FreeBSD 7.0 manual page repository

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mem, kmem - memory files

 

NAME

      mem, kmem - memory files
 

SYNOPSIS

      device mem
 

DESCRIPTION

      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 cur‐
      rently 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.
      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
 
      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 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 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 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 struc‐
      tures, 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.
      [EOPNOTSUPP]       Memory range operations are not supported on this
                         architecture.
 
      [ENXIO]            No memory range descriptors are available (e.g.
                         firmware has not enabled any).
 
      [EINVAL]           The memory range supplied as an argument is invalid or
                         overlaps another range in a fashion not supported by
                         this architecture.
 
      [EBUSY]            An attempt to remove or update a range failed because
                         the range is busy.
 
      [ENOSPC]           An attempt to create a new range failed due to a
                         shortage of hardware resources (e.g. descriptor
                         slots).
 
      [ENOENT]           An attempt to remove a range failed because no range
                         matches the descriptor base/length supplied.
 
      [EPERM]            An attempt to remove a range failed because the range
                         is permanently enabled.
 

FILES

      /dev/mem
      /dev/kmem
      kvm(3), memcontrol(8)
 

HISTORY

      The mem and kmem files appeared in Version 6 AT&T UNIX.  The ioctl inter‐
      face for memory range attributes was added in FreeBSD 3.2.
 

BUGS

      Busy range attributes are not yet managed correctly.
 
      This device is required for all users of kvm(3) to operate.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.