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Display Timing Modes

Macintosh graphics drivers have always sensed the type of display attached to the graphics card. They did this with three lines on the connector to perform a hardware sense code algorithm. This algorithm is detailed in the Macintosh New Technical Note HW-30, described in Apple Publications. Once the sense code was determined, the graphics driver trimmed its list of available timing modes to those that it calculated were possible.

Having the driver determine which timing modes are possible is very unflexible. New displays have required new sense codes that old drivers do not recognize and new technologies, such as the Display Data Channel (DDC) technology, provide additional information that old drivers do not know how to interpret.

Thus, the graphics driver strategy for Mac OS changed in PCI-based Power Macintosh computers. This new strategy emphasizes timing mode decisions done through the Display Manager instead of the graphics driver. This approach has these advantages:

It gives display designers maximum flexibility to create displays that support multiple timing modes.
It lets card desgners focus on hardware and be less concerned with the display that is attached.
It supports the Video Electronics Standards Association (VESA) DDC standard (Level 2B), but does not force cards to interpret DDC content.

Display Timing Modes

Display Manager Requirements

Responding to GetConnectionInfo

Connection Information Flags

Timing Information