What is zero fill?

There are two ways a smaller integer type can be converted to a larger one:

The method employed depends on the instruction that is using the operand.

For zero-fill, all bits above the range of the smaller type are filled with 0.

For example, SINT: 16#87 = -121 converted to a DINT yields 16#00000087 = 135

For sign-extension, all bits above the range of the smaller type are filled with the sign bit of the smaller type.

For example:SINT: 16#87 = -121 converted to a DINT yields 16#FFFFFF87 = -121