Efficiently handling signed overflow in PASM?

ericball · 2009-11-11 19:32

I'm trying to calculate C = (A+B)/2 and C = (A-B)/2 in PASM, where A, B and C are 32 bit signed integers.· I'd like to gracefully and efficiently handle overflow of the intermediate result.· For unsigned addition I'd code it as:

mov c, a
add c, b   wc
rcr c, #1

But handling signed values and subtraction is making my head hurt.

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Ale · 2009-11-11 20:37

You should use adds and sar instead, and subs

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Alsowolfman · 2009-11-11 21:08

i don't think 3 operations is possible. the simplest way i can figure out is C=(A-B)/2=(A/2)-(B/2). that should be free of head pain.

mov c, a
sar c, #1
mov variable, b
sar variable, #1
subs c, variable

overflow is avoided instead of handled.

Post Edited (Alsowolfman) : 11/11/2009 9:15:07 PM GMT

BR · 2009-11-11 21:42

Would this work?:

      [b]mov[/b]   c,a
      [b]adds[/b]  c,b  [b]wc[/b]
[b]if_nc[/b] [b]sar[/b]   c,#1

Phil Pilgrim (PhiPi) · 2009-11-11 22:01

It will take four instructions:

sum           [b]mov    [/b]   c,b            ' 7fff     ffff     0001     8001
              [b]adds [/b]     c,a [b]wc[/b]         '+7fff    +ffff    +0001    +8001
              [b]       [/b]                  '=fffe c  =ffff nc =0002 nc =0002 c
              [b]sar    [/b]   c,#1           '=ffff    =ffff    =0001    =0001
        [b]if_c[/b]  [b]xor[/b]       c,_0x8000_0000 '=7fff    =ffff    =0001    =8001


dif           [b]neg    [/b]   c,b            
              [b]adds [/b]     c,a [b]wc[/b]         
              [b]sar    [/b]   c,#1           
        [b]if_c[/b]  [b]xor[/b]       c,_0x8000_0000

_0x8000_0000  [b]long[/b]      $8000_0000

It's easiest, thanks to the neg instruction, to treat both as sums. In this case overflow occurs iff two positive numbers sum to a negative or two negative numbers sum to a positive.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 11/11/2009 10:17:33 PM GMT

ericball · 2009-11-12 01:35

Thanks Phil, that's exactly what I was looking for. It handles the overflow (unlike a simple ADD+SAR) and doesn't lose precision (unlike SAR+ADD).

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Efficiently handling signed overflow in PASM?

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