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PitfallJ
 Post subject: Modulo 80
PostPosted: Sat Mar 31, 2012 11:04 am 
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Joined: Fri Nov 07, 2008 2:28 pm
Posts: 65
Has anybody got a good modulo 80?

I currently use this code but have just found the number can exceed 160 so it fails:

Code:
MACRO   WRAP80
      CMP   #80
      BCC   ok781
      SEC
      SBC   #80
.ok781:
ENDMACRO


I guess I could use a table but don't want to use up 256 bytes:

Code:
  TAX
  LDA mod80,X


- PJ
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PaulDv
 Post subject: Re: Modulo 80
PostPosted: Sat Mar 31, 2012 12:13 pm 
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Joined: Sun Jun 28, 2009 11:37 pm
Posts: 55
You could just repeat the test a second time to catch values over 160.

The routine can also be optimised slightly by removing the redundant SEC:

Code:
MACRO   WRAP80
      CMP   #80
      BCC   ok781
      SBC   #80
      CMP   #80
      BCC   ok781
      SBC   #80
.ok781:
ENDMACRO
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RichTW
 Post subject: Re: Modulo 80
PostPosted: Sat Mar 31, 2012 10:18 pm 
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Joined: Mon Jan 07, 2008 6:46 pm
Posts: 380
Location: Málaga, Spain
In a similar vein is this, which will work for any value of A:

Code:
MACRO MOD80
    CMP #160
    BCC lessthan160
    SBC #160
    .lessthan160
    CMP #80
    BCC lessthan80
    SBC #80
    .lessthan80
ENDMACRO
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PitfallJ
 Post subject: Re: Modulo 80
PostPosted: Sun Apr 01, 2012 3:03 pm 
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Joined: Fri Nov 07, 2008 2:28 pm
Posts: 65
Thanks for that - with the idea of testing it twice I've come up with this variation:

Code:
MACRO   WRAP80
.again:
      CMP   #80
      BCC   ok781
      SEC
      SBC   #80
      BNE   again
.ok781:
ENDMACRO
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RichTW
 Post subject: Re: Modulo 80
PostPosted: Sun Apr 01, 2012 6:01 pm 
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Joined: Mon Jan 07, 2008 6:46 pm
Posts: 380
Location: Málaga, Spain
You don't need the SEC, as C is obviously already set if it gets there. Other than that, yep, works a treat!
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