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Inside Macintosh: Processes
Chapter 4 - Vertical Retrace Manager / About the Vertical Retrace Manager

VBL Tasks Installed by the Operating System

The Operating System uses the Vertical Retrace Manager to accomplish a number of repetitive tasks at uniform intervals. These are some of the VBL tasks installed by the Operating System, grouped by the intervals at which they execute:

Some VBL routines may execute only on certain computers or only in certain versions of system software. For example, on early Macintosh computers, a VBL task checks every other interrupt to determine whether the state of the mouse button has changed from its previous state and then remained unchanged for at least four interrupts. If so, that task posts a mouse-down or mouse-up event, as appropriate. In Macintosh computers equipped with Apple Desktop Bus mouse devices, the Operating System uses a different mechanism for posting mouse-down and mouse-up events.

VBL tasks installed by the Operating System are not maintained in the same queue used for application-defined VBL tasks. ·

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© Apple Computer, Inc.
17 JUN 1996