GetVBLQHdr
GetVBLQHdr Get the address of the VBL queue header
#include <Retrace.h> Vert. Retrace Mgr
returns address of the VBL queue header
Use GetVBLQHdr if you want to examine the queue of vertical retrace tasks.
Returns: a 32-bit QHdrPtr (the address of a 10-byte QHdr structure). The
qHead field of that structure points to a VBL queue element (a
VBLTask structure).

Notes: C programmers may prefer to access the global variable VBLQueue (at
0x0160).
Apple tells us that bit 6 of the VBLTask.qFlags is set if any of the VBL tasks
are currently executing. Thus:
printf( "a VBL task %s executing\n",
(VBLQueue.qHead & 0x40) ? "IS" : "IS NOT");
qep = (VBLTask *)VBLQueue.qHead; /* get global variable */
while ( qep != 0 ) {
printf( "Task at %lx has %d ticks to go\n", qep, qep->vblCount );
qep = qep->qLink;
}