The sign-bit of the sign/magnitude number (b3) is used to selectively
complement the bits of the magnitude (b2, b1, and b0) as well as add 1
via the carry-in of the full-adder of the least significant bit (thus converting
the sign/magnitude number into a 2s complement number. This scheme
also works correctly for both forms of 0 (0000 and 1000) in sign/magnitude
notation.
A = 0, B = 1 set the latch so that Q = 1 and Q' = 0. The input combination A = 0, B = 0 should not be used as it sets both Q and Q' to 1 with unpredictable results (race condition) if the inputs are then switched to hold (A = 1, B = 1). The analysis is analogous to that of the cross-coupled NOR gates done in lecture and in the text.