2-6. Shift Right Signed from Unsigned
If your machine does not have the shift right signed instruction, it may be computed using the formulas shown below. The first formula is from [GM], and the second is based on the same idea. Assuming the machine has mod 64 shifts, the first four formulas hold for 0 ≤ n ≤ 31, and the last holds for 0 ≤ n ≤ 63. The last formula holds for any n if by “holds” we mean “treats the shift amount to the same modulus as does the logical shift.”
When n is a variable, each formula requires five or six instructions on a basic RISC.
In the first two formulas, an alternative for the expression is 1 << 31 − n.
If n is ...
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