Analog Devices ADSP-BF535 Blackfin, Hardware Reference Manual

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ADSP-BF535 Blackfin Processor Hardware Reference 4-55
Program Sequencer
Deferring Exception Processing
Exception handlers are usually long routines, because they must discrimi-
nate among several exception causes and take corrective action
accordingly. The length of the routines may result in long periods during
which the interrupt system is, in effect, suspended.
To avoid lengthy suspension of interrupts, write the exception handler to
identify the exception cause, but defer the processing to a low priority
interrupt. To set up the low priority interrupt handler, use the Force
Interrupt / Reset instruction (
RAISE
).

When deferring the processing of an exception to lower priority
interrupt
IVGx
, the system must guarantee that
IVGx
is entered
before returning to the application-level code that issued the excep-
tion. If a pending interrupt of higher priority than
IVGx
occurs, it is
acceptable to enter the high priority interrupt before
IVGx
.
Example Code for an Exception Handler
Listing 4-4. Exception Routine Handler With Deferred Processing
/* Determine exception cause by examining EXCAUSE field in SEQ-
STAT (first save contents of R0, P0, P1 and ASTAT in Supervisor
SP) */
[--SP] = R0 ;
[--SP] = P0 ;
[--SP] = P1 ;
[--SP] = ASTAT ;
R0 = SEQSTAT ;
/* Mask the contents of SEQSTAT, and leave only EXCAUSE in R0 */
R0 <<= 26 ;
R0 >>= 26 ;
/* Using jump table EVTABLE, jump to the event pointed by R0 */
P0 = R0 ;