VBLTask

VBLTask structure

#include <Retrace.h>

typedef struct VBLTask { Size Offset Description

QElemPtr qLink; 4 0 Address of next item in VBL task list

short qType; 2 4 Queue type (always vType = 1)

ProcPtr vblAddr; 4 6 Address of routine to process the

task

short vblCount; 2 10 Number of 1/60th-sec ticks

between tasks

short vblPhase; 2 12 Phase offset for multiple tasks

(usually 0)

typedef VBLTask *VBLQElPtr;

Notes: The VBLTask structure is used in calls to VInstall and VRemove.

The vblAddr field is the address of a routine to be called during the vertical

retrace blanking interrupt. See VInstall for guidelines in writing such a

routine.

The vblCount value is the number of ticks to skip between calls to the

routine whose address is in vblAddr. Each 16.66 ms (1/60-th of a

second), this field is decremented. When it reaches 0, the VBL task routine

is called. The task routine is responsible for setting this back to a non-zero

value to restart the timing cycle. If left at 0, the task is removed from the

queue.

In the event that you add more than one VBL task at the same time, and both

have the same vblCount interval, you may want them to to be executed on

different ticks (so as not to load up the system too much). In that case, set

vblPhase to some non-zero value (but less than vblCount) in the second and

subsequent VBLTasks which you install.

A call to GetVBLQHdr returns the address of a QHdr which contains a

pointer to the first of the VBLTask structures in the vertical retrace task

queue. Or you can simply examine the global variable VBLQueue (at

0x0160).