Problem

Assume that registers R1 and R2 in Figure hold two unsigned numbers. When select input X i...

Assume that registers R1 and R2 in Figure hold two unsigned numbers. When select input X is equal to 1, the adder-subtractor circuit performs the arithmetic operation “R1 + 2s complement of R2.” This sum and the output carry Cn are transferred into R1 and C when K1 = 1 and a positive edge occurs on the clock.

(a) Show that if C = 1, then the value transferred to R1 is equal to R1 - R2,but if C = 0, the value transferred to R1 is the 2s complement of R2 - Rl.


(b) Indicate how the value in the C bit can be used to detect a borrow after the subtraction of two unsigned numbers.

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