16F877a ASM Interrupt Confused

[mister_e]

Each PIC datasheet explain black on white how to save the context variable.

PIC16F877a section 14.12

[Darrel Taylor]

For a 16F877A, and for many others, it's very simple.

Just change the $20 to $70.

There is a small amount of ram space that is common between all BANKS.   It ranges from $70 to $7F.   Any one of those locations can be used.

That way you don't need the wsave1 - wsave3 stuff
<br>

[modifyit]

Here is the code example from the 877a data sheet:

--------
MOVWF W_TEMP ;Copy W to TEMP register
SWAPF STATUS,W ;Swap status to be saved into W
CLRF STATUS ;bank 0, regardless of current bank, Clears IRP,RP1,RP0
MOVWF STATUS_TEMP ;Save status to bank zero STATUS_TEMP register
MOVF PCLATH, W ;Only required if using pages 1, 2 and/or 3
MOVWF PCLATH_TEMP ;Save PCLATH into W
CLRF PCLATH ;Page zero, regardless of current page
:
:(ISR) ;(Insert user code here)
:
MOVF PCLATH_TEMP, W ;Restore PCLATH
MOVWF PCLATH ;Move W into PCLATH
SWAPF STATUS_TEMP,W ;Swap STATUS_TEMP register into W
;(sets bank to original state)
MOVWF STATUS ;Move W into STATUS register
SWAPF W_TEMP,F ;Swap W_TEMP
SWAPF W_TEMP,W ;Swap W_TEMP into W
--------

I guess my question is if I define the following variables at the begining of my PBP code:

	w_temp  var	byte $20 system
	w_temp1 var	byte $a0 system   ' If device has RAM in bank1
	w_temp2 var	byte $120 system	' If device has RAM in bank2
	w_temp3 var	byte $1a0 system	' If device has RAM in bank3

how do I tell if I am in bank 0,1,2,or 3 when the interrupt occurs, so I know what w_temp variable(0,1,2, or3) to save using the following assembly code?

MOVWF W_TEMP ;Copy W to TEMP register
SWAPF STATUS,W ;Swap status to be saved into W

Thanks again

[modifyit]

For a 16F877A, and for many others, it's very simple.

Just change the $20 to $70.

Thanks Darrel, I will give that a shot. I am still interested how to make the wsave1,2,3 stuff work tho.

[Darrel Taylor]

Ok, easy to explain.

When a particular bank is selected with the RP0 and RP1 bits in the STATUS register, any opcode trying to access that bank is limited to 128 bytes. So, $20, $A0, $120 and $1A0 all point to a location that is 32 bytes into their respective BANK. &nbsp; The Variable declarations wsave1-3 are set that way to reserve the same locations in each bank.

That way you don't need to worry which bank you are in. The "MOVWF W_TEMP" always puts the byte in the correct location, no matter which bank is selected.

Then, notice that the STATUS register is restored prior to restoring the W register. That way you always have the same bank selected that was used to save the W reg. to begin with.

HTH,
&nbsp;&nbsp;&nbsp;Darrel

[modifyit]

So even tho I may be in bank 2 the "MOVWF W_TEMP" command will save to W_Temp2 in this case automatically?

[Darrel Taylor]

Correcto Mundo!

I have no idea what I just said, but it sounds right.