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0.12.1.1 Source Code

The first piece of code on this page is a typical checksum generation routine that takes a pointer to the data in question and returns a checksum value which, when added to the data, will yield a multiple of ten.

int checksum (char *data, int data_length) {

  int sum, i = 0;

  while (i < data_length) {
    sum += *data++;
    i++;
  }
  return(10 - (sum % 10));
}

The following code was Bill Sasina. It takes as input an Intel hex file and computes a checksum for it. While the utility of this code is somewhat limited, it demonstrates the idea of how checksums work very well. This code is not covered by the copyright statement in the introduction of this document; it was donated by the author to the public domain.

/*
                 Checksum Calculator Utility
 		Written by Bill Sasina, 1995
 */
 
 #include 
 #include 
 #include 
 #include 
 #include 
 
 /*the system error list is known at link time*/
 extern char *sys_errlist [];
 
 /*prototype definitions --*/
 void main (int, char **);
 void sum_it (long int *, char *, long int *);
 char *fgetstr (char *, int, FILE *);
 void errexit (unsigned);
 long int prompt_for_size(void);
 
 /* Global Variables */
 int line_cnt = 0;
 
 /*Main - open the first and second arguments and proceed as in Prog1*/
 void main (int argc, char *argv[]) 
 {
 	FILE *input;
 	char string[256];
 	long int cksum1 = 0;
 	long int cksum2 = 0;
 	long int byte_cnt = 0;
 	long int prom_size;
 	char *junk;
 
 	clrscr();
 	printf("%18c Bill's Checksum Utility  Version 2.2 \n\n", ' ');
 
 	if (argc < 2 || argc > 3)
 	{
 		errexit(1);
 	}
   
 	if (argc == 3)
 	{
 		prom_size = ((strtol(argv[2], &junk, 10)) * (long) 1024) - 1;
 	}
 	else
 	{
 		prom_size = prompt_for_size();
 	}
 	
 	/*get input file*/
 	if ((input = fopen (argv[1], "r")) == 0) 
 	{
 		errexit(2);
 	}
   
 	printf("\n%s\t", argv[1]);
 
 	while ((fgetstr(string, 255, input)) && (byte_cnt < prom_size)) 
 	{
 		line_cnt++;
 		sum_it(&cksum1, string, &byte_cnt);
 	}
   
 	cksum2 = cksum1;
   
 	if (byte_cnt < prom_size)
 	{
 		for ( ; byte_cnt <= prom_size; byte_cnt++) 
 		{
 			cksum1 += 255;
 		}
 	}
 
 	printf("\n\nFill Character\t FF\t\t 00\n");
 	printf("Checksum =\t%4X\t\t%4X\n", (int) cksum1, (int) cksum2);
   
 	if (fclose(input))
 	{
 		errexit (6);
 	}
   
 	/*exit normally*/
 	exit (0);
 }
 
 
 /*sum_it - Calculate the checksum */
 
 void sum_it (long int *sum, char *stringptr, long int *bytes) 
 {
 	int i;
 	long len = 0;
 	long line_sum = 0;
 	long temp = 0;
 	char tempstr[] = {"0XFF"};
 	char *junk;
 
 	if (*stringptr == 'S')
 	{
 		errexit(4);
 	}
   
 	if (*stringptr == ':')
 	{
 		stringptr++;
 	}
   
 	tempstr[2] = *stringptr++;
 	tempstr[3] = *stringptr++;
 
 	len = strtol(tempstr, &junk, 16);
 	line_sum = len;
 
 	for (i = 0; i < 3; i++) 
 	{
 		tempstr[2] = *stringptr++;
 		tempstr[3] = *stringptr++;
 
 		temp = strtol(tempstr, &junk, 16);
 		line_sum += temp;
 	}
 
 	if ( ( temp != 2 ) && ( temp != 3 ) )
 	{
 		*bytes += len;
 	}
 	else
 	{
 		if ( ( *stringptr != '0' ) && ( *stringptr != '1' ) ) 
 		{
 			printf("\nWarning\t--\tFile uses extended addressing.");
 			printf("\nOffset\t=\t%c%c%c%c", 
 				   stringptr[0], stringptr[1], stringptr[2], stringptr[3]);
 		}
 	}
 
 	for ( ; len; len--) 
 	{
 		tempstr[2] = *stringptr++;
 		tempstr[3] = *stringptr++;
 		
 		line_sum += strtol(tempstr, &junk, 16);
 		if ( ( temp != 2 ) && ( temp != 3 ) )
 		{
 			*sum += strtol(tempstr, &junk, 16);
 		}
 	}
 	tempstr[2] = *stringptr++;
 	tempstr[3] = *stringptr++;
 
 	if (((unsigned char) (~line_sum + 1)) != 
 		((unsigned char) strtol(tempstr, &junk, 16))) 
 	{
 		printf("Line number %d", line_cnt);
 		printf(" %X %X\n", (~((unsigned char) line_sum) + 1), 
 			   (unsigned char) strtol(tempstr, &junk, 16));
 		errexit(3);
 	}
 }
 
 /*fgetstr - 'gets' does not return '\n' -- 'fgets' does.
             this routine makes 'fgets' like 'gets'*/
 char *fgetstr (char string[], int n, FILE *filptr) 
 {
 	char *retval, *ptr;
 	if ((retval = fgets (string, n, filptr)) != NULL)
 	{
 		for (ptr = string; *ptr; ptr++)
 		{
 			if (*ptr == '\n') 
 			{
 				*ptr = '\0';
 				break;
 			}
 		}
 	}
 	return retval;
 }
 
 
 /*Errexit - handle errors as they arise*/
 char *errlist[] =
 {"invalid error",
  
  "wrong number of arguments."
  "\n Try: chksum  ",
  
  "input file does not exist",
  "checksum error in HEX file",
  "File is a Motorola S-Record File.",
  "error on output file write",
  "error on closing input file",
  "error on closing output file",
  "debug error"};
 
 void errexit (unsigned errnum) 
 {
 	if (errnum > 7)
 	{
 		errnum = 7;
 	}
 
 	fprintf (stderr, "\nchecksum utility error:  %s\n"
 			 "system error:          %s\n",
 			 errlist[errnum],
 			 sys_errlist [errno]);
 	exit (errnum);
 }
 
 
 long int prompt_for_size() {
 	long int retval;
   
 	printf("Select PROM Size:\n\n");
 	printf("1       8 KBytes x 8 (2764/87C528)\n");
 	printf("2      16 KBytes x 8 (27128)\n");
 	printf("3      32 KBytes x 8 (27256)\n");
 	printf("4      64 KBytes x 8 (27512)\n");
 	printf("5     128 KBytes x 8 (28F010/27C1024)\n");
 
 	while (!kbhit()) {
 	};
   
 	switch (getch()) 
 	{
 		case '1': 
 			retval = 8191;
 			break;
 		case '2': 
 			retval = 16383;
 			break;
 		case '3': 
 			retval = 32767;
 			break;
 		case '4': 
 			retval = 65535;
 			break;
 		case '5': 
 			retval = 131071;
 			break;
 		default: 
 			retval = 0;
 			break;
 	}
   
 	return( retval );
 }