Arm Development Studio forum Why nested interrupt corrupt Link Register?

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Why nested interrupt corrupt Link Register?

Note: This was originally posted on 13th June 2012 at http://forums.arm.com

hi,
Sorry for this basic question.

I find hard to understand why the Link Register can be corrupted in this below scenario:
1. IRQ interrupt occur.
2. IRQ ISR call a function foo(). (assume inside ISR, the IRQ interrupt is reenabled)
3. foo() is interrupted by another IRQ interrupt.

From what I understand:

*During (1):

STACK content:
some general purpose registers
LR_irq_1 (let call like that to indicate that the content is point to address of interrupted function by scenario-1)

LR_irq = address-1 (see LR_irq_1)

*During (2):

STACK content:
some general purpose registers
LR_irq_2
some general purpose registers
LR_irq_1

LR_irq = address-2 (address of a line within ISR)

*During (3):

STACK content:
some general purpose registers
LR_irq_3
some general purpose registers
LR_irq_2
some general purpose registers
LR_irq_1

LR_irq = address-3 (address of a line within foo() where second IRQ interrupt occur)

Assuming that we have enough IRQ stack size, then from above context, seem no reason for LR to be corrupted.

Any body could help me explain what causing LR to be corrupted?

Thanks!