ranger@ecsvax.UUCP (Rick N. Fincher) (05/11/87)
Apple II SCSI Card: Interrupt problem
System crashes on the Apple IIGS may occur with programs that use
interrupts
during access of SCSI disk drives connected to the Apple II SCSI card.
Once
interrupts are enabled, it's possible that an interrupt may occur during a
SCSI drive data transfer. A program initiates this data transfer by passing
control to the SCSI card.
On the SCSI card, the SCSI firmware places its own code on its own stack
for
executing. If taking an interrupt also places some code on the same stack,
wiping out some or all of the previous SCSI firmware code, then returning
from the interrupt returns to a portion of the stack that the interrupt has
corrupted, which results in some kind of unidentifiable system crash.
This implies that crashes are possible if BOTH of the following conditions
apply:
1. The SCSI card is used with an interrupt driven program, like
AppleTalk.
2. This application program calls the SCSI card directly, NOT using
ProDOS.
There are currently VERY FEW programs that may exhibit this problem. It
is
foreseen, though, that the programs that developers are currently working
on
may produce more errors. This is due to use of features in the Apple IIGS
that use interrupts heavily.
We highly recommend that customers follow a regular practice of backing
up
valuable data.
The SCSI card ROM is currently being fixed and readied for release. The
January AppleGram to the dealers will have a general announcement on
the issue.
Apple Technical Communications