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???
06/07/07 15:14
Modified:
  06/07/07 15:16
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 		 #140377 - Division / modulus not always slow
Responding to: ???'s previous message
Bob said:
Some discussion, though: division and modulus operations take a nice chunk of code, even with precompiled libraries and unsigned integers.

Ooh. You can't assume such things. Here's the code generated by my 8051 C compiler for a simple test:
Save This Code
    321          void test() {
   \   023E            test:
    322          
    323              unsigned char       a, b, c;
    324          
    325              a = b / c;
   \   023E  7800              MOV     R0,#$LOCBI test+1
   \   0240  E6                MOV     A,@R0
   \   0241  7800              MOV     R0,#$LOCBI test+2
   \   0243  86F0              MOV     B,@R0
   \   0245  84                DIV     AB
   \   0246  7800              MOV     R0,#$LOCBI test
   \   0248  F6                MOV     @R0,A
    326              a = b % c;
   \   0249  7800              MOV     R0,#$LOCBI test+1
   \   024B  E6                MOV     A,@R0
   \   024C  7800              MOV     R0,#$LOCBI test+2
   \   024E  86F0              MOV     B,@R0
   \   0250  84                DIV     AB
   \   0251  E5F0              MOV     A,B
   \   0253  7800              MOV     R0,#$LOCBI test
   \   0255  F6                MOV     @R0,A
    327          
    328              }
   \   0256  22                RET
    329
Your mileage may vary with a different compiler and/or a processor that doesn't have a divide instruction. It also gets worse on an 8-bit CPU if the variables are wider than 8 bits.

-- Russ


List of 20 messages in thread
TopicAuthorDate
Software design problem            01/01/70 00:00      
   Something like this, maybe?            01/01/70 00:00      
      Well, yes, actually...            01/01/70 00:00      
      Duhr and a question            01/01/70 00:00      
         Which ones did you look at ?            01/01/70 00:00      
         Duhr and an answer            01/01/70 00:00      
   Is it not possible to            01/01/70 00:00      
      Horses for courses            01/01/70 00:00      
         have fun            01/01/70 00:00      
   A Queue?            01/01/70 00:00      
      I used what I called a \'cache\'            01/01/70 00:00      
         Gah, code repost            01/01/70 00:00      
            you need to read it all, THEN process            01/01/70 00:00      
            Your approach is much more general...            01/01/70 00:00      
               Prioritizing?            01/01/70 00:00      
            Some suggestions Bob            01/01/70 00:00      
               Thank you, sir!            01/01/70 00:00      
                  Division / modulus not always slow            01/01/70 00:00      
         Are we making this too difficult?            01/01/70 00:00      
            I don't think so, it seems to work pretty well...            01/01/70 00:00      

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