Well, the thing is that the hex can have different meanings depending on whether it is representing a signed number or an unsigned number. If it is representing an unsigned number, $00 is zero, $7F is 127, $80 is 128, and $FF is 255. If it is representing a signed number, then $00 is zero, $7F is 127, $80 is -128, and $FF is -1 (although I think there are more systems for signed numbers, this is the one that I'm most familiar with).
Moogles FTW