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
/dev/mem
are allowed.
Kernel virtual memory is accessed through the interface
/dev/kmem
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
UPAGES
long, and ends at virtual
address 0xf0000000.
IOCTL INTERFACE
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
In sys/memrange.h .
The specific attributes, and number of programmable ranges may vary between
architectures.
The full set of supported attributes is:
MDF_UNCACHEABLE
The region is not cached.
MDF_WRITECOMBINE
Writes to the region may be combined or performed out of order.
MDF_WRITETHROUGH
Writes to the region are committed synchronously.
MDF_WRITEBACK
Writes to the region are committed asynchronously.
MDF_WRITEPROTECT
The region cannot be written to.
Memory ranges are described by
Vt 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 */
char mr_owner[8];
In addition to the region attributes listed above, the following flags
may also be set in the
Fa mr_flags
field:
MDF_FIXBASE
The region's base address cannot be changed.
MDF_FIXLEN
The region's length cannot be changed.
MDF_FIRMWARE
The region is believed to have been established by the system firmware.
MDF_ACTIVE
The region is currently active.
MDF_BOGUS
We believe the region to be invalid or otherwise erroneous.
MDF_FIXACTIVE
The region cannot be disabled.
MDF_BUSY
The region is currently owned by another process and may not be
altered.
Operations are performed using
Fa struct mem_range_op :
struct mem_range_desc *mo_desc;
int mo_arg[2];
The
MEMRANGE_GET
ioctl is used to retrieve current memory range attributes.
If
mo_arg[0]
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 less.
The
MEMRANGE_SET
ioctl is used to add, alter and remove memory range attributes.
A range
with the
MDF_FIXACTIVE
flag may not be removed; a range with the
MDF_BUSY
flag may not be removed or updated.
mo_arg[0]
should be set to
MEMRANGE_SET_UPDATE
to update an existing or establish a new range, or to
MEMRANGE_SET_REMOVE
to remove a range.
RETURN VALUES
Bq Er EOPNOTSUPP
Memory range operations are not supported on this architecture.
Bq Er ENXIO
No memory range descriptors are available (e.g. firmware has not enabled
any).
Bq Er EINVAL
The memory range supplied as an argument is invalid or overlaps another
range in a fashion not supported by this architecture.
Bq Er EBUSY
An attempt to remove or update a range failed because the range is busy.
Bq Er ENOSPC
An attempt to create a new range failed due to a shortage of hardware
resources (e.g. descriptor slots).
Bq Er ENOENT
An attempt to remove a range failed because no range matches the descriptor
base/length supplied.
Bq Er EPERM
An attempt to remove a range failed because the range is permanently
enabled.
