C Mouse Input

Mouse input lets your program react to where you move and click the mouse, like a game reacting to your hand. You can use it to make buttons.

In this page:

Detecting Mouse Support
Getting Mouse Coordinates
Handling Mouse Clicks
Creating Interactive Buttons
Hiding the Mouse Cursor

Syntax

union REGS in, out;
in.x.ax = function_code;   // 0 = init, 1 = show, 3 = position/buttons
int86(0x33, &in, &out);    // call the mouse interrupt
// results in out.x.ax, out.x.bx, out.x.cx, out.x.dx

Detecting Mouse Support

On the classic DOS-era BGI setup, the mouse isn't accessed through a modern event API but through low-level interrupts: populating a union REGS structure and issuing interrupt 0x33 with the appropriate function number is what initializes and communicates with the mouse driver.

Example: Detecting Mouse Support

#include <dos.h>
int main() {
	union REGS in, out;
	in.x.ax = 0;
	int86(0x33, &in, &out);
	return 0;
}

Getting Mouse Coordinates

Requesting mouse status with the interrupt function code for 'get position' (operation value 3) returns the cursor's current X and Y screen coordinates in the interrupt's output registers, which your program then reads out of that same union REGS structure.

Example: Getting Mouse Coordinates

#include <dos.h>
// Program execution starts in main()
int main() {
	union REGS in, out;
	in.x.ax = 3;
	int86(0x33, &in, &out);
	// Declare x and set it to out.x.cx
	int x = out.x.cx;
	// Declare y and set it to out.x.dx
	int y = out.x.dx;
	// Return 0 to signal that the program finished successfully
	return 0;
}

Handling Mouse Clicks

Mouse button state is reported through a value in the interrupt's output register after the status call: a value of 1 typically indicates the left button is pressed and 2 indicates the right button, letting you branch your program's behavior based on which button (if any) is down.

Example: Handling Mouse Clicks

#include <dos.h>
int main() {
	union REGS in, out;
	in.x.ax = 3;
	int86(0x33, &in, &out);
	if (out.x.bx == 1) {
		// left button pressed
	}
	return 0;
}

Creating Interactive Buttons

To build a clickable button, you define a rectangular region in screen coordinates representing that button's bounds, then on every detected click compare the reported mouse coordinates against that rectangle's edges to decide whether the click landed inside it.

Example: Creating Interactive Buttons

#include <dos.h>
int main() {
	union REGS in, out;
	in.x.ax = 3;
	int86(0x33, &in, &out);
	int x = out.x.cx, y = out.x.dx;
	if (x >= 50 && x <= 150 && y >= 50 && y <= 100) {
		// click inside button bounds
	}
	return 0;
}

Hiding the Mouse Cursor

Before your program exits (or whenever you want the cursor visually hidden), calling the mouse interrupt with operation value 2 hides the mouse cursor, which matters because leaving it visible while your program tears down its graphics mode can leave a stray cursor artifact on screen.

Example: Hiding the Mouse Cursor

#include <dos.h>
int main() {
	union REGS in, out;
	in.x.ax = 2;
	int86(0x33, &in, &out);
	return 0;
}

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