␡
- 2-1 Manipulating Rightmost Bits
- 2-2 Addition Combined with Logical Operations
- 2-3 Inequalities among Logical and Arithmetic Expressions
- 2-4 Absolute Value Function
- 2-5 Average of Two Integers
- 2-6 Sign Extension
- 2-7 Shift Right Signed from Unsigned
- 2-8 Sign Function
- 2-9 Three-Valued Compare Function
- 2-10 Transfer of Sign Function
- 2-11 Decoding a "Zero Means 2 **n" Field
- 2-12 Comparison Predicates
- 2-13 Overflow Detection
- 2-14 Condition Code Result of Add, Subtract, and Multiply
- 2-15 Rotate Shifts
- 2-16 Double-Length Add/Subtract
- 2-17 Double-Length Shifts
- 2-18 Multibyte Add, Subtract, Absolute Value
- 2-19 Doz, Max, Min
- 2-20 Exchanging Registers
- 2-21 Alternating among Two or More Values
- 2-22 A Boolean Decomposition Formula
- 2-23 Implementing Instructions for All 16 Binary Boolean Operations
This chapter is from the book
This chapter is from the book
This chapter is from the book
This chapter is from the book
2–7 Shift Right Signed from Unsigned
If your machine does not have the shift right signed instruction, it can be computed using the formulas shown below. The first formula is from [GM], and the second is based on the same idea. These formulas hold for 0 ≤ n ≤ 31 and, if the machine has mod-64 shifts, 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 a constant, the first two formulas require only three instructions on many machines. If n = 31, the function can be done in two instructions with −
.