Thread: signed vs unsigned value when seen in assembly code

Hi Everyone,

Below is the snippet of code that I was trying to execute

Code:

typedef char s8;

typedef unsigned char u8;

int main(void)
{
//range for char is -128 to +127
//range for unsigned char is 0 to 255 


// range for int is    -2,147,483,648 to 2,147,483,647
// range for unsigned int is 0 to 4,294,967,295


    s8 *ptr, var1=110;


    u8 *ptr2, var2 = 250;


    ptr = &var2;


    ptr2 = &var1;


    printf("Values when datatypes are interchanged are %d and %d\n", *ptr, *ptr2);


    return 0;
}

The idea was to check how this code works at background when playing with different datatypes with pointer and here is the output of the program which was expected.

Code:

Values when datatypes are interchanged are -6 and 110

Here is the assembly code compiled in GCC 9.3

Adding code for just 2 lines of code as my query is limited to it:

Code:

        push    rbp
        mov     rbp, rsp
        mov     BYTE PTR [rbp-1], 110
        mov     BYTE PTR [rbp-2], -6
        mov     eax, 0
        pop     rbp
        ret

As seen from above , the value is initialized with -6 and not 250. My consideration was that value should initialized with 250 value since its unsigned char datatype and its range is between 0 to 255.

Again, I tried something new and added one more variable of datatype unsigned int with value 2147483647 (0x7FFFFFFF) and its assembler is as below:

Code:

        push    rbp
        mov     rbp, rsp
        mov     BYTE PTR [rbp-1], 110
        mov     BYTE PTR [rbp-2], -6
        mov     DWORD PTR [rbp-8], 2147483647
        mov     eax, 0
        pop     rbp
        ret

and when I added 1 into it:

Code:

    char *ptr, var1=110;
    unsigned char *ptr2;
    
    unsigned char var2= 250;
    unsigned int var3 = 2147483647+1;

its assembler code changes to

Code:

        push    rbp
        mov     rbp, rsp
        mov     BYTE PTR [rbp-1], 110
        mov     BYTE PTR [rbp-2], -6
        mov     DWORD PTR [rbp-8], -2147483648
        mov     eax, 0
        pop     rbp
        ret

in which it is initialized with -2147483648 and not with 2147483648.



Can anybody explain that why the signed range follows even the variables are unsigned?

Thanks in advance