MGMT 1030 Lecture Notes - Lecture 21: Arithmetic Shift, Instruction Set, Logical Shift
6 May 2018
School
Department
Course
Professor
MGMT 1030 Lecture 21 Notes – CPU Instruction
Introduction
• Shifts and rotate operations have other programming applications, and CPU instruction
sets commonly provide a variety of different shift and rotate instructions for the
programmer to use.
• As shown, shift instructions move the data bits left or right one or more bits.
• Rotate instructions also shift the data bits left or right, but the bit that is shifted out of
the end is placed into the vacated space at the other end.
• Depending on the design of the particular instruction set, bits shifted out the end of the
word may be shifted into a different register or into the carry or overflow flag bit.
• They ay siply fall off the ed ad e lost.
• Two different kinds of shifts are usually provided.
• The data word being shifted might be logical or it might be numeric.
• Logical shift instructions simply shift the data as you would expect, and zeros are shifted
in to replace the bit spaces that have been vacated.
• Arithmetic shift instructions are commonly used to multiply or divide the original value
by a power of 2.
• Therefore, the instruction does not shift the leftmost bit, since that bit usually
represents the algebraic sign of the numeric value
• Obviously the sign of a number must be maintained.
• Left arithmetic shifts do not shift the left bit, but zeros replace the bits from the right as
bits are moved to the left.
• These will effectively double the numeric value for each shift of one bit.
• On the other hand, right arithmetic shifts fill the space of moved bits with the sign bit
rather than with zero.
• This has the effect of halving the value for each bit shifted, while maintaining the sign of
the value.
• It may not seem obvious to you that this works correctly
find more resources at oneclass.com
find more resources at oneclass.com
