4 Design examples using combinational building blocks (decoders, encoders, multiplexers, comparators, three-state devices, adders).Barrel shifterDual-priority encoderCascading comparators, mode-dependent comparator
5 Barrel shifter design example n data inputs, n data outputsControl inputs specify number of positions to rotate or shift data inputsExample: n = 16DIN[15:0], DOUT[15:0], S[3:0] (shift amount)Many possible solutions, all based on multiplexers
6 Multiplexers (mux)Eg. Put between Processor’s registers and ALUA 16-bit processor where 3-bit field specifies on of 8 registers.The 3-bit field is connected to the select inputs of an 8-input, 16-bit mux.Select one of n sources of data to transmit on a bus.
14 Parallel Cascading Comparators 74x85: serial cascading scheme, delay of propagating the cascading signals through a cascade of comparators.74x682: Parallel approach, used to build very wide comparators.Build 24-bit comparator, using 3 74x682PEQQ=EQ2.EQ1.EQ0PGTQ=GT2+EQ2.GT1+EQ2.EQ1.GT0
15 8 bit Comparator +5V 74x85 74x85 A<B A<BIN A<BOUT A<BIN Least Significant bitsMost Significant bits
17 Mode-dependent Comparator Design a c.c. whose inputs are two 8-bit unsigned binary integers(X,Y), and a control signal(MIN/MAX). The output is an 8-bit unsigned binary integer Z, such thatZ=min(X,Y) if MIN/MAX=1;Z=max(X,Y) otherwise