Commodore 128 Programmer's Reference Manual page 603

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C128 HARDWARE SPECIFICATIONS 593
RSEL NUMBER OF ROWS CSEL NUMBER OF COLUMNS
^ 0 24 rows 0 38 columns
, 1 25 rows 1 40 columns
The RSEL bit (bit 3) is in register 53265 ($D011), and the CSEL bit (bit 3) is in
register 53270 ($D016). For standard display, the larger display window is normally
used, while the smaller display window is normally used in conjunction with scrolling.
SCROLLING
The display data may be scrolled up to one character in both the horizontal and vertical
directions. When used in conjunction with the smaller display window (above), scrolling
can be used to create a smooth panning motion of display data while updating the
system memory only when a new character row (or column) is required. Scrolling is
also used for centering a display within the border. An example of horizontal smooth
scrolling is found in Chapter 8.
BITS REGISTER FUNCTION
0-2 53270 ($D016) Horizontal Position
0-2 53265 ($D011) Vertical Position
LIGHT PEN
The light pen input latches the current screen position into a pair of registers (LPX,
LPY) on a low-going edge. Since the X position is defined by a 9-bit counter (53267
($D013)), resolution to 2 horizontal dots is provided. Similarly, the Y position is latched
in register 53268 ($D014) with 8 bits providing unique raster resolution within the
visible display. The light pen latch may be triggered only once per frame, and subse
quent triggers within the same frame will have no effect.
For more information on programming the VIC (8564) chip, see Chapter 8, The
Power Behind Commodore 128 Graphics.