#202 How Multiply Big numbers ? [STM8]

[Hafez Shirmohamadli]

Hi,
My problem is with multiplying large numbers in SDCC.
for example :

void main (void){
  uint32_t a;
  uint64_t b;
  a = 101000;
  b = a * a;
  /* Other Codes ... */
  }

When I read the value of the variable "b" after the above code, I get the incorrect value 1611065408 .
Correct value of Multiply 101000 in 101000 is 10,201,000,000
What is the cause of the problem and its solution?

[Philipp Klaus Krause]

This is not specific to SDCC or the STM8.

a * a is of type uint32_t, so you only get the lowest 32 bits of the result. 10201000000 is bigger than 4294967295, the maxium value for an uint32_t. If you want the result to have 64 bits, you need to cast at least one operand to that width first, e.g. b = (uint64_t)a * a;.

[Hafez Shirmohamadli]

Thanks for Answer,
I tested above Code with TypeCasting ( before Create the Ticket ) And it did not solve the problem.

[Hafez Shirmohamadli]

Another question that might help solve my problem:
How can I enable "long long int" support in sprintf (at SDCC)?

[Philipp Klaus Krause]

AFAIK, you currently can't. As a workaround, I suggest printing an unsigned long long as two hexadecimal unsigned long, one for the upper 32 bits, one for the lower 32 bits.

[Hafez Shirmohamadli]

Hi again,
my Problem is Solved !
The problem was that I was reading the values of the variables through the "sprintf" function as follows:

void main (void){
  uint8_t Buffer[50];
  uint32_t a;
  uint64_t b;
  a = 101000;
  b = (uint64_t)a * a;
  sprintf(Buffer,"Value of a : %lu \n and b : %llu",a,b);
  UART_Send_String(Buffer);
  /* Other Codes ... */
 }

And I didn't know that SDCC doesn't support "long long int" format in "sprintf" function.
Thanks again for your kind support.