Specifications

CHAPTER 4
Software
42 System Software
Screen Control Panel 4
The Screen control panel has sliders for adjusting brightness and contrast. The user can
adjust either slider by dragging with the mouse and can adjust the contrast using the
arrow keys or the number keys. Figure 4-3 shows the Screen control panel.
Figure 4-3 Screen control panel
Note
The Screen control panel uses minimum and maximum values supplied
by the Screen driver so as not to set the screen so dark that the user
cannot see the control panel to make further adjustments. ◆
The Screen Driver 4
Applications can read and set the brightness and contrast of the screen by making
appropriate status and control calls to the Screen driver (.BCScreen). The system
startup code uses the Screen driver to set the initial screen values.
The current values of brightness and contrast are stored as byte values in parameter
RAM. If the startup software finds a value below the minimum startup value, it sets
it to a default midrange value.
IMPORTANT
The Screen Driver is machine specific; it does not support
all Macintosh models. ▲
Screen Driver Calls 4
Screen driver calls use the ParamBlockRec method as described in the Device Manager
chapter of Inside Macintosh. The parameter passing conventions are the standard ones: the
calling program passes a pointer to the parameter block in register A0.