How assembly knows if register contains positive or negative number?

Question

int main() {
    int64_t di = -10;
    uint64_t ui = UINT64_MAX - 9;

    return 0;
}

When I disassemble main it gives me this output:

Dump of assembler code for function main:
   0x0000000000001119 <+0>:     push   rbp
   0x000000000000111a <+1>:     mov    rbp,rsp
   0x000000000000111d <+4>:     mov    QWORD PTR [rbp-0x10],0xfffffffffffffff6
   0x0000000000001125 <+12>:    mov    QWORD PTR [rbp-0x8],0xfffffffffffffff6
   0x000000000000112d <+20>:    mov    eax,0x0
   0x0000000000001132 <+25>:    pop    rbp
   0x0000000000001133 <+26>:    ret    

Both of them are 0xfffffffffffffff6 I understand one bit in signed variable is to determine its sign but how it knows it is signed or unsigned variable. Maybe it sets some flag idk

Answer 1

In this processor architecture, and commonly in today’s processors, there is no indication of whether the bits in a register are a signed integer, an unsigned integer, an address, or something else. There is no type information associated with the register; it is simply a sequence of bits.

When you write additional C code to use the data, such as a test if (di < 3) …, the compiler will generate instructions that depend on the type of data. For example, there is a compare instruction that produces several bits describing the relationship between two numbers, and there are branch instructions that use those bits to perform branches such as “branch if the first number is less than the second number using a signed interpretation.” Thus, the type information is built into the assembly instructions that use the data.