2-16. Double-Length Shifts
Let (x1, x0) be a pair of 32-bit words to be shifted left or right as if they were a single 64-bit quantity, with x1 being the most significant half. Let (y1, y0) be the result, interpreted similarly. Assume the shift amount n is a variable ranging from 0 to 63. Assume further that the machine’s shift instructions are modulo 64 or greater. That is, a shift amount in the range 32 to 63 or -32 to -1 results in an all-0 word, unless the shift is a signed right shift, in which case the result is 32 sign bits from the word shifted. (This code will not work on the Intel x86 machines, which have mod-32 shifts.)
Under these assumptions the shift left double operation may be accomplished as follows (eight instructions):
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