Please take a look at this function

uint16_t val;

uint16_t func2(void)
{
uint16_t t = 18000;
uint32_t w = (uint32_t)(t) * 10;
w /= val;
return (uint16_t)w;
}

In this case, val = 10. That means that the function should return 18000 * 10 / 10 = 18000. But it actually return 4892. It is easy to understand how it could be 4892 since 18000*10 = 180000 = 0x0002BF20, cut the 2 most significant bytes and you have 0xBF20, divide by 10 and we have 0x131c = 4892 .

If we change one line in the function from “w /= val; “ to “w = w / val;” the return value is correct (18000). So there seems to be something strange with the '/=' operator!

The generated assembler code look like this

; FUNCTION ?func2 (BEGIN)
; SOURCE LINE # 17
0000 5204 SUB SP,#004H
; SOURCE LINE # 20
0002 AEBF20 LDW X,#0BF20H
0005 1F03 F LDW (003H,SP),X ; [ w + 02H ]
0007 5F CLRW X
0008 1F01 F LDW (001H,SP),X ; [ w ]
; SOURCE LINE # 21
000A 1E03 F LDW X,(003H,SP) ; [ w + 02H ]
000C 89 PUSHW X
000D 1E03 F LDW X,(003H,SP) ; [ w ]
000F 89 PUSHW X
0010 CE0000 F LDW X,val
0013 BF03 F LDW ?CH,X
0015 5F CLRW X
0016 BF03 F LDW ?BH,X
0018 CD0000 F CALL ?C?swap_skbc
001B CD0000 F DNF CALL ?C?divu3232
001E 4F CLR A
001F CD0000 F CALL ?C?mv4_bc2isa
; SOURCE LINE # 22
0022 1E03 F LDW X,(003H,SP) ; [ w + 02H ]
0024 5B04 ADD SP,#004H
0026 81 RET

This does not look right to me. It seems that the function calculates the result of 0xBF20 / 10 which is what we get though it is incorrect.

This is the complete source file if you want to compile it yourself.

#include

typedef unsigned char uint8_t;
typedef unsigned long uint32_t;
typedef unsigned short uint16_t;

uint16_t val;

char buf[100];
static int count = 0;
int putchar(char c)
{
buf[count++] = c;
return 1;
}

uint16_t func2(void)
{
uint16_t t = 18000;
uint32_t w = (uint32_t)(t) * 10;
w /= val;
return (uint16_t)w;
}

void main(void)
{
val = 10;
printf("%d\n", func2());
}

My tools:
STM8 compiler/linker ver 2.32.10.0307

I am compiling for the STM8 family, SCINAME(STM8). Target CPU is STM8S207M8.

Best Regards
Andreas Hansson