16F877a ASM Interrupt Confused

[modifyit]

I've been trying to get a handle on how to write an assembly interrupt, but I feel like I am missing something simple.

The example given in the Pic Basic Pro manual makes sense, but I'm not sure exactly what to do since the 877a has 4 banks of RAM. I realize I need to save the W and Status register and that I am supposed to reserve a system variable in each bank similar to the following:

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	wsave  var	byte $20 system
	wsave1 var	byte $a0 system   ' If device has RAM in bank1
	wsave2 var	byte $120 system	' If device has RAM in bank2
	wsave3 var	byte $1a0 system	' If device has RAM in bank3
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The following is the example given in the PBP manual, but it appears it is an example for a chip with a single bank of RAM. How do I convert the following code so it will work on the 877a and save "wsave1" and "wsave2" etc when needed?

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	' Assembly language interrupt example

	led	 var	PORTB.1

	wsave var	byte $20 system
	ssave var	byte bank0 system
	psave var	byte bank0 system


	Goto start	' Skip around interrupt handler

	' Define interrupt handler
	define INTHAND myint

	' Assembly language interrupt handler
	asm
	; Save W, STATUS and PCLATH registers
	myint	movwf	wsave
		swapf	STATUS, W
		clrf	STATUS
		movwf	ssave
		movf	PCLATH, W
		movwf	psave

	; Insert interrupt code here
	; Save and restore FSR if used

		bsf	_led	; Turn on LED (for example)

	; Restore PCLATH, STATUS and W registers
		movf	psave, W
		movwf	PCLATH
		swapf	ssave, W
		movwf	STATUS
		swapf	wsave, F
		swapf	wsave, W
		retfie
	endasm

	' PICBASIC PRO™ program starts here
	start: Low led	' Turn LED off

	' Enable interrupt on PORTB.0
		INTCON = %10010000

	loop:	Goto loop	' Wait here till interrupted
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Thanks in advance for any help