- ld r0,X+
- ld r1,X
- out SPL-__SFR_OFFSET,r0 ; SP = *new_sp
- out SPH-__SFR_OFFSET,r1
+ ld r18,X+
+ ld r19,X
+
+; Set new stack pointer.
+; AVR is an 8 bit processor so
+; care must be taken when updating
+; SP that is a 16 bit reg.
+; Two instructions are required to update SP
+; so an IRQ can sneak in between them.
+; So IRQ *MUST* be disabled and then restored.
+ cli ; Disable interrupt
+ out SPL-__SFR_OFFSET,r18 ; SP = *new_sp
+ out SPH-__SFR_OFFSET,r19
+ out SREG-__SFR_OFFSET,r0 ; Restore previous IRQ state