GCBASIC documentation
Bitwise

Syntax:

    BitsOut = FnLSL(BitsIn, NumBits)

Command Availability:

Available on all microcontrollers.

Explanation:

FnLSL (Logical Shift Left) performs a bitwise left shift. It returns BitsIn shifted left by NumBits places, equivalent to the C operation:

    BitsOut = BitsIn << NumBits

Each left shift is equivalent to multiplying BitsIn by 2. 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 right; bits shifted out of the left end are discarded and lost — there is no overflow flag or carry-out from this function, so do not rely on FnLSL to detect that a value has grown too large for its variable type.

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 DS1(RC0) to
    ' DS4(RC3) 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 = 0b0001       ' Initialise the Patten
    Dir LEDPORT Out        ' Enable the LED Port.

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

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

See Also: