I'm working with some old binary code where data values are read from a single byte as bits.

In this particular case, I have a value that is stored in a byte. The first (x0000000) bit represents whether the remaining bits (0xxxxxxx) are a positive or negative number from 1-128. If the value is negative (x0000000) = 1, the bits are flipped. (no, I don't know why they thought they needed to flip the bits as well as indicating if it was positive or negative).

I can tell whether I need to flip the bits (if the first bit is 1 or 0):     $z = $x & (1 << 7);

but I can't quite get right the flipping of the remaining 7 bits into a number. My goal is getting back a decimal number that is the inverse of the right-most 7 bits.

I support I could look at the decimal value, and, if > 127, simply subtract 256 from it, but that seems inelegant.

In reply to Flipping partial bits by rickss

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