Sign-Extend 7 or Post-Clear '~'

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 7 form of the operator appears to the left of a variable and the Post-Clear form appears to the right of a variable. 

The following is an example of the Sign-Extend 7 operator form:

Y := ~X + 25

The Sign-Extend 7 operator in this example extends the sign of the value, X in this case, from bit 7 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 byte-sized values that should be treated as a signed integer in the range of -128 to +127. When you need to perform further calculations with those byte-sized values, use the Sign-Extend 7 operator to convert the number into the proper 32-bit signed integer form. In the above example, assume X represents the value -20, which in 8-bit twos-complement form is actually the value 236 (%11101100). The ~X portion of the expression extends the sign bit from bit 7 all the way up to bit 31, converting the number to the proper 32-bit twos-complement form of -20 (%11111111 11111111 11111111 11101100). Adding that sign-extended value to 25 results in 5, the intended result, whereas it would have resulted in 261 without the proper sign extension.

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

Y := X~ + 2

The Post-Clear operator in this example clears the variable to 0 (all bits low) after providing its current value for the next operation. In this example if X started out as 5, X~ would provide the current value for the expression (5 + 2) to be evaluated later, then would store 0 in X. The expression 5 + 2 is then evaluated and the result, 7, is stored into Y. After this statement, X equals 0 and Y equals 7.

Since Sign-Extend 7 and Post-Clear are always assignment operators, the rules of Intermediate Assignments apply to them (see Intermediate Assignments).