The special file /dev/mem
is an interface to
the physical memory of the computer. Byte offsets in this file are interpreted
as physical memory addresses. Reading and writing this file is equivalent to
reading and writing memory itself. Only offsets within the bounds of
Kernel virtual memory is accessed through the interface
in the same manner as
. Only kernel virtual addresses
that are currently mapped to memory are allowed.
On ISA the I/O memory space begins at physical address 0x000a0000 and runs to
0x00100000. The per-process data size for the current process is
long, and ends at virtual address
Several architectures allow attributes to be associated with ranges of physical
memory. These attributes can be manipulated via
() calls performed on
. Declarations and data types are
to be found in
The specific attributes, and number of programmable ranges may vary between
architectures. The full set of supported attributes is:
- The region is not cached.
- Writes to the region may be combined or performed out of order.
- Writes to the region are committed synchronously.
- Writes to the region are committed asynchronously.
- The region cannot be written to.
Memory ranges are described by struct
uint64_t mr_base; /∗ physical base address ∗/
uint64_t mr_len; /∗ physical length of region ∗/
int mr_flags; /∗ attributes of region ∗/
In addition to the region attributes listed above, the following flags may also
be set in the mr_flags
- The region's base address cannot be changed.
- The region's length cannot be changed.
- The region is believed to have been established by the system
- The region is currently active.
- We believe the region to be invalid or otherwise erroneous.
- The region cannot be disabled.
- The region is currently owned by another process and may not be
Operations are performed using struct
struct mem_range_desc *mo_desc;
ioctl is used to retrieve
current memory range attributes. If mo_arg
is set to 0, it will be updated with the total number of memory range
descriptors. If greater than 0, the array at
will be filled with a corresponding
number of descriptor structures, or the maximum, whichever is less.
ioctl is used to add, alter
and remove memory range attributes. A range with the
flag may not be removed; a
range with the
flag may not be
removed or updated.
should be set to
to update an existing
or establish a new range, or to
to remove a range.
- Memory range operations are not supported on this architecture.
- No memory range descriptors are available (e.g. firmware has not enabled
- The memory range supplied as an argument is invalid or overlaps another
range in a fashion not supported by this architecture.
- An attempt to remove or update a range failed because the range is
- An attempt to create a new range failed due to a shortage of hardware
resources (e.g. descriptor slots).
- An attempt to remove a range failed because no range matches the
descriptor base/length supplied.
- An attempt to remove a range failed because the range is permanently
files appeared in
Version 6 AT&T UNIX
. The ioctl interface
for memory range attributes was added in FreeBSD 3.2
Busy range attributes are not yet managed correctly.
This device is required for all users of