates an attention-grabbing software state such as a program alarm. White indicates an operator-relevant state such as "key release requested" or "operator error." The lamps are arrayed in two columns of six.

The displays

Six seven-segment displays, all in five-hundred-thirty-nanometer electroluminescent green with appropriate glow. Three of them are two-digit fields at the top of the panel; three are signed five-digit fields in the middle.

The signed five-digit registers display three components of whatever the current noun represents. NOUN 17 (attitude) shows roll, pitch, and yaw. NOUN 36 (mission timer) shows hours, minutes, and seconds. NOUN 62 (velocity, altitude, altitude-rate) shows speed in feet per second, altitude in feet, and altitude rate in feet per second.

A small green COMP ACTY lamp at the upper-left indicates that the AGC is currently executing — its analog of a CPU activity light. It pulses subtly during normal operation.

The keyboard

Nineteen keys in a four-row layout. The numeric keys are arranged as a three-by-three-plus-one block from 0 through 9. The action keys ring around them with iconic color coding: VERB and NOUN at the upper-left in white; + and − on the upper-right; ENTR in green at the lower-right; RSET in red at the lower-right; CLR, KEY REL, and the gold PRO arrayed on the left side.

Keying a command.

The basic unit of input on the AGC is the verb-noun pair. Almost every operation the astronaut performs through the DSKY consists of: press VERB, key two digits, press NOUN, key two digits, press ENTR. That sequence — five logical keystrokes for two digits each, plus mode and execute — is the entire grammar.

Verbs are actions

A verb tells the AGC what to do. V06 means "display once." V16 means "monitor — refresh continuously." V21 through V25 are the load verbs: load one, two, or three component values into the noun's data fields. V35 is the lamp test. V37 is the "load major program" verb — the one you use to dispatch from idle to a specific program. The verb is keyed first because it determines what the noun means.

Nouns are data references

A noun tells the AGC what data the verb refers to. N17 is attitude (roll, pitch, yaw). N36 is the mission timer. N62 is velocity-altitude-altitude-rate. The same noun can be combined with different verbs to mean different things: V06 N36 displays the timer once; V16 N36 monitors it continuously; V25 N36 would load three values into it.

The execute key

ENTR commits the just-keyed command. Until you press it, the verb and noun digits in the display are flashing — they have not been accepted by the AGC, only shown back to you. ENTR stops the flash and runs the command. If the verb and noun combination is invalid, the AGC lights OPR ERR and rejects the input; clear with RSET and key the command again.

Special keys

Three keys exist outside the verb-noun grammar. PRO (gold) means "proceed" — the program asked the astronaut a yes/no question and the answer is yes. RSET (red) acknowledges and clears the most recent program alarm or operator error. KEY REL releases a held display so a new command can be keyed. CLR erases the most recent register entry during a load operation. None of these are part of a verb-noun sequence; they fire immediately on press.

Reference tables.

These four tables enumerate every verb the emulator implements, every noun it knows, every major program that can be loaded, and every key on the panel. Bookmark this section. You will scroll back to it any time you sit down with the DSKY.

The eleven verbs

The seven nouns

The major programs

The keyboard

The major programs.

The AGC's flight software is organized as a collection of major programs, each addressing one phase of the mission. Only one program runs at a time. You change programs with V37E nnE, where nn is the program number. Eight programs are wired up in this emulator; below is what each one does and how to operate it.

P00 — idle

The default program. The PROG display reads 00; the VERB and NOUN displays are blank. The IMU is powered but no guidance integration is happening. To leave P00, key V37E followed by the program number you want.

P01 — pre-launch

Pre-launch initialization. In a real mission, P01 ran for hours on the launch pad while the inertial platform aligned and the launch coordinates were verified. In the emulator, P01 is a static mode you can enter to confirm pre-launch behavior; the AGC sits ready and waits.

P11 — Earth-orbit insertion monitor

P11 is the workhorse. During the Saturn V's first-stage and second-stage burns, the crew watched V16N62 on P11 — velocity, altitude, altitude rate. The emulator integrates state continuously: load P11 with V37E 11E, then monitor with V16E 62E, and the registers will show numbers rising through the boost profile. This is the most representative single demonstration of the AGC's operating mode in a real mission outside of the lunar descent.

P63 — braking phase

The first of the three lunar-descent programs. The descent engine is ignited at full thrust; the lunar module pitches over and the AGC steers it through a high-velocity braking maneuver to bring the vehicle down from orbital speed. P63 transitions automatically into P64 when the vehicle reaches the high gate — about seven thousand feet of altitude and two thousand feet per second.

P64 — approach phase

Pitchover. The lunar module rotates its nose forward so the astronauts can look through the window at the approaching landing site. The AGC continues automated guidance toward a designated point. The crew can re-designate the landing site by slewing a reticle on the window with the rotational hand controller; the emulator does not implement landing-site re-designation but the program transition is correct. P64 hands off to P66 at the low gate.

P66 — landing autopilot

Manual rate-of-descent control. The astronaut commands altitude rate via the throttle; the AGC holds attitude and translates throttle inputs into engine commands. In the emulator, P66 runs to touchdown automatically as part of the Apollo 11 demo; outside of the demo, you can dispatch to P66 directly with V37E 66E to inspect the display behavior.

P12 — lunar ascent

The reverse of P63 — powered ascent from the lunar surface back to orbit. Wired up in the emulator as a behavioral target for V37E 12E to demonstrate program transitions; the actual ascent profile is summarized rather than fully integrated.

Common operations.

Six routines cover almost everything an astronaut did with the DSKY in flight. None involves more than seven keystrokes. Each is short. Each composes with the others. Internalize these and the rest of AGC operation is mostly arrangement.

Test the panel

Before relying on the DSKY for anything, light it up. V35E turns on every lamp and every digit segment simultaneously. If anything is dark, the panel has a fault. This was the first thing astronauts did on entering the spacecraft and the first thing operators do on first opening the emulator.

Display once

Use V06 to display a noun's data exactly once. The display freezes on whatever value held at the moment of execution. V06 N17 E shows the current attitude as a static reading; the numbers do not update.

Monitor

Use V16 to refresh continuously. The display updates with live data as long as the verb stays active. This is how the crew watched ascent and descent — the registers change in real time. To leave monitor mode, press KEY REL and key a different command.

Run a program

Use V37 followed by the program number to dispatch. The verb display shows 37 while the AGC waits for the program number; key it and press ENTR again. To return to idle, key V37E 00E.

Acknowledge an alarm

When the PROG lamp lights and an alarm code shows, press RSET. The lamp goes out and the alarm code is cleared. The program continues if the alarm was non-fatal — which most are. The emulator's Apollo 11 demo specifically demonstrates this: the 1202 fires, you press RSET, and the descent continues.

Proceed

The PRO key answers "yes" to a program-issued prompt. P63 issues a throttle-up prompt at engine ignition; P66 takes PRO as a touchdown command in the manual phase; P00 in standby is woken with PRO.

Worked sequences.

Three full button-press sequences worth practicing on the emulator. The first two verify the panel and demonstrate live state integration; the third is the headline Apollo 11 lunar-descent simulation, and is the reason the project exists.

Lamp test (~5 seconds)

Confirms that every lamp, every digit segment, and the COMP ACTY light are functional. Run this first whenever you sit down with the panel.

If any lamp does not illuminate during V35, the lamp is broken and the panel cannot be relied on for that signal. On the real spacecraft this was a critical pre-mission check; on the emulator it is a quick sanity confirmation of the DSKY's rendering.

Earth-orbit ascent monitor (~30 seconds)

Run P11 and watch the velocity, altitude, and altitude-rate numbers rise as the AGC simulates a Saturn V boost profile. This is the cleanest single demonstration of the AGC's monitor mode — three live numbers updating in real time on a panel of glowing green digits, exactly as the crew saw them.

Apollo 11 lunar descent (~90 seconds)

The headline demo. A scripted ninety-second simulation of the lunar-descent sequence that the LM Eagle flew on July 20, 1969. The descent runs on its own once started; you observe and respond when the alarms fire.

The two alarms fire on a script timed to the actual Apollo 11 telemetry. The 1202 alarm at T+12 s means "executive overflow — no vacant areas"; it indicates the AGC's executive scheduler ran out of work slots, which on Apollo 11 was caused by the rendezvous radar being switched to the wrong mode and dumping cycle-stealing data into the computer. The AGC's response was the BAILOUT routine — discard the lowest-priority queued work, restart the executive, keep the descent program running. The astronauts and Mission Control saw the alarm light, the guidance officer (Steve Bales, age 24) decided in seconds it was survivable, and the call "Go!" went up the loop. The descent continued. The Eagle landed.

The second alarm at T+48 s is a 1201 — same family as 1202, a different overflow flavor. Again recoverable, again acknowledged with RSET, again the descent continued.

Alarms & limits.

A short list of things to know — what the alarm codes mean, what BAILOUT actually does, and the boundaries of this emulator's faithfulness.

The 1201 / 1202 alarms

Both are executive overflow alarms. 1202 means the executive scheduler ran out of vacant areas — slots in which to record pending work. 1201 means it ran out of core sets — slots in which to record the running state of currently-executing tasks. They are family-related; both indicate that more work is being requested of the AGC than it can simultaneously track.

The recovery mechanism is BAILOUT. When an executive-overflow alarm fires, the AGC discards the lowest-priority queued tasks, restarts its executive from a clean state, lights the RESTART lamp, and resumes. The currently-executing critical task — in 1969, the descent guidance — is unaffected. This is why the descent was able to continue: the alarm was not a software bug but a software-controlled response to a genuine overload, and the response prioritized the right thing.

Other alarm codes the emulator may show

What this emulator simulates — and does not

The DSKY operator's experience is faithful. The verb-noun grammar, the eleven verbs and seven nouns wired up, the major programs and their phase transitions, the lamp colors and behavior, the seven-segment electroluminescent green at the right wavelength, the 1202 alarm timing in the Apollo 11 demo — all of these are reconstructed accurately to what an astronaut would have seen.

What is not simulated is the AGC's actual instruction set. The real AGC ran its software out of core-rope ROM in a sixteen-bit ones-complement instruction set with opcodes like TC (transfer control), CCS (count, compare, and skip), AD (add), MASK (logical AND), and the EXTEND prefix that doubled the available opcode space. The flight software — Colossus for the command module, Luminary for the lunar module — was assembled by the YUL assembler and woven into core-rope by hand at Raytheon. None of that is here. The emulator implements the AGC behaviorally: when you key a verb-noun command, it responds the way the real AGC's flight software would have responded, but the response is implemented in JavaScript rather than emulated through the AGC's own instruction stream.

Differences from the historical Block II DSKY

A few simplifications. The historical AGC supported around a hundred verbs and over a hundred nouns; the emulator wires up eleven verbs and seven nouns covering the most commonly-exercised flight operations. The historical interrupt-driven uplink and downlink to ground (UPLINK ACTY) was bidirectional; here it is decorative. The Block II DSKY had a slightly different lamp arrangement than Block I; the emulator uses the Block II layout (the configuration flown from Apollo 7 on). The Apollo 11 demo's timeline is compressed for demonstration — the actual descent took roughly twelve minutes, the demo runs ninety seconds.