Device Numbers in Linux PCI Device Drivers : Practice MCQ Questions & Answers with Explanations

The Linux kernel uses the major number to identify and dispatch requests to the correct device driver. When a process accesses a device file, the kernel looks up the major number in its driver table (cdev_map for character devices) to find the registered driver and invoke the appropriate file operations.

In Linux kernel 2.4, minor numbers were used to represent the sub-functionalities of a driver. For example, a single driver for a multi-partition disk would use different minor numbers to represent each partition. The major number identifies the driver, while the minor number is interpreted by the driver itself to determine which device or sub-function is being accessed.

In Linux kernel 2.4, major numbers are 8-bit values (0–255), allowing up to 256 major numbers. Since character and block devices have separate major number spaces, it is possible to have 256 character drivers and 256 block drivers simultaneously. In kernel 2.6+, the dev_t type was expanded to 32 bits, greatly increasing the number of supported devices.

Both statements in option C are false: (1) Dynamic allocation of major numbers IS possible in Linux using alloc_chrdev_region() — the kernel assigns a free major number at runtime. (2) Major numbers CAN be shared among drivers in some cases (e.g., multiple drivers using the same major with different minor numbers). Therefore, option C (which combines two false statements) is the correct answer to 'which is NOT true'.

In Linux (kernel 2.4 and earlier), the minor number range is 0 to 255, giving 256 possible minor numbers per major number. In kernel 2.6 and later, the dev_t type was expanded to support a much larger range (up to 2^20 minor numbers), but the traditional range is 0 to 255.

In Linux, the major number identifies the driver associated with a device. When the kernel receives a request for a device file, it uses the major number to dispatch the request to the appropriate driver. The minor number, on the other hand, is used by the driver internally to distinguish individual devices or sub-functionalities it controls.