SCREEN HORIZONTAL SCROLL

The SCREEN HORIZONTAL SCROLL command is designed to scroll the screen horizontally, but its interpretation and the meaning of the parameter supplied vary significantly depending on the hardware platform on which the program is running. This peculiarity makes it essential to understand the specifics of each system to correctly use this command and achieve the desired effect.

On Commodore computers, the numeric parameter supplied to the SCREEN HORIZONTAL SCROLL command takes integer values between 0 and 7. In this range, the value 4 represents the center position of the scroll. Values less than 4 move the screen content to the left, while values greater than 4 move it to the right. The amount of scrolling is proportional to the distance from the center value of 4. For example, a value of 0 or 7 will produce maximum scrolling to the left or right, respectively, while a value of 3 will cause a slight shift to the left of the center.

The SCREEN HORIZONTAL SCROLL command behaves differently on the TRS-80 Color Computer 3. On this platform, the numeric parameter indicates the starting position of the screen. A value of 0 specifies that the display begins at the leftmost available column. As the parameter value increases, the screen is progressively moved to the left, revealing portions of the screen that were previously located to the right of the visible area. In other words, a larger parameter value causes the displayed content to scroll to the left, revealing what was beyond the right edge of the screen. This approach is more intuitive for those who expect an increasing value to correspond to a movement in a specific direction.

This divergence in the semantics of the SCREEN HORIZONTAL SCROLL command across platforms highlights one of the inherent challenges in developing cross-platform software such as ugBASIC. The developer must be aware of the target machine in order to properly use this command and achieve the desired horizontal scrolling effect. Often, in ugBASIC programs that aim for cross-platform compatibility, conditional procedures (PROCEDURE ... ON target) are used to apply different parameter values to the SCREEN HORIZONTAL SCROLL command, adapting the behavior to the specifics of each system and thus ensuring a consistent user experience or desired visual effect on each machine.

EXAMPLE

 SCREEN HORIZONTAL SCROLL 3