Description: Apple II SCSI Card: Interrupt Problem (11/96)
Header: Apple II SCSI Card: Interrupt Problem (11/96)

 Article Created: 26 November 1986
 Article Reviewed/Updated: 15 November 1996

 TOPIC -----------------------------------------------------------

 This article discusses a potential problem with the Apple IIGS, Apple II SCSI
 card and programs using interrupts.

 DISCUSSION ------------------------------------------------------

 Apple IIGS system crashes may occur with programs using interrupts during
 access of SCSI disk drives connected to the Apple II SCSI card. Once
 interrupts are enabled, it is possible an interrupt may occur during a SCSI
 drive data transfer. Programs initiate data transfer by passing control to
 the SCSI card.

 The SCSI card firmware places code in its own stack for executing. If taking
 an interrupt also places code on the same stack it may wipe out some or all
 of the previous SCSI firmware code. Returning from the interrupt, returns to
 the portion of the stack corrupted by the interrupt, which results in some
 kind of unidentifiable system crash.

 Crashes are possible if BOTH of the following conditions apply:

 - The SCSI card is used with an interrupt driven program, like AppleTalk.

 - This application program calls the SCSI card directly, NOT using ProDOS.

 To work around this possibility, make sure your applications use ProDOS to
 handle any interrupts. This may decrease SCSI disk data transfer rates but
 will help reduce system crashes.

 Article Change History:
 15 Nov 1996 - Reviewed for technical accuracy, revised formatting.
 Copyright 1986-96, Apple Computer, Inc.
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