Sign-Extend 15 or Post-Set '~~'

This operator is a special, immediate operator that has a dual purpose depending on which side of the variable it appears on. It can only be used in run-time variable expressions. The Sign-Extend 15 form of the operator appears to the left of a variable and the Post-Set form appears to the right of a variable.

The following is an example of the Sign-Extend 15 operator form:

Y := ~~X + 50

The Sign-Extend 15 operator in this example extends the sign of the value, X in this case, from bit 15 up to bit 31. A 32-bit signed integer is stored in twos-complement form and the most significant bit (31) indicates the sign of the value (positive or negative). There may be times where calculations on simple data result in word-sized values that should be treated as a signed integer in the range of -32768 to +32767. When you need to perform further calculations with those word-sized values, use the Sign-Extend 15 operator to convert the number into the proper 32-bit signed integer form. In the above example, assume X represents the value -300, which in 16-bit twos-complement form is actually the value 65,236 (%11111110 11010100). The ~~X portion of the expression extends the sign bit from bit 15 all the way up to bit 31, converting the number to the proper 32-bit twos-complement form of -300 (%11111111 11111111 11111110 11010100). Adding that sign-extended value to 50 results in -250, the intended result, whereas it would have resulted in 65,286 without the proper sign extension.

The following is an example of the Post-Set operator form.

Y := X~~ + 2

The Post-Set operator in this example sets the variable to -1 (all bits high) after providing its current value for the next operation. In this example if X started out as 6, X~~ would provide the current value for the expression (6 + 2) to be evaluated later, then would store -1 in X. The expression 6 + 2 is then evaluated and the result, 8, is stored into Y. After this statement, X equals -1 and Y equals 8.

Since Sign-Extend 15 and Post-Set are always assignment operators, the rules of Intermediate Assignments apply to them (see Intermediate Assignments).