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written 2.1 years ago by |

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.