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Step of 4 9.017E The circuit is a 16 bit full adder/subtracter, it contains 32 input bits with carry input and mode control. Therefore, the total number of input bits is the sum of 32 input bits, carry bit and mode control. The sum gives 34 bits.. The realization of the full adder/subtracter requires 2 same size ROM's. The 2 same size ROM's are used to implement the 16 bit adder/ subtracter operation. The carry output bit of one ROM is carry input bit of other ROM. Mode control input bit is common to both the ROM's. The number of input bits for the each ROM is shown in Table 1 and is as follows: Table 1 Augend/ Minuend bits 8 Addend/ Subtrahend bits 8 Carry input bit 1 Mode control bit 1 Total bits 18 Step 2 of 4 There are 8 output data bits and one overflow/ carry output bit. Overflow bit and carry output bit are same. The both bits are equal to The number of output bits for each ROM is shown in Table 2 as follows: Table 2 Outout data bits 8 Carry/Two's complement 1 Overflow output Total bits 9 Step of 4 The number of input bits and output bits required by each ROM is 18 and 9 The size of each ROM required to design the circuit is as follows: Size of each ROM = 256 kx9 By this design approach, the size of each ROM is minimized, but the overall size of the circuit is not minimized. The design of the 16 bit adder/subtracter with two ROM's of equal size is shown in Figure 1. Cout X31 D17 A17 D16 S15 A16 256K X 9 ROM X16 A2 Y16 A1 D9 A0 S8 mode control C7 B17 D8 D7 S7 X15 B16 Y15 B15 256K X 9 ROM X0 B2 D0 Cin B0 Figure 1 Step 4 of 4 The number of ROM bits can be reduced by using two different size ROM's. The reduction in the ROM bits is only for the individual ROM. The total number of ROM bits required in the design of 16 bit adder/subtracter is not reduced. Therefore, the number ROM bits cannot be reduced.