PIC16C641 datasheet(68/136 Pages) MICROCHIP

68 / 136 page

PIC16C64X & PIC16C66X

DS30559A-page 68

Preliminary

9.6

Context Saving During Interrupts

During an interrupt, only the return PC value is saved

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

isters during an interrupt e.g. W register and STATUS

Example 9-1 stores and restores the STATUS and W

registers. The user register, W_TEMP, must be deﬁned

in both banks and must be deﬁned at the same offset

from the bank base address (i.e., W_TEMP is deﬁned

at 0x70 - 0x7F in Bank 0). The user register,

STATUS_TEMP, must be deﬁned in Bank 0.

Example 9-1:

• Stores the W register regardless of current bank

• Stores the STATUS register in Bank 0

• Executes the ISR code

• Restores the STATUS (and bank select bit

• Restores the W register

EXAMPLE 9-1:

SAVING THE STATUS AND W REGISTERS IN RAM

MOVWF

W_TEMP

; Copy W to a Temporary Register regardless of current bank

SWAPF

STATUS,W

; Swap STATUS nibbles and place into W register

BCF

STATUS,RP0

; Change to Bank 0 regardless of current bank

MOVWF

STATUS_TEMP

; Save STATUS to a Temporary register in Bank 0

: (Interrupt Service Routine)

SWAPF

STATUS_TEMP,W

; Swap original STATUS register value into W (restores original bank)

MOVWF

STATUS

; Restore STATUS register from W register

SWAPF

W_TEMP,F

; Swap W_Temp nibbles and return value to W_Temp

SWAPF

W_TEMP,W

; Swap W_Temp to W to restore original W value without affecting STATUS

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