From: Dwayne Reid
...here is an example that takes the value in R1H,M,L and adds it to the value contained in AD2H,M,L (result in AD2H,M,L). You should be able to see how to extend it to more than 24 bits.
There is a similar version that does subtraction. I often extend to 40 bits or more - its easier to do this when manipulating integer values than it is to try and figure out which bits I can discard.
;now add new sample
movfw R1L ;
addwf AD2L,F ;LS byte
movfw R1M ;middle byte
skpnc
incfsz R1M,W
addwf AD2M,F ;
movfw R1H ;MS byte
skpnc
incfsz R1H,W
addwf AD2H,F ;