Editing Apollo Guidance Computer (section)

===Interrupts and involuntary counters===
The AGC had five vectored [[interrupt]]s:
* ''Dsrupt'' was triggered at regular intervals to update the user display (DSKY).
* ''Erupt'' was generated by various hardware failures or alarms.
* ''Keyrupt'' signaled a key press from the user's keyboard.
* ''T3Rrupt'' was generated at regular intervals from a hardware timer to update the AGC's [[real-time clock]].
* ''Uprupt'' was generated each time a 16-bit word of uplink data was loaded into the AGC.
The AGC responded to each interrupt by temporarily suspending the current program, executing a short interrupt service routine, and then resuming the interrupted program.

The AGC also had 20 involuntary [[Counter (digital)|counters]]. These were memory locations which functioned as up/down counters, or shift registers. The counters would increment, decrement, or shift in response to internal inputs. The increment (''Pinc''), decrement (''Minc''), or shift (''Shinc'') was handled by one subsequence of microinstructions inserted between any two regular instructions.

Interrupts could be triggered when the counters overflowed. The T3rupt and Dsrupt interrupts were produced when their counters, driven by a 100&nbsp;Hz hardware clock, overflowed after executing many Pinc subsequences. The Uprupt interrupt was triggered after its counter, executing the Shinc subsequence, had shifted 16&nbsp;bits of uplink data into the AGC.