This Technical Note discusses anomalies in the way GS/OS handles interrupts.
Changes since May 1990: Added discussions about changes to GS/OS interrupt handling since System Software 5.0.2.
If your application calls ALLOC_INT to install an interrupt handler for an external interrupt source, it works fine unless the SCSI Manager (GS/OS file SCSI.Manager) is installed, in which case the system eventually grinds to a halt with a message about 65536 unclaimed interrupts.
If any interrupt handlers are bound (using BindInt) to reference number $17 (IRQ.OTHER), the unclaimed interrupt count gets incremented if none of the BindInt routines claims the interrupt, even though any handlers installed with ALLOC_INT routines still need a chance to claim it. The 5.0.2 SCSI.Manager triggers this problem because it calls BindInt with vector reference number $17.
In addition, if one or more interrupt handlers are bound to the IRQ.OTHER vector (VRN $17), the interrupt is passed to the ALLOC_INT handler even if it was already claimed by a BindInt routine. If no ALLOC_INT routine claims the interrupt, the unclaimed-interrupt count is incremented. As documented in Apple IIgs Technical Note #18, Do-It-Yourself SCC Interrupts, you cannot successfully call BindInt with vector reference number $0009.
An application may install both a BindInt routine and an ALLOC_INT routine. If they both claim the external interrupt, the unclaimed count does not get incremented. The solution is compatible with future System Software releases, since it does not depend upon the ALLOC_INT routine ever getting called.
Your application's BindInt routine sees the interrupt before your ALLOC_INT routine does, so the BindInt routine should figure out whether the interrupt was caused by your external device, and claim it if so. Your ALLOC_INT routine should claim an interrupt it sees if and only if your BindInt routine claimed the last interrupt it saw.
Starting with GS/OS version 3.2 (released with the Apple II High-Speed SCSI Card), the system no longer treats too many unclaimed interrupts as a fatal error. However, before version 6.0, it still counts the unclaimed interrupts so it can do something like display a dialog asking you to restart even though choosing "restart" returns you to the application unharmed (GS/OS version 3.2), or sometimes display a dialog box sending you to your dealer and sometimes not (version 3.3), or do nothing about it at all (version 4.0 and later). This is obviously as confusing to most of us as it was to the system itself, so fortunately GS/OS now ignores unclaimed interrupts and doesn't even bother counting them.
The GS/OS Reference suite says that device drivers may make BindInt and UnbindInt calls, noting this as an exception to the general rule that drivers may not make GS/OS system calls. What the references fail to note is that these calls may fail for an incredibly annoying reason -- the OS may be busy.
GS/OS takes special pains to avoid this while starting and while switching to ProDOS 8, but it does not avoid this condition during an OSShutDown -- a real shutdown of the OS, not a switch to ProDOS 8.
Driver authors can work around this problem by using a new system service call provided in GS/OS version 3.2 and later. The call, named UNBIND_INT_VECTOR, provides the functionality of UnbindInt to FSTs and drivers only to avoid the OS reentrancy issue. The vector is at $01/FCD8 and takes an interrupt identification number (as returned from BindInt) in the accumulator.
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