Shift

Syntax:

    Shift variable [,] { Left | Right }

Command Availability:

Available on PIC and AVR microcontrollers.

Explanation:

The Shift command performs a logical shift of variable by one bit in the direction given. variable can be a Byte, Word or Long and is changed in place.

  • A 0 is always shifted into the vacated bit: the least significant bit on Left, the most significant bit on Right.
  • The bit shifted out is discarded. Unlike Rotate, Shift does not feed the carry bit (STATUS.C) back into the variable, and the carry flag should not be relied on after the command.
  • A Left shift multiplies an unsigned value by 2, and a Right shift divides it by 2.
  • For a Word or Long the shift runs through every byte, so the bit shifted out of one byte is shifted into the next.
  • Left and Right can be written L and R. The comma, and brackets round the variable, are optional: Shift Value Left, Shift Value, Left and Shift (Value) Left are the same.

Shift moves one bit each time it is used. To shift by several places, repeat it: 

    Repeat 4
        Shift Value, Left
    End Repeat

Shift is faster than FnLSL and FnLSR for a shift of one place or a few places, because it is compiled in line and does not call a function. Use FnLSL or FnLSR when the number of places is held in a variable and the shifted value is needed inside an expression.

Example:

    ' Shift a byte and a word, left then right.
    ' Look at the values in the simulator, or send them to a terminal.

    #chip 16F18855, 32

    Dim TestByte As Byte
    Dim TestWord As Word

    TestByte = 0b10010010
    Shift TestByte, Left        ' <<< the Shift instruction: 0b00100100, the top bit is lost
    Shift TestByte, Right       ' 0b00010010, a 0 enters at the top

    TestWord = 0x0080
    Shift TestWord, Left        ' 0x0100: the bit moves from the low byte to the high byte
    Shift TestWord, Right       ' 0x0080

    End

Key line: Shift TestWord, Left — moves every bit of the Word one place left through both bytes, a 0 entering at the bottom, with no carry fed back.

See Also:

  • Rotate — rotates a variable through the carry bit instead of shifting in a 0
  • FnLSL — a left shift by any number of places, as a function
  • FnLSR — a right shift by any number of places, as a function
  • Bitwise Operations Overview — background on GCBASIC’s bitwise operations
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