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Card handling in the AE

At least in the Sketch, the card handling is remarkably vague.

As opposed to electronic computers, the many kinds of cards is astonishing, in particular, as there are no explanation on how their movement is coordinated.

So far, these card types have been used:

  • operation cards
  • variable cards
  • number cards

Operation cards

It seems fairly clear, that the operation cards just set the mill to one out of four arithmetic modes, and that the operation cards never start an action of the mill: >The Operation-cards merely determine the succession of operations in a general manner. They in fact throw all that portion of the mechanism included in the mill into a series of different states, which we may call the adding state, or the multiplying state, &c. respectively.

Throughout the text, it is often stressed that only a few operation cards are needed. This seems to be rather strange, as the operation cards are at most 25% of all cards, and they are small, as at most four holes are needed.

Variable cards

These control the transfer of numbers between the mill and the store, and come in to major flavors:

  • Supplying-cards: transfer from the store to the mill
  • Receiving-cards: transfer from the mill to the store

Each also comes – only hinted in the Sketch – for two destinations (normal and primed ingress) as well as sources, and the Supplying cards also may either leave the storage cell cleared, or have the value restored (see Destructive and retaining ingress operation).

As the mill has no unary operations, in every operation of the mill, two Supplying-cards and one Receiving-card is needed (unless the result is only needed to control a loop).

A hard to understand remark is: >It should be understood that the Variable-cards are not placed in immediate contiguity with the columns. Each card is connected by means of wires with the column it is intended to act upon.

Number Cards

Provided the AE shall calculate the value of an analytical function, it has to receive the arguments and constant numerical values used in the formula or algorithm.

In the Sketch, nothing was found to provide the initial values of the variables. There, it was assumed that all numbers needed in a calculation are put into the respective colums before the machine starts.

In the Passages…, Babbage refers to number cards, but these are not cards sending a number encoded on a card to a storage column; the represent argument-value pairs, which could be requested – by means not explained – during the run of the machine, and supplied by an operator. It was not explained where the number goes.

John Walker in his AE simulator (see http://fourmilab.ch/babbage/cards.html) has introduced number cards that allow to send a constant to a specified storage cell. It origins are left open.

His cards have the disadvantage that they are rather large, as they have to contain the cell number as well as the constant.

From today's point of view, it might be also an option just to transfer the number from a stack of cards via the ingress axis to the mill, and to be used instead of a variable card.

One might even be tempted to provide that column 0 is just the next card from the constants card, and no physical column 0 exists.

Note that while Babbage claims that results may be printed or punched, neither the Sketch nor the Passages give any indication how this is managed.


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