Slick way to generate Modulus 3 in PASM - Any ideas?

Beau Schwabe · 2009-11-01 06:53

Anyone know of a slick way to generate Modulus 3 or any other user defined Modulus value in PASM?

I could use a lookup table or partial lookup table and shift it up or down, but yuck!

This will only be needed for BYTE wide variables, and of course the fastest solution the better.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi) · 2009-11-01 07:30

By modulus 3, do you mean the base 3 equivalent of BCD, i.e. 00, 01, and 10, but not 11 in every two-bit digit?

-Phil

Beau Schwabe · 2009-11-01 14:18

Phil Pilgrim,

Yes, basically just division by 3 where all I'm interested in is the remainder, I don't care about the whole number. I can put something together that works pretty good for just Modulus 3, but I wanted to provide an option that doesn't confine it to a specific number.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi) · 2009-11-01 21:36

Would this work?

mod           [b]shl[/b]       modulus,#7
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
              [b]shr[/b]       modulus
              [b]cmpsub[/b]    dividend,modulus
mod_ret       [b]ret[/b]

-Phil

Beau Schwabe · 2009-11-01 23:22

Phil,

Thanks!... I think that will work just fine. Although I think you meant to add a ,#1 after each shr modulus.

TestCode:

[b]CON[/b]

  [b]_CLKMODE[/b] = [b]XTAL1[/b] + [b]PLL16X[/b]
  [b]_XINFREQ[/b] = 5_000_000


[b]VAR[/b]

[b]long[/b]    modulus,dividend,remainder1,remainder2


[b]PUB[/b] MOD_Test| ERR_fail

    modulus := 3

    ERR_fail := 0

    [b]cognew[/b](@MODtest, @modulus)

    [b]dira[/b][noparse][[/noparse]&#173;16..23&#093;~~
   
    [b]repeat[/b] dividend [b]from[/b] 0 to 255

      remainder2 := dividend // modulus

      [b]waitcnt[/b]([b]cnt[/b]+clkfreq/20000)

      [b]if[/b] remainder1 <> remainder2
         ERR_fail := 1
         
    [b]if[/b] ERR_Fail == 1
       [b]outa[/b][noparse][[/noparse]&#173;16..23&#093; := %11111111                           'ALL LEDs on = FAIL
    [b]else[/b]
       [b]outa[/b][noparse][[/noparse]&#173;16..23&#093; := %10101010                           'Alternating LED pattern = PASS
    [b]repeat[/b]

[b]DAT[/b]
              [b]org[/b]        
MODtest       [b]mov[/b]       _t1,                    [b]par[/b]
              [b]rdlong[/b]    _modulus,               _t1
              [b]add[/b]       _t1,                    #4
              [b]rdlong[/b]    _dividend,              _t1
              [b]add[/b]       _t1,                    #4
'----------------------------------------------------------
mod           [b]shl[/b]       _modulus,               #7
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
              [b]shr[/b]       _modulus,               #1
              [b]cmpsub[/b]    _dividend,              _modulus
'----------------------------------------------------------              
              [b]wrlong[/b]    _remainder,             _t1

              [b]jmp[/b]                               #MODtest              

_modulus      [b]long[/b]      0

'-----------------------------------------------------------------------------------
_dividend                       'alias dividend and remainder to same long variable
_remainder
              [b]long[/b]      0
'-----------------------------------------------------------------------------------


_t1           [b]long[/b]      0

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi) · 2009-11-02 00:43

Beau Schwabe said...
I think you meant to add a #1 after each shr modulus.

Oops! SX and PIC habits don't fade easily, do they? That'll teach me not to post something I didn't even try to compile.

-Phil

Slick way to generate Modulus 3 in PASM - Any ideas?

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