GCBASIC documentation
Variables Operations

Syntax:

    Rotate variable {Left | Right} [Simple]

Command Availability:

Available on all microcontrollers.

Explanation:

The Rotate command will rotate variable one bit in a specified direction. The bit shifted will be placed in the Carry bit of the Status register (STATUS.C). STATUS.C acts as a ninth bit of the variable that is being rotated.

variable supports Bytes, Word and Long variables.

When a variable is rotated right, the bit in the STATUS.C location is placed into the MSB of the variable being rotated, and the LSB of the variable is placed into STATUS.C location.

When rotated left the opposite occurs. The MSB of the variable is shifted to the STATUS.C bit and the LSB of the variable will contain what was previously in the STATUS.C bit location.

This table shows the operation of the Rotate Left command

As you may notice the STATUS.C bit added a 0 to the rotation. So this will take 9 shifts left to get back to the original value.

Simple option

Many times you want to rotate the variable around like the STATUS.C bit wasn’t there so the MSB of the variable fills the LSB of the variable on Rotate Left or the LSB fills the MSB on Rotate Right. That is where the SIMPLE option comes in. It adds a hidden step that shifts the STATUS.C bit twice so the bit moves from one end of the variable to the other.

Notes: The carry is also called SREG bit C, or simply C flag on AVR.

In some cases the Status.C or C flag may already be set because of prior operations in your program. Therefore, it may be necessary to clear the C flag before using Rotate. Use Set C Off before using the Rotate command to clear the flag.

Example:

    'This program will use Rotate to show a chasing LED.
    '8 LEDs should be connected to PORTB, one on each pin.

    #chip 16F819, 8

    'Set port direction
    Dir PORTB Out

    'Set initial state of port (bits 0 and 4 on)
    PORTB = b'00010001'

    'Chase
    C = 0
    Do
        Rotate PORTB Right Simple
        Wait 250 ms
    Loop