Computer Organization and Design: The Hardware/Software Interface

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1 University of California, Berkeley College of Engineering Computer Science Division EECS Spring 1999 John Kubiatowicz Homework Quiz (HW #2) Solutions February 16, 1999 CS152 Computer Architecture and Engineering This quiz covers one of the problems from homework #2. Good Luck! Your Name: SID Number: Discussion Section: Total:
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2 A better multiply for the Single Instruction Computer In the homework, you were introduced to the “Subtract and branch if negative” instruction: sbn a, b, c # Mem[a] = Mem[a] – Mem[b]; If Mem[a] < 0 goto c; Problem 3.30 asked you to develop a multiply routine that was not particularly efficient. Consider the following alternative for a 16-bit computer. This performs c = a * b, modifying b in the process: c = 0; for (index = 32768; index >= 1; index = index / 2) { If (b >= index) { b = b – index; c = c - index * a; } } c = -c; To implement this, we will assume a macro facility that lets us generate groups of instructions many times. 1. Create a macro called “ inner ” which performs the inner instructions of our multiply. This takes 4
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