Mouse #2
Varying VBL Interrupt Rate

Revised by Matt Deatherage (November 1988)
Written by Rilla Reynolds (November 1985)

This Technical Note describes a method to make the AppleMouse peripheral card interrupt at a rate other than the default 60 Hz. This method does not work on the Apple IIc or IIGS.


This Technical Note describes a previously undocumented call to the AppleMouse II firmware which allows the user to set the interrupt rate to 50 or 60 Hz. (The default is 60 Hz, which keeps the card-generated VBL interrupts synchronized with the actual VBL rate on standard North American Apples; European Apples use 50 Hz as a standard.)

Call:
TimeData
Offset Location:
Cn1C
Input:
Accumulator bit 0:
Note: All other accumulator bits are reserved, and must be set to 0.
Output:
carry bit clear = screen holes unchanged
>[?You must make this call just prior to calling InitMouse to be effective. If you want to change the interrupt rate in the middle of an application, you must call TimeData with the appropriate value in the accumulator, then call InitMouse (which generates an interrupt). InitMouse resets the mouse position, mode, clamps, etc. to their default values. If you fail to call TimeData, InitMouse will use a default interrupt rate of 60 Hz.

Note: This call exists only on the AppleMouse card for the IIe or ][+ and should only be used when you know you are working with a IIe or ][+. A user may configure a IIGS to 50 Hz by holding down the Option key while rebooting. The standard North American Apple IIc will not generate 50 Hz VBL interrupts.

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