11.1.2.5 Fast Register Stack
A Fast Register Stack is provided for the STATUS, WREG and BSR
registers, to provide a “fast return” option for interrupts. The Stack for each register is
only one level deep and is neither readable nor writable. It is loaded with the current
value of the corresponding register when the processor vectors for an interrupt. All
interrupt sources will push values into the Fast Register Stack. The values in the
registers are then loaded back into their associated registers if the RETFIE, FAST
instruction is used to return from the
interrupt.
If both low and high-priority interrupts are enabled, the Stack registers cannot be used reliably to return from low-priority interrupts. If a high-priority interrupt occurs while servicing a low-priority interrupt, the Stack register values stored by the low-priority interrupt will be overwritten. In these cases, users must save the key registers by software during a low-priority interrupt.
If interrupt priority is not used, all interrupts may use the Fast
Register Stack for returns from interrupt. If no interrupts are used, the Fast Register
Stack can be used to restore the STATUS, WREG and BSR registers at the end of a subroutine
call. To use the Fast Register Stack for a subroutine call, a CALL label, FAST
instruction must be executed to
save the STATUS
, WREG and BSR registers to the Fast
Register Stack. A RETURN, FAST
instruction is then executed to restore
these registers from the Fast Register Stack.
The following example shows a source code example that uses the Fast Register Stack during a subroutine call and return.
Fast Register Stack Code
CALL SUB1, FAST ;STATUS, WREG, BSR SAVED IN FAST REGISTER STACK
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SUB1:
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RETURN, FAST ;RESTORE VALUES SAVED IN FAST REGISTER STACK