- 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
2–15 Rotate Shifts
These are rather trivial. Perhaps surprisingly, this code works for n ranging from 0 to 32 inclusive, even if the shifts are mod-32.
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If your machine has double-length shifts, they can be used to do rotate shifts. These instructions might be written
shldi RT,RA,RB,I shrdi RT,RA,RB,I
They treat the concatenation of RA and RB as a single double-length quantity, and shift it left or right by the amount given by the immediate field I. (If the shift amount is in a register, the instructions are awkward to implement on most RISCs because they require reading three registers.) The result of the left shift is the high-order word of the shifted double-length quantity, and the result of the right shift is the low-order word.
Using shldi, a rotate left of Rx can be accomplished by
shldi RT,Rx,Rx, I
and similarly a rotate right shift can be accomplished with shrdi.
A rotate left shift of one position can be accomplished by adding the contents of a register to itself with “end-around carry” (adding the carry that results from the addition to the sum in the low-order position). Most machines do not have that instruction, but on many machines it can be accomplished with two instructions: (1) add the contents of the register to itself, generating a carry (into a status register), and (2) add the carry to the sum.