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====== Arithmetic Calculations ======

  * Avoid using **multiplications** and **divisions** whenever possible. These complex mathmatic calculations need a lot processing power and (what's even worser) a huge library to be compiled at all.
  * If you need to multiply with and divide through even numbers like 2, 4, 8, 16, 32 ... you can make use of the bitshifting operators ">>" and "<<" <code c>
unsigned char c;
c = 12 >> 1;  // c is 6 (division through 2)
c = 12 >> 2;  // c is 3 (division through 4)
c = 12 << 1;  // c is 24 (multiplication by 2)
c = 12 << 2;  // c is 48 (mulitplication by 4)
c = 1023 >> 3; //  c is 127 (10bit to 7bit ;)
</code>
  * There's an excellent thread in the forum that discusses bitoperations: http://www.midibox.org/forum/index.php?topic=6981.0
  * If this is not enough, you could search for ASM optimized custom functions. You'll find some in code examples of TK, the [[ACSensorizer]] and a lot of PIC-Specialized Webpages – or of course the forum.
  * If that still is not enough or you have no time and a lot of processing power / space available on your PIC, you can include the **libsdcc library**: 

>> //if multiplications, divisions, pointer operations, etc. are used in the .c code, the linker may fail due to missing functions, which are part of the libsdcc.lib library. The common library for pic16 derivatives is not compatible to MIOS, therefore I've created a special one which can be downloaded from [[http://www.ucapps.de/mios/mios_libsdcc_v2_5_0.zip|here]]. Read the README.txt file for further details. [[http://www.ucapps.de/mios_c.html|TK on the C-Page]] //


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====== C Functions ======

  * MIOS_*_SRSet and _SRGet Functions refer to the pins in [[http://en.wikipedia.org/wiki/Endianness|Little-Endian]] order, so for example: \\
MIOS_DOUT_SRSet(1, 00000001) Will set the 1st pin (aka Pin 0).... or\\
MIOS_DOUT_SRSet(1, 01000000) Will set the 7th pin (aka Pin 6)\\


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====== C Optimizations ======

  * [[How to mix C and ASM]]
  * [[Compiled C Code Size]]

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====== C Variables ======

  * Declaring a [[http://en.wikipedia.org/wiki/Variable|variable]] as type 'const' will cause the [[http://en.wikipedia.org/wiki/Compiler|compiler]] to store the variable in the PIC's program [[http://en.wikipedia.org/wiki/Flash_memory|flash memory]], not the [[http://en.wikipedia.org/wiki/Static_random_access_memory|SRAM]].

  * Adding the keyword '[[http://en.wikipedia.org/wiki/Volatile_variable|volatile]]' to a [[http://en.wikipedia.org/wiki/Variable|variable]] is a good idea when this variable can be changed or altered outside the sourcefile that declared this variable.

  * Always use 'unsigned' if you are sure you don't need negative values. As it's not clear how C treats a 'char' (signed: -128 to 127; unsigned: 0 to 255), it's better to be clear here. 

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====== SDCC Bugs/Workarounds ======
Some of these bugs have first been described in a [[http://www.midibox.org/forum/index.php?topic=7463.msg50464#msg50464|german thread in the forum]].

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===== Array Access =====
Sometimes the transfer of an array between modules does not work properly, e.g. file 1: 
<code c>
unsigned char MIDIValues[8];
</code> 
file 2: 
<code c>
MIOS_MIDI_TxBufferPut(MIDIValues[1]);
</code>  
Instead, you need to do something like 
<code c>
unsigned char value = MIDIValues[1]; //explicit temp variable
MIOS_MIDI_TxBufferPut(value);
</code>

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===== Large Arrays =====
Arrays with more than 256 elements will produce compile (in fact linker) errors:
<code c>
unsigned char myArray[256]; // will work
unsigned char myArray[257]; // will not be linked!

unsigned char myArray[64][4]; // will work
unsigned char myArray[64][5]; // will not be linked!
</code>

This is due to the fact that the PIC's RAM has been segmented into 256-byte banks in the linker script, and an array's contents may not span across more than one bank.

The linker script can be modified to work around the 256-byte limitation by creating larger banks, as per the [[using_pic18f4620#linker_script|Linker Script]] and [[using_pic18f4620#application_code|Application Code]] tips on the 4620 page.

Thanks to Thomas for [[http://www.midibox.org/forum/index.php?topic=6814.msg43501#msg43501|testing some workarounds with multiple single-dimensional arrays]]. These methods would be recommended if possible.
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===== Bit Copy Operations =====
There is potential trouble with bit copy operations (See [[http://www.midibox.org/forum/index.php?topic=7925.msg54675#msg54675|this posting]]). Instead of 
<code c>
  app_flags.SRAM_CARD_STATUS = PORTEbits.RE2;
</code>
you should use
<code c>
  if( PORTEbits.RE2 ){
    app_flags.SRAM_CARD_STATUS = 1;
  }else{
    app_flags.SRAM_CARD_STATUS = 0;
  }
</code>
It is less elegant, but it works safely.

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===== Parenthesis =====
Always use parenthesis around expressions like <code c>myarray[a+b];</code> instead use <code c>myarray[(a+b)];</code>

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===== Preprocessor #ifs =====
Avoid #ifdef and #if preprocessor-statements wrapped around declarations and function prototypes. Even if the preprocessor's #if statement is true (eg defined as '1'), any access to it's vars and functions from outside these wrapped statements produce a compile-warning: <code c>
#define TEST 1

#if TEST
  unsigned char testvar;
#endif /* TEST */

void testfunction(void) {
  unsigned char c = testvar + 1;  // access to testvar produces compiler error!
}
</code>

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===== Zero Compare =====
Avoid comparisons of ''unsigned char'' with ''0'', e.g. <code c>
unsigned char i;
   for (i = 0; i < 0; i+ü) {
      //body
   }  </code> ''0'' could be a constant that was defined using ''#define'', e.g. the number of motorized faders. But you have no motorized faders... The main problem consists in the fact that your code depends on what else is done around the comparison or in the body. This provokes completely erratic behaviour.

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