Low Pin Count Demo Board User’s Guide
Restore Context
Once the peripheral is serviced, it needs to restore the context and resume the main
program. Restoring the context is a little harder than it might seem at first. The obvious
method doesn’t work because the MOVF W_Temp,w may affect the Z flag, which was
restored in the previous instruction. Instead, a pair of SWAPF instructions can restore
Wreg without affecting the flags in the Status register. SWAPF exchanges the high and
low nibbles. The first SWAPF switches the nibbles in the file register and the second one
switches them back and puts the result in Wreg.
EXAMPLE 3-9:
CONTEXT RESTORE
;incorrect context restore
MOVF STATUS_Temp,w
MOVWF STATUS
MOVF W_Temp ;this may change the Z bit
;in the Status register
;good context restore
MOVF STATUS_Temp,w
MOVWF STATUS
SWAPF W_Temp,f
SWAPF W_Temp,w
;swap in place
;swap with Wreg destination
Finally, RETFIE transfers control back to the original program and sets the GIE bit,
re-enabling interrupts.
FIGURE 3-10:
SWAPF INSTRUCTION
DS51556A-page 30
Before
After
1 0 1 0
0 0 1 1
0 0 1 1
1 0 1 0
? 2005 Microchip Technology Inc.
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