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One trick for swapping variables does not appear all that necessary in 'C', but can be very useful in limited memory cases:
In 'C':    
     int a,b;
     a = 5;
     b = 22;
     //to swap
     a^=b^=a^=b;
     //now a=22 and b=5

The same trick in asm may look like:
     mov a,5
     mov b,22
     ;now swap
     xor a,b
     xor b,a
     xor a,b
     ;now a=22 and b=5

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Decimal Number to BCD:

In this example, the firmware revision number is in decimal, but the USB revision number must be provided in BCD with switched bytes:

#define FW_REV_MSB        6       
#define FW_REV_LSB         42  //REV 6.42

//Assuming (int) is a 16-bit value.
#define USB_REV   (int)( ((((int)FW_REV_MSB / 10 ) * 16 ) & 0xF0 )      | \
                       ( (int)FW_REV_MSB - (( (int)FW_REV_MSB /10) *10) ) | \
                             (((( (int)FW_REV_LSB /10 ) * 16 ) & 0xF0 ) << 8)  | \
                 (( (int)FW_REV_LSB - (( (int)FW_REV_LSB /10) *10))  << 8 ) \
                                  )  


// Afterwards, USB_REV equal 0x4206