Assembly Language Logical Shift

Logical Shift Left

A Logical Shift Left will shift the binary pattern and add a zero at the least significant place value. For example the binary for 4 is '00100' a logical shift to the left of one place will give '01000' which is now 8. A logical shift to the left by one digit will multiply by 2. A logical shift of 2 places on '00100' will give '10000' which is 16, therefore a logical shift to the left by two digits will multiply by 4.

Logical Shift Right

A Logical Shift Right by a single digit will half the value, this essentially removes the least significant place value and shifts the other digits to the right. If the value represented is 13 ie '01101' a logical shift to the right of one place will remove the least significant place value so it will now be '00110' which is 6. 13 divided by 2 is 6 remainder 1. A logical shift to the right of two place will turn 13 '01101' into '00011' which is 3, ie 13 divided by 4 (the remainder will be 1).

Multiplication example

This example uses a Logical Shift to perform multiplication. R2 is used to store the second value of the multiplication, and for every shift we subtract 2 from this. We then compare it with #1, because if it is greater we still need to do a shift, if it is less than we have finished the calculation. However if the comparison is equal, we need to finally add the first number to the total.

      INP R0,2
      INP R1,2
      MOV R2, R1
      MOV R3, R0
LOOP: LSL R3,R3,#1
      SUB R2,R2,#2
      CMP R2,#1
      BGT LOOP
      BLT END
      ADD R3,R3,R0
END:  OUT R3,4
      HALT