Build a 3-to-8 decoder by "stacking" two 2-to-4 decoders with an active low enables. See page 312. Use the most significant function variable "a" and an external inverter to enable the correct decoder. Variables "b" and "c" are connected to decoder lines "A (MS)" and "B". Label the decoder outputs m0 to m3 (top) and m4 to m7 (bottom).

s(a,b,c)=m(1,2,4,7)

cout(a,b,c)=m(3,5,6,7)

ENT A B 0 1 2 3 1 X X 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 1 1 0 0 0 1 EN