GCBASIC documentation
Bitwise

Syntax:

    BitsOut = FnLSR(BitsIn, NumBits)

Command Availability:

Available on all microcontrollers.

Explanation:

FnLSR (Logical Shift Right) performs a bitwise right shift. It returns BitsIn shifted right by NumBits places, equivalent to the C operation:

    BitsOut = BitsIn >> NumBits

Each right shift is equivalent to dividing BitsIn by 2 (discarding any remainder). BitsIn and NumBits can each be a variable, constant, or another function’s result, of type Bit, Byte, Word, or Long. Zeros are shifted in from the left; bits shifted out of the right end are discarded and lost — there is no rounding, so shifting an odd number right always truncates rather than rounds.

It is useful for mathematical and logical operations, as well as for creating serial data streams or manipulating I/O ports one bit-pattern at a time.

Example:

    ' This program will shift the LEDs on the Microchip PIC Low Pin Count Demo Board
    ' from Right to Left, that is DS4(RC3) to DS1(RC0) and repeat.

    #chip    16f690        ' declare the target Device

    #define  LEDPORT PORTC ' LEDs on pins 16, 15, 14 and 7

    Dim LEDMask as Byte    ' Pattern to be displayed
    LEDMask = 0b1000       ' Initialise the Patten
    Dir LEDPORT Out        ' Enable the LED Port.

    Do
      LEDPORT = LEDMask    ' Display the Pattern
      wait 500 ms
      LEDMask = FnLSR(LEDMask, 1) & 0x0F ' <<< the FnLSR instruction
      if LEDPORT.0 then LEDMask.3 = 1    ' Restart the sequence
    Loop
    End

Key line: FnLSR(LEDMask, 1) & 0x0F — shifts the lit LED one position right, then masks the result down to 4 bits so the pattern wraps within `LEDPORT’s lower nibble instead of drifting into unrelated bits.

Constants and the result type: the result is as wide as BitsIn, and a constant is a Byte unless it is larger than 255. So WordVariable = FnLSR(0x01, 13) is a Byte shift: the bits shifted past bit 7 are lost, and WordVariable is set to 0. Give the constant its type with a cast: WordVariable = FnLSR([word]0x01, 13). A variable is shifted as its own type, so this applies to constants only.

Faster alternative: for a shift of one place, or a few places, Shift is faster. It is compiled in line, with no function call, and changes the variable in place: Shift Value, Right. Shift moves one bit each time, so it does not replace FnLSR when the number of places is held in a variable.

See Also: