I remember it was quite short code to read mouse position in C in DOS. Mouse was connected by serial RS232 interface. I used this technique in DOS in Turbo C with BGI interface.

https://ic.unicamp.br/~ducatte/mc404/Lampiao/docs/dosints.pdf

I dont have this code anymore but look this link above I think I used interrupt 33 but it was almost 30 years ago. (INT 33h)

here is example code

#include <dos.h> // Or a similar header for interrupt functions

void main() {
    int mouse_installed;
    int x, y, buttons;

    // Initialize the mouse driver
    _AX = 0;
    geninterrupt(0x33); // Call INT 33h

    mouse_installed = _AX; // Check the return value in AX

    if (mouse_installed) {
        // Turn the mouse on
        _AX = 1;
        geninterrupt(0x33);

        // Get the mouse position and button status
        _AX = 3;
        geninterrupt(0x33);
        buttons = _BX; // Get button info
        x = _CX;      // Get X-coordinate
        y = _DX;      // Get Y-coordinate

        printf("Mouse is installed. Cursor at %d,%d, button %d\n", x, y, buttons);

        // Turn the mouse off (optional)
        _AX = 2;
        geninterrupt(0x33);
    } else {
        printf("Mouse driver not found.\n");
    }
}