Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Thu, 10 Aug 2006 14:58:19 -0500

Till Straumann wrote:

This behaviour is not possible with the Tempe chip - VME read cycles
that terminate with a Bus Error just return 0xFFFF values (all one
bits), and by default there is no indication that anything unusual
happened. It is possible to enable the VME Exception interrupt, which
will eventually be serviced by the processor and most of the time
would allow the BSP to suspend the correct task (assuming that the Bus
Error wasn't caused by an Interrupt Service Routine), but that
interrupt could be delayed for quite a long time by other pending
interrupts.

So why would you care? The faulting task would still not be able
to make any progress and hence be suspended right at the faulting
instruction.

Interrupts may not be as quick at actually getting to the CPU as a
Target Abort - I don't know whether modern CPUs finish off any/all
instructions that they've already started running before they actually
switch to processing the exception, but it's likely that there will be a
number of instructions pending. This also supposes that interrupts are
enabled at the time the bus error gets flagged.
If the Bus Error occurs inside an interrupt service routine, the ISR
will get to run to completion, making use of the dud all-1's read data
in the process (the vxWorks mv6100 BSP doesn't support nested
interrupts, and the hardware doesn't appear to make it easy to do so).
If this does occur, the task that happened to be running before the
faulting ISR was entered might be incorrectly blamed for causing the
error and suspended.
While it might be possible to work around some of these effects it won't
be easy to do, and the "unreliable instruction location" behaviour is
not something I'd want when debugging a driver.
- Andrew
--
Not everything that can be counted counts,
and not everything that counts can be counted.
  -- Albert Einstein

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System