What is a Device Driver?


Definition: A device driver is software that allows the operating system to communicate with hardware devices.

How Drivers Work

graph TD App[Application] --> OS[Operating System] OS --> Driver[Device Driver] Driver --> Hardware[Hardware Device] Hardware --> Driver Driver --> OS OS --> App

Drivers act as translators between the OS and hardware, converting generic OS commands into device-specific instructions.

Types of Drivers

1. Kernel-Mode Drivers
2. User-Mode Drivers

Driver Architecture

Windows Driver Model (WDM)
NTSTATUS DriverEntry(
    PDRIVER_OBJECT DriverObject,
    PUNICODE_STRING RegistryPath
)
{
    // Initialize driver
    DriverObject->MajorFunction[IRP_MJ_CREATE] = DeviceCreate;
    DriverObject->MajorFunction[IRP_MJ_CLOSE] = DeviceClose;
    DriverObject->MajorFunction[IRP_MJ_READ] = DeviceRead;
    DriverObject->MajorFunction[IRP_MJ_WRITE] = DeviceWrite;
    
    return STATUS_SUCCESS;
}
Linux Driver Model
static int __init my_driver_init(void)
{
    printk(KERN_INFO "Driver loaded\n");
    
    // Register character device
    major = register_chrdev(0, DEVICE_NAME, &fops);
    
    return 0;
}

static void __exit my_driver_exit(void)
{
    unregister_chrdev(major, DEVICE_NAME);
    printk(KERN_INFO "Driver unloaded\n");
}

module_init(my_driver_init);
module_exit(my_driver_exit);

Driver Communication

I/O Request Packets (IRPs)
sequenceDiagram participant App participant OS participant Driver participant Device App->>OS: Read request OS->>Driver: Create IRP Driver->>Device: Hardware command Device->>Driver: Data Driver->>OS: Complete IRP OS->>App: Return data
IOCTL (Input/Output Control)
// User space
int fd = open("/dev/mydevice", O_RDWR);
int value = 42;
ioctl(fd, SET_VALUE, &value);
close(fd);

// Kernel space driver
static long device_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    switch(cmd) {
        case SET_VALUE:
            copy_from_user(&device_value, (int*)arg, sizeof(int));
            break;
        case GET_VALUE:
            copy_to_user((int*)arg, &device_value, sizeof(int));
            break;
    }
    return 0;
}

Common Driver Operations

Operation Purpose Example
Open Initialize device Allocate resources
Read Get data from device Read sensor value
Write Send data to device Control LED
IOCTL Device-specific control Set baud rate
Close Release device Free resources

Interrupt Handling

// Register interrupt handler
request_irq(IRQ_NUMBER, my_interrupt_handler, IRQF_SHARED, "mydevice", dev_id);

// Interrupt Service Routine (ISR)
static irqreturn_t my_interrupt_handler(int irq, void *dev_id)
{
    // Read device status
    status = read_device_register(STATUS_REG);
    
    // Handle interrupt
    if (status & DATA_READY) {
        data = read_device_register(DATA_REG);
        // Process data
    }
    
    // Clear interrupt
    write_device_register(STATUS_REG, CLEAR_INT);
    
    return IRQ_HANDLED;
}

Driver Development Process

graph LR Design[Design Driver] --> Write[Write Code] Write --> Compile[Compile] Compile --> Load[Load Driver] Load --> Test[Test] Test --> Debug{Works?} Debug -->|No| Write Debug -->|Yes| Deploy[Deploy]

Real-World Examples

Graphics Driver
Network Driver
USB Driver

Driver Signing

Modern operating systems require drivers to be digitally signed to ensure security and stability.

Best Practices