Post by thread:Word (or more) subtract on 16F84

Hi, I'm new to this list so this may have been answered before. I'm looking for a neat code snippet for word subtract for the 16F84. I've found some info for word add (on Dejanews) but I can't seem to extend this simply to subtract. for word add I've got:- MOVF Word1Low,W ADDWF Word2Low,F MOVF Word1High,W BTFSC STATUS,C INCFSZ Word1High,W ADDWF Word2High,F ... which produces Word2 += Word1, with carry correctly set! Any ideas? Also, does anyone have the URL of a searchable acrhive for this list? Thanks Ian

Hello Ian. Why not use the same idea as for addition. At final the Cy=1 if no borrow was taken else Cy=0. movfw Word1Low subwf Word2Low,f movfw Word1High skpc ;if Cy=1 then simply subwf incfsz Word1High,w ;if Cy=0 then subwf with borrow subwf Word2High,f ;if Word1High+1 ==0 then skip SUBWF ;command to save the Cy flag You may easy to add one more byte subtraction like a second block in example above. Be luck in your research ! ;-) WBR Dmitry. Ian Parish wrote: > > I'm new to this list so this may have been answered before. > I'm looking for a neat code snippet for word subtract for the 16F84. I've foun d some info for word add (on Dejanews) but I can't seem to extend this simply to subtract. {Quote hidden}> > for word add I've got:- > > MOVF Word1Low,W > ADDWF Word2Low,F > MOVF Word1High,W > BTFSC STATUS,C > INCFSZ Word1High,W > ADDWF Word2High,F > > ... which produces Word2 += Word1, with carry correctly set! > Any ideas?

Ian Parish wrote: > I'm looking for a neat code snippet for word subtract for the 16F84. I've foun d some info for word add (on Dejanews) but I can't seem to extend this simply to subtract. {Quote hidden}> > for word add I've got:- > > MOVF Word1Low,W > ADDWF Word2Low,F > MOVF Word1High,W > BTFSC STATUS,C > INCFSZ Word1High,W > ADDWF Word2High,F > > ... which produces Word2 += Word1, with carry correctly set! > > Any ideas? > > Also, does anyone have the URL of a searchable acrhive for this list? Hi, There is a complete set of 16bit routines in Myke Predko's site and explaining this routine. Check http://www.myke.com Reggie

Go to http://www.myke.com. Myke Predko has 16 bit code examples for addition, subtraction, multiplication, division, and compare. I am using all 5 operations in an RC servo mixer and they work great. spam_OUTIanTakeThisOuTALLANON.DEMON.CO.UK on 10/04/98 03:40:00 AM Please respond to .....PICLISTKILLspam@spam@MITVMA.MIT.EDU To: PICLISTKILLspamMITVMA.MIT.EDU cc: (bcc: Adam Bryant/PEAK/MOORE) Subject: Word (or more) subtract on 16F84 Hi, I'm new to this list so this may have been answered before. I'm looking for a neat code snippet for word subtract for the 16F84. I've found some info for word add (on Dejanews) but I can't seem to extend this simply to subtract. for word add I've got:- MOVF Word1Low,W ADDWF Word2Low,F MOVF Word1High,W BTFSC STATUS,C INCFSZ Word1High,W ADDWF Word2High,F ... which produces Word2 += Word1, with carry correctly set! Any ideas? Also, does anyone have the URL of a searchable acrhive for this list? Thanks Ian

Adam Bryant Wrote: >Go to http://www.myke.com. Myke Predko has 16 bit code examples for addition, >subtraction, multiplication, division, and compare. I am using all 5 >operations in an RC servo mixer and they work great. Wow! That sounds like an interesting application. If you get some time, I'd be interested in seeing it. Just a note about the 16 bit operations, they produce the correct 16 bit value, but the carry (ie set after addition result > 65,536) may not be correct. This is an issue in some applications; most typically, I see other add/subtract code snippets which keep the carry correct for noting that an out of bound condition exists - the problem is that they don't produce the correct 16 bit result. Hmmm... Maybe I should come up with a snippet that does both. Regarding the HP 16C calculator thread; I guess I'm a genius. When I was at school, I bought two of them (one for work, one for home). They are the best programmer's calculators any developed - I would be lost without mine. I just wish HP would come up with an updated version. Sorry, I've been away for a few days; if you've sent me a note and are looking for a reply, I'll get to it in the next few days. Toodles, myke Look at the accomplishments and people that made them happen over the past 1,000 years in "The LIFE Millennium". http://www.myke.com/Book_Room/book1a.htm

Yes, the solutions below are a smart way to always have carry bit to tell if we had an overflow. Only problem is that in case when the add/sub of the lower byte resulted in a carry that made the inc/dec to wrap: the high byte does not get added/subtracted! (or am I just too tired this late on a friday?) And this effect will cause problem if we try to expand to more bytes. Is there a smart solution to both get the carry correct, and also always get the addition/subtraction correct? Regards /Morgan At 19:57 1998-10-04 +0300, you wrote: {Quote hidden}>Hello Ian. > >Why not use the same idea as for addition. >At final the Cy=1 if no borrow was taken else Cy=0. > > movfw Word1Low > subwf Word2Low,f > > movfw Word1High > skpc ;if Cy=1 then simply subwf > incfsz Word1High,w ;if Cy=0 then subwf with borrow > subwf Word2High,f ;if Word1High+1 ==0 then skip SUBWF > ;command to save the Cy flag > >You may easy to add one more byte subtraction like a second block >in example above. > >Be luck in your research ! ;-) > >WBR Dmitry. > >Ian Parish wrote: >> >> I'm new to this list so this may have been answered before. >> I'm looking for a neat code snippet for word subtract for the 16F84. I've found some info for word add (on Dejanews) but I can't seem to extend this simply to subtract. {Quote hidden}>> >> for word add I've got:- >> >> MOVF Word1Low,W >> ADDWF Word2Low,F >> MOVF Word1High,W >> BTFSC STATUS,C >> INCFSZ Word1High,W >> ADDWF Word2High,F >> >> ... which produces Word2 += Word1, with carry correctly set! >> Any ideas? > > Morgan Olsson ph +46(0)414 70741 MORGANS REGLERTEKNIK fax +46(0)414 70331 H€LLEKS (in A-Z letters: "HALLEKAS") SE-277 35 KIVIK, SWEDEN .....mrtKILLspam.....iname.com ___________________________________________________________

Morgan Olsson wrote: >Yes, the solutions below are a smart way to always have carry bit to tell >if we had an overflow. > >Only problem is that in case when the add/sub of the lower byte resulted in >a carry that made the inc/dec to wrap: the high byte does not get >added/subtracted! (or am I just too tired this late on a friday?) =============================== Yes, the only time the last add or subtract is skipped is when the value to add or subtract is zero. The carry bit and the 16-bit results are both correct in all cases. Chip.