Slide 15.8: Implementing arithmetic expressions Slide 16.2: SAL/SHL instruction Home

Shift and Rotate Instructions

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Shifting means to move bits right and left inside an operand. Rotating shifts each bit and copies the shifted-out bit into the lowest bit if rotate left or the highest bit if rotate right. There are two basic ways to shift bits in a number:

Command   Description  SHL Shift left SHR Shift right SAL Shift arithmetic left SAR Shift arithmetic right ROL Rotate left ROR Rotate right RCL Rotate carry left RCR Rotate carry right SHLD Double-precision shift left SHRD Double-precision shift right

Bit Position  7 6 5 4 3 2 1 0  Bit Value  v7 v6 v5 v4 v3 v2 v1 v0

CF

• Logical shift: It fills the newly created bit position with zero. For example, logically shift one position to the right.
  
  

  Bit Position  7 6 5 4 3 2 1 0  Bit Value  0 v7 v6 v5 v4 v3 v2 v1

  CF   v0

  
  
• Arithmetic shift: The newly created bit position is filled with a copy of the original number's sign bit. For example, arithmetically shift one position to the right.
  
  

  Bit Position  7 6 5 4 3 2 1 0  Bit Value  v7 v7 v6 v5 v4 v3 v2 v1

  CF   v0

^†Note that shift arithmetic left is identical to shift left instruction.