Nortel DMS 100 Series Maintenance Manual page 70

2-54 Maintenance overview
this stage, the recovery process sets up the trapped task for recovery. The
recovery process allows the trapped task for recovery to resume. Or, the
recovery process initiates a maintenance action to start or set the unit again.
If the trap is not fatal, the process restores convicted task to a point of
execution. This point of execution allows the task to restore to a known state.
The process of recovery does not consist of starting the task again. This
process forces the task to perform a multi-level exit back to the task mainline.
The task recovery model specifies a task to provide a mainline. The mainline
is a do-forever loop that only calls the main body of the steady state code. An
optional call to a recovery procedure follows the call to the main body of the
steady state code. The task resumes after a recoverable trap. The process
forces the task to return to the next instruction that follows the call to the main
body of code. The next instruction must be a call to a recovery procedure that
the task uses. The task uses this recovery procedure to realize the task takes a
fault. The procedure allows the task to take special recovery action, or branch
back to the start of the loop.
An improvement to exception recovery deals with the Direct Memory Access
Memory Management Unit (NTMX77 DMA MMU). The system must take
special action when the system attempts to recover from an NTMX77 DMA
MMU error. Before this feature, when a DMA MMU error occurred, the
exception processing system convicted the task that was interrupted. The
software on the NTMX77 does not apply to the error but the system is able to
convict the task. The convicted task determines if the error was fatal. An
MMU error is not always fatal. The results of the fatal evaluation depend on
the trapped task.
The processor that caused the fault does not know a problem occurred. If the
access is for a read from the NTMX77 memory, the external processor receives
invalid data. If the access is to write to NTMX77 memory, data is not written.
The trap is considered fatal and maintenance action initiates to start or set the
unit again.
Traps (XPM exception handling)
Feature AF5680, XPM Processing System Improvements, provides
improvements in exception handling and in exception services. Exception
handling includes the following activities:
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297-8223-550 Standard 04.02 November 2000
trap detection
trap data capture
recovery action