PIC17C44 datasheet(27/241 Pages) MICROCHIP

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DS30412C-page 27

PIC17C4X

5.9

Context Saving During Interrupts

During an interrupt, only the returned PC value is saved

on the stack. Typically, users may wish to save key reg-

isters during an interrupt; e.g. WREG, ALUSTA and the

BSR registers. This requires implementation in soft-

ware.

Example 5-1 shows the saving and restoring of infor-

mation for an interrupt service routine. The PUSH and

POP routines could either be in each interrupt service

routine or could be subroutines that were called.

Depending on the application, other registers may also

need to be saved, such as PCLATH.

EXAMPLE 5-1:

SAVING STATUS AND WREG IN RAM

;

; The addresses that are used to store the CPUSTA and WREG values

; must be in the data memory address range of 18h - 1Fh. Up to

; 8 locations can be saved and restored using

; the MOVFP instruction. This instruction neither affects the status

; bits, nor corrupts the WREG register.

;

PUSH

MOVFP

WREG, TEMP_W

; Save WREG

MOVFP

ALUSTA, TEMP_STATUS

; Save ALUSTA

MOVFP

BSR, TEMP_BSR

; Save BSR

ISR

; This is the interrupt service routine

POP

MOVFP

TEMP_W, WREG

; Restore WREG

MOVFP

TEMP_STATUS, ALUSTA

; Restore ALUSTA

MOVFP

TEMP_BSR, BSR

; Restore BSR

RETFIE

; Return from Interrupts enabled

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