take a look to my programm: When counting how often a unsigned long long integer could be divided by ten, i got strange results. Then i realised, a printf function call repairs the fault. Who can explain the strange effect?
//------------------------------------------------------------------- // a little C Programm (CARM) just to demonstrate: // printf function call repairs corrupted long long arithmetic.. // // sometimes i awake in the middle of the night // sweat flooded // my heart beating like a drum // is C just an abbreviation for Crashing airplanes? //------------------------------------------------------------------- #include <ADuC7026.h> #include"stdio.h" void RS232_init(int); int main (void) { unsigned long long int A; unsigned int n; RS232_init(9600); while(1) { printf("\nStart\n"); A=100000000000000; n=0; while(A) { printf(""); n++; A /= 10; } printf("%d \n",n); //expected 15, got 15 A=100000000000000; n=0; while(A) { n++; A /= 10; } printf("%d \n",n); //expected 15, got 2 printf("\nPress a key\n"); scanf("%*c"); } } void RS232_init(int Baudrate) { // 3% error on 115200 Baud !!! GP1CON = 0x011; // Setup tx & rx pins on P1.0 and P1.1 COMCON0 = 0x080; // Setting DLAB // Setting DIV0 and DIV1 to DL calculated COMDIV0 = 32768*1275/32/Baudrate; COMDIV1 = 0x000; COMCON0 = 0x007; // Clearing DLAB }
How about having a look at the assembly code ? I would guess that some attempt by the compiler at optimizing the loop went awry. If you don't want to examine the assembly code, try disabling the optimizations or declare n and A volatile and see what happens.
Yes, a look to the assembler code is a good idea. ;-) Making n volatile does'nt change a thing. Making A volatile changes result n from two to one. (15 expected) So volatile has an effect.
looking to assembler code is an endless story... for example:
?C?UQDIV?A: 0x0008102C E92D00F0 STMDB R13!,{R4-R7} 0x00081030 E3A0C040 MOV R12,#0x00000040 0x00081034 E2626000 RSB R6,R2,#0x00000000 0x00081038 E2E37000 RSC R7,R3,#0x00000000 0x0008103C E3A02000 MOV R2,#0x00000000 0x00081040 E0923002 ADDS R3,R2,R2 0x00081044 E0B00000 ADCS R0,R0,R0 0x00081048 E0B11001 ADCS R1,R1,R1 0x0008104C E0B22002 ADCS R2,R2,R2 0x00081050 E0A33003 ADC R3,R3,R3 0x00081054 E0924006 ADDS R4,R2,R6 0x00081058 E0B35007 ADCS R5,R3,R7 0x0008105C 21A02004 MOVCS R2,R4 0x00081060 21A03005 MOVCS R3,R5 0x00081064 E24CC001 SUB R12,R12,#0x00000001 0x00081068 E11C000C TST R12,R12 0x0008106C 1AFFFFF4 BNE 0x00081044 0x00081070 E0B00000 ADCS R0,R0,R0 0x00081074 E0B11001 ADCS R1,R1,R1 0x00081078 E8BD00F0 LDMIA R13!,{R4-R7} 0x0008107C E12FFF1E BX R14