Digi NS7520 Exception entry/exit summary, Exception entry/exit by exception type, Reset Undef

Models: NS7520

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W o r k i n g w i t h A R M e x c e p t i o n s

Note: An explicit switch back to Thumb state is never needed. Restoring the CPSR from the SPSR automatically sets the T bit to the value it held immediately before the exception.

Exception entry/exit summary

In the variable R14_x, R14 is the Link register; _x is the previous state of the processor.

Return/

 

Previous state

Previous state

 

exception

Return instruction

ARM R14_x

Thumb R14_x

Notes

 

 

 

 

 

BL

MOV PC, R14

PC+4

PC+2

1

 

 

 

 

 

RESET

NA

NA

NA

4

 

 

 

 

 

UNDEF

MOVS PC, R14_und

PC+4

PC+2

1

 

 

 

 

 

SWI

MOVS PC, R14_svc

PC+4

PC+2

1

 

 

 

 

 

ABORT P

SUBS PC, R14_abt, #4

PC+4

PC+4

1

 

 

 

 

 

ABORT D

SUBS PC, R14_abt, #8

PC+8

PC+8

3

 

 

 

 

 

IRQ

SUBS PC, R14_irq, #4

PC+4

PC+4

2

 

 

 

 

 

FIRQ

SUBS PC, R14_firq, #4

PC+4

PC+4

2

 

 

 

 

 

Table 18: Exception entry/exit by exception type

Notes:

1Where PC is the address of the BL/SWI/undefined instruction fetch that had the prefetch abort. BL is a branch with link instruction.

2Where PC is the address of the instruction that was not executed since FIRQ or IRQ took priority.

3Where PC is the address of the load or store instruction that generated the data abort.

4The value saved in R14_svc upon reset is unpredictable.

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N S 7 5 2 0 H a r d w a r e R e f e r e n c e , R e v . D 0 3 / 2 0 0 6

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Digi NS7520 manual Exception entry/exit summary, Exception entry/exit by exception type, Reset Undef, Abort P, Abort D