The CWD instruction in the x86 and x64 assembly sign extends the content of AX and converts a word to a double word. The sign-extended value is stored in DX:AX registers where the DX register stores the most significant 16 bits and the AX register stores the least significant 16 bits.

Similarly, the CDQ instruction in the x86 and x64 assembly sign extends the content of the EAX register and the 64-bit result is stored in EDX:EAX register. The most significant 32 bits are stored in the EDX register and the least significant 32 bits are stored in the EAX register.

And, the CQO instruction in the x86 and x64 assembly sign extends the content of the RAX register. The 128-bit result is stored in RDX:RAX registers where the RDX register stores the most significant 64 bits and the RAX register stores the least significant 64 bits.

These CWD, CDQ, and CQO instructions in x86 and x64 assembly are often used in division operations. For example, let’s look at the following C code and the corresponding x64 assembly instructions:

#include <stdio.h>

int main() {
        int a = 1, b = 2, c;
        c = a / b;
        return 0;
}

The corresponding x64 assembly code is: