Shift Operations

UMBC

CMSC 313 -- Shift Operations Segment

Shift Operations

Shifting the contents of a word or byte left or right can also be a useful operation. With binary numbers, left shifts are the the same as multiplying by two for each position shifted left. Likewise, shifting right is like dividing by two (assuming it is an unsigned integer!)

Logical Shifts

The simplest type of shift instruction is to shift zeroes into the vacated positions and to throw away the bits shifted out the other end. (However, a copy of the thrown away bit is stored in the Carry Flag!) The instructions are:

The last bit shifted out is stored in the Carry Flag so that it can be tested. The form is:

shl/shr mem/reg, count/CL ; unitl the 80386, the only constant was a 1

To shift more than one position, you had to use the CL register:

    mov  CL, 4
    shl  AX, CL

Examples

        1010 1010 1010 1010
    SHR                   1 
        0101 0101 0101 0101   => 0 in Carry Flag

        0001 0010 0011 0100
    SHR                   4   ; Remember to use the CL register!!!
        0000 0001 0010 0011   => 0 in Carry Flag ; the 0 came from bit 3

Shift Arithmetic

The is also a arithmetic shift, where the only difference is during a right shift, inside of the high order bit becoming zero, it keeps the value that it had, which is useful with signed integers.

Examples

        1010 1010 1010 1010
    SAR                   1 
        0101 0101 0101 0101   => 0 in Carry Flag

        0001 0010 0011 0100
    SAR                   4   ; Remember to use the CL register!!!
        0000 0001 0010 0011   => 0 in Carry Flag ; the 0 came from bit 3

Rotates

The last shifts are called rotates. In this type of shift, the bit that comes out one side is put back into the other side.

Examples

        1010 1010 1010 1010
    ROR                   1
        0101 0101 0101 0101   => 0 in Carry Flag and bit 15

        0001 0010 0011 0100
    ROR                   4   ; Remember to use the CL register!!!
        0100 0001 0010 0011   => 0 in Carry Flag ; the 0 came from bit 3

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