For certain devices, such as frame buffers, it is more efficient for application programs to have direct access to device memory. Applications can map device memory into their address spaces using the mmap(2) system call. To support memory mapping, device drivers implement segmap(9E) and devmap(9E) entry points. For information on devmap(9E), see Chapter 11, Mapping Device or Kernel Memory. For information on segmap(9E), see Chapter 9, Drivers for Character Devices.
Drivers that define an devmap(9E) entry point usually do not define read(9E) and write(9E) entry points, as application programs perform I/O directly to the devices after calling mmap(2).