Linux Device Drivers, Second Edition

The mmap Device Operation

Memory mapping is one of the most interesting features of modern Unix systems. As far as drivers are concerned, memory mapping can be used to provide user programs with direct access to device memory.

A definitive example of mmap usage can be seen by looking at a subset of the virtual memory areas for the X Window System server:

cat /proc/731/maps
08048000-08327000 r-xp 00000000 08:01 55505    /usr/X11R6/bin/XF86_SVGA
08327000-08369000 rw-p 002de000 08:01 55505    /usr/X11R6/bin/XF86_SVGA
40015000-40019000 rw-s fe2fc000 08:01 10778    /dev/mem
40131000-40141000 rw-s 000a0000 08:01 10778    /dev/mem
40141000-40941000 rw-s f4000000 08:01 10778    /dev/mem
     ...

The full list of the X server’s VMAs is lengthy, but most of the entries are not of interest here. We do see, however, three separate mappings of /dev/mem, which give some insight into how the X server works with the video card. The first mapping shows a 16 KB region mapped at fe2fc000. This address is far above the highest RAM address on the system; it is, instead, a region of memory on a PCI peripheral (the video card). It will be a control region for that card. The middle mapping is at a0000, which is the standard location for video RAM in the 640 KB ISA hole. The last /dev/mem mapping is a rather larger one at f4000000 and is the video memory itself. These regions can also be seen in /proc/iomem:

000a0000-000bffff : Video RAM area f4000000-f4ffffff : Matrox Graphics, Inc. MGA G200 AGP fe2fc000-fe2fffff : ...