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 /dev/mem
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 0xf0000000.
Several architectures allow attributes to be associated with ranges of physical
memory. These attributes can be manipulated via ioctl
calls performed on /dev/mem
. Declarations and data types are
to be found in <sys/memrange.h>
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
- 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 mem_range_desc
u_int64_t mr_base; /∗ physical base address ∗/
u_int64_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
- The region is currently active.
- We believe the region to be invalid or otherwise
- The region cannot be disabled.
- The region is currently owned by another process and may
not be altered.
Operations are performed using struct mem_range_op
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 mo_desc
will be filled with a
corresponding number of descriptor structures, or the maximum, whichever is
ioctl is used to add, alter and remove
memory range attributes. A range with the
flag may not be removed; a range with
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
- Memory range operations are not supported on this
- No memory range descriptors are available (e.g. firmware
has not enabled any).
- The memory range supplied as an argument is invalid or
overlaps another range in a fashion not supported by this
- An attempt to remove or update a range failed because the
range is busy.
- 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
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 kvm(3)