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We're Going to Look at Some Ways in Which Binary

Question 2

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We're going to look at some ways in which binary arithmetic can be unexpectedly useful. For this problem, all numbers will be 8-bit, signed, and in 2's complement.
(a) For x = 8, compute x & (−x). (& here refers to bitwise-and, and − refers to arithmetic negation.)
(b) For x = 36, compute x & (−x).
(c) Explain what the operation x & (−x) does.
(d) In some architectures (such as the PowerPC), there is an instruction adde rX=rY,rZ, which performs the following:
rX = rY + rZ + CA
where CA is the carry flag. There is also a negation instruction, neg rX=rY which performs:
rX = 0 - rY
Both adde and neg set the carry flag. gcc (the GNU C Compiler) will often use these instructions in the following sequence in order to implement a feature of the C language:
neg r1=r0 adde r2=r1,r0
Explain, simply, what the relationship between r0 and r2 is (Hint: r2 has exactly two possible values), and what C operation it corresponds to. Be sure to show your reasoning.

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