Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/30



1    Q, 0000,0100,0000,0000     2 2          TS 6 — TS 2          3 3          ADD ‘A’              4 4          ROTATE 16            5 5          TS 4 - TS 6          6 6          TS 6 - TS 9          7 7          TS 27 - TS 6         8 8          TS 6 - TS 10         9 9               OR             10 10          TS 8 - TS 6         11 11          B,1, INDEXIN 11 12          TS 6 - TS 28        13 13          B, BURY

The first column gives the popular form of the name of the instruction, and the last column that of the next instruction to be followed. In most cases this could in theory be omitted because of the instructions being of type A. When the instructions are of type A the middle column describes them in abbreviated form. For instance TS 6 - TS 3 describes the operation of transferring the content of TS 6 into TS 3. Expressions of form Q, mean an instruction of type Q, and the expression after the comma describes what is in columns 17-32. ADD ‘A’ is to mean ‘Add TS 2 into TS 4 cancelling the partial sums’, ROTATE 16 means ‘Rotate the content of TS 4, TS 5 forwards 16 places’, OR is a logical operation.

The expression B, 1, INDEXIN 11 is intended to stand for B in column 3, 1 in column 17 and INDEXIN 11 in columns 17-32.

Outline of operation (INDEXIN). From 1 to 10 we are constructing the instruction which tells us to make the appropriate transfer and putting that instruction into TS 6. The instruction B, 1, INDEXIN 11 requires us to carry out the instruction in TS 6. The new IN formed will be 0, INDEXIN 12 so that we then continue with instruction INDEXIN 12.

The table for INDEXIN is shown in full in Fig. 38.

We use the convention that no digit is shown if the value of the digit is not significant. Both 0 and 1 are shown if either value is possible, and significant.

DISCRIM (General Description). If TS 8 contains any digit 1 then TS 15 is passed into TS 24, otherwise TS 16 is passed into TS 24. The contents of TS 2, TS 3, TS 4, TS 5, TS 8 are altered. Outline/