---
name: "mem"
section: "4freebsd"
platform: "linux"
package: "freebsd-manpages"
version: "12.2-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/kmem/4freebsd"
---

# mem(4freebsd)

memory files

Part of [kmem(4freebsd)](/kmem/4freebsd.md).

## 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 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 ⟨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 *struct* *mem\_range\_desc*:

```
uint64_t	mr_base;	/∗ physical base address ∗/
uint64_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 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

- \[EOPNOTSUPP\]

  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*

  ## SEE ALSO

[kvm(3)](/kvm/3.md), [memcontrol(8)](/memcontrol/8.md)

## HISTORY

The **mem** and **kmem** files appeared in Version 6 AT&T UNIX. The ioctl interface 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)](/kvm/3.md) to operate.