You can always count on them

Usage examples of abaci.

To match up the numbers on the scratch paper with the numbers left on the abaci, and thus to compile a flash-frozen image of the calculations that were underway in that room when the apocalypse struck, is not that difficult—at least, by the standards of difficulty that apply during wartime, when, for example, landing several thousand men and tons of equipment on a remote island and taking it from heavily armed, suicidal Japanese troops with the loss of only a few dozen lives is considered to be easy.

They just happen to be exactly the same numbers that were frozen on certain abaci down in the room of the computer slaves.

Two abaci rattling in tandem the whole day long, one changing the shape of the world and the other one deploring the cost.