SXB 2.0 16bit add/sub and overflow/underflow

Peter Verkaik · 2009-04-02 19:35

@Bean,

Adding 2 words generates

· 5488· 01B8· 0219······· ADD dacValue_MSB,adcValue_MSB· ;dacValue = dacValue + adcValue
······· 01B9· 01F7
· 5489· 01BA· 0218······· ADD dacValue_LSB,adcValue_LSB
······· 01BB· 01F6
· 5490· 01BC· 0603······· ADDB dacValue_MSB,C··········
······· 01BD· 02B7

If adding the MSB parts generates overflow (C set) but adding the LSB parts does not,
then the overflow is not detected.

If the code was changed to

··mov·w,adcValue_LSB
··add·dacValue_LSB,w
··mov·w,adcValue_MSB
··snc
··movsz·w,++adcValue_MSB·;leaves C intact
··add·dacValue_MSB,w

then we can detect 16bit overflow by testing carry.
This takes the same amount of code.
(code was found on sxlist.com)

regards peter

Post Edited (Peter Verkaik) : 4/2/2009 7:51:05 PM GMT

Peter Verkaik · 2009-04-02 19:46

For subtracting this is generated

· 5488· 01B8· 0218······· SUB dacValue_LSB,adcValue_LSB· ;dacValue = dacValue - adcValue
······· 01B9· 00B6
· 5489· 01BA· 0703······· SUBB dacValue_MSB,/C·········
······· 01BB· 00F7
· 5490· 01BC· 0219······· SUB dacValue_MSB,adcValue_MSB

But again underflow may be missed.

Alternative code from sxlist.com
··mov W, adcValue_LSB
· sub dacValue_LSB,w
· mov w,adcValue_MSB
· sc
· movsz w,++adcValue_MSB
· sub dacValue_MSB,w ;· dest=dest-source, with valid carry (although the Z flag is not valid).

regards peter

pjv · 2009-04-03 00:13

Hi Peter;

I will have to check again, but I use the same code, and I believe that Bean has the sequence correct, that is to say adding the 'lsb, then dealing with any carry out, and then adding the 'msb.

Cheers,

Peter (pjv)

Peter Verkaik · 2009-04-03 01:02

Hi pjv,

I think I made an error regarding the subtraction. The sx clears the C for underflow
and sets C when there is no underflow. So the subtract sequence appears right.

But the addition fails

dacValue = $FF00
adcValue = $0100
dacValue + adcValue = $0000 with C set

Using the sxb sequence
add the MSB yields 0 with C set
add the LSB yields 0 with C clear
so the ADDB dacValue_MSB,C leaves C clear

regards peter

PS. Have you seen my latest reply on the IFDEF issue.
I thought you were looking for a macro to autocreate a jumptable.
http://forums.parallax.com/showthread.php?p=795495

pjv · 2009-04-03 15:56

Hi Peter;

On confirming this, you are absolutely correct in the case of adding two words together. Subtracting two words is done correctly. Adding or subtracting a byte to a word are also both done correctly. In the case of adding two words, Bean simply has the order backwards, the '_lsb needs to be added and "carry corrected"·before adding the '_msb.

Here is the compiled results on adding/subtracting two words as well as a word with a byte:

 
   342  0000  0215        ADD WordOne_MSB,WordTwo_MSB    ;WordOne = WordOne + WordTwo
        0001  01F5
   343  0002  0214        ADD WordOne_LSB,WordTwo_LSB   
        0003  01F4
   344  0004  0603        ADDB WordOne_MSB,C            
        0005  02B5
   345                  
   346  0006  0214        SUB WordOne_LSB,WordTwo_LSB    ;WordOne = WordOne - WordTwo
        0007  00B4
   347  0008  0703        SUBB WordOne_MSB,/C           
        0009  00F5
   348  000A  0215        SUB WordOne_MSB,WordTwo_MSB   
        000B  00B5
   349                  
   350  000C  021C        ADD WordOne_LSB,ByteOne        ;WordOne = WordOne + ByteOne
        000D  01F4
   351  000E  0603        ADDB WordOne_MSB,C            
        000F  02B5
   352                  
   353  0010  021C        SUB WordOne_LSB,ByteOne        ;WordOne = WordOne - ByteOne
        0011  00B4
   354  0012  0703        SUBB WordOne_MSB,/C           
        0013  00F5

Regarding the IFDEF, I have not had the time recently to mess with that, but I hope to look into it soon.

Thanks for your interest.

Cheers,

Peter (pjv)

Post Edited (pjv) : 4/3/2009 4:01:27 PM GMT

NetHog · 2009-04-04 01:34

One frustrating thing about add/sub with addb/subb is that you lose the final carry/borrow and cannot check for overflow, regardless of which way you go about it.

Last thing I compiled with SXB/2.0, the compiler·turned on CARRYX, which allows overflow checking.

Without CARRYX, you have to take a very convoluted approach.

Post Edited (NetHog) : 4/4/2009 1:39:38 AM GMT

SXB 2.0 16bit add/sub and overflow/underflow

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