Legacy Document

Inside Macintosh: Processes

Chapter 4 - Vertical Retrace Manager / Vertical Retrace Manager Reference

Vertical Retrace Manager Routines / Utility Routines

DoVBLTask

Slot interrupt handlers for video cards should call the DoVBLTask function to handle the execution of VBL tasks.

FUNCTION DoVBLTask (theSlot: Integer): OSErr;

theSlot

Slot number corresponding to the vertical retrace queue whose tasks are to be executed.

DESCRIPTION

The DoVBLTask function decrements the vblCount field of each task in the vertical retrace queue corresponding to the theSlot parameter (except for tasks whose vblCount field already contains the value 0). The function executes a task if decrementing the vblCount field for a task record results in a value of 0.

If theSlot designates the slot of the primary video device, the position of the cursor is also updated.

Slot interrupt handlers for video cards need to call this function to execute any tasks in the queue for that slot. You can also call this function if you need to simulate vertical retrace interrupts.

ASSEMBLY-LANGUAGE INFORMATION

The registers on entry and exit for DoVBLTask are

Registers on entry
D0	Slot number

Registers on exit
D0	Result code

To reduce overhead at interrupt time, instead of executing the _DoVBLTask trap, you can load the jump vector jDoVBLTask into an address register and execute a JSR instruction using that register.

RESULT CODES

noErr	0	No error
slotNumErr	-360	Invalid slot number