ALU is very widely used and popular combinational ckt. It is capable of performing the arithmetic as well as logical operations, ALU is heart of any $\mu p$

• 74181 is 24 pis IC dual in package.

• [$A_0 – A_3] [B_0 – B_3$] two i/p’s of ALU, each 4 bit in size.

• $s_0 – s_3 \rightarrow$ select i/p so that 16 different operations can be performed.

• M is the mode control bit.

M = 0 Arithmetic operations

M = 1 Logic operations.

• Ct and p o/p’s are used when a number of 74181 ckts are to be used in cascade along with 74182 the look ahead carry generator ckt to make the arithmetic operations faster.

• When the mode control is high (M = 1) then logic operations are performed on the individual bits.

• When the mode control bit is low (M = 0) then arithmetic operations are performed on the two 4 bit words and are internal carries are enabled. IC – 874181 incorporate full internal carry look ahead. This enhances the speed of operation to great extent.

• It provides a ripple carry between the devices using $C_n + 4$ o/p.

• A = B o/p: It indicates that the logical equality of two operands. This o/p goes high when the unit is in the subtract mode and A = B.

• The o/p also goes high when all the four function o/p are high.

• It is possible to write AND the A = B o/ps used. The wire ANDing becomes possible became A = B is an open collector o/p. this enables us to compare words which are longer than 4 – bits.