Negative Numbers In Binary
Negative numbers in Binary are almost always displayed in "2's Complement" form.
For a negative number, invert all the bits of it's positive counterpart and add one. For Instance, in 16-bit decimal numbers:
1 = %0000 0000 0000 0001
Inverting the bits gives: %1111 1111 1111 1110
Adding 1 gives: %1111 1111 1111 1111 = -1
See :
http://www.rz.uni-hohenheim.de/rz/sys/basics/csc102/ch2.html#bin_not