Re: [Tinyos-help] tmote sky : difference between large integers

Thu, 17 Nov 2005 01:58:45 -0800

Thank Mr. Polastre (and Mr. Siirtola)
the error is really the (weird for me) behaviour of SUBC.
For who is interested a working large integer subtraction is here:


    for (i = 0; i < digits; i++) {
      asm volatile ("rrc %1 \n\t"
                    "jc .Llab%= \n\t"
                    "setc \n\t"
                    "jmp .Lcon%= \n\t"
                    ".Llab%= : clrc \n\t"
                    ".Lcon%=: subc %3, %2 \n\t"
                    "mov %2,%0 \n\t"
                    "rlc %1 \n\t"           
                    :"=r"(a[i]), "+r"(borrow)
                    :"r"(b[i]), "r"(c[i])
                    );
   }

          
Where a ,b and c are integer array of digits elements.
Code could be simpler if some instruction for negate CARRY bit
(if CARRY == 1 then CARRY =0 ; and viceversa) exists but
I didn't find any.




Joe Polastre wrote: 
This is a task for the handy-dandy MSP430 x1xx User's Guide.

If you look up the ADDC and SUBC instructions, ADDC does carry,
whereas SUBC will borrow/.NOT. carry.  This may be a source of
confusion.

Don't know how that helps you, but I wanted to point it out :) 
Relevant section is page 3-67, but you probably already know that.

-Joe

On 11/16/05, Andrea Pacini <[EMAIL PROTECTED]> wrote:
  
 I am going crazy about why this assembly code works :

 /* SUM BETWEEN LARGE INTEGERS */

  for (i = 0; i < digits; i++) {
       asm volatile ("rrc %1 \n\t"
                                "addc %3, %2 \n\t"
                                "mov %2, %0 \n\t"
                                "rlc %1 \n\t"

                                 :"=r"(a[i]),"+r"(carry)
                                 :"r"(b[i]), "r"(c[i])
                     );
    }


 and this other doesn't work :

 /* DIFFERENCE BETWEEN LARGE INTEGERS */
  for (i = 0; i < digits; i++) {
       asm volatile ( "rrc %1 \n\t"
                                 "subc %3, %2 \n\t"
                                 "mov %2,%0 \n\t"
                                 "rlc %1 \n\t"

                                 :"=r"(a[i]), "+r"(borrow)
                                 :"r"(b[i]), "r"(c[i])
                     );
    }

 The first does sum between two large integer (array),  b and c, and result
is stored in a , and it works fine
 The second does difference but it doesnt' work !!!
 The only difference is that in the second I use subc instead of addc.


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