Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/24

 We have a similar tree system for the selection of temporary storages.

 12. Detailed Description of the Arithmetic Part (CA).

We shall divide the CA operations into a number of types. We shall make provision for 16 types, but for the present will only use nine. The types are distinguished by digits CI 5-8.

Type K. Pass the content of TS 6 into a given minor cycle.

Type L. Pass the content of a given minor cycle into TS 6.

Type M. Pass the content of a given TS into TS 6.

Type N. Pass the content of TS 6 into a given TS other than TS 4 or TS 5, or TS 8 or TS 1.

Type O. Pass the content of the first 12 minor cycles of a given DL out onto a card via the reproducer.

Type P. Pass the content of the card at present in the card reader on to a given DL.

Type Q. Pass CI 17-32 into TS 6.

Type R. Various logical operations and others yielding results forming one minor cycle, to be performed on the contents of TS 9 and TS 10 and transferred to TS 8.

Type S. Arithmetical operations yielding a result requiring more than one minor cycle for its retention. Results go into TS 4 and TS 5.

Type T. Stimulate a given valve element.

A trigger circuit is associated with each type. With the exception of Q these are all excited for a period consisting of a number of complete minor cycles beginning with a P1 and ending with a P32.

The main components of CA are the 32 temporary storages TS 1-32. Of these TS 1-12 have some special duties.

TS 1 is used to carry the retiring data, i.e. the CD which applied just before the last instruction of type B.

TS 2 and TS 3 contain the arguments for the purely arithmetical operations, or most of them, and for the logical operations.

TS 4 and 5 contain the results of the arithmetical operations. They are frequently connected up in series to form a DL 64. This is because the results of most of the arithmetical operations are sequences of more than 32 but not more than 64 digits.

TS 6 is used as a shunting station for the transfer of information from place to place.

TS 7 is used to carry the digits of a number m when it is proposed to multiply by 2m.

TS 8 is used to carry the result of logical operations and other operations not requiring more than one minor cycle.

TS 9 and TS 10 are the inputs for the logical operations.

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