ram -- Device Driver

Driver for manipulating RAM

The COHERENT ram devices let  you allocate and use the random-access memory
(RAM) of  the computer system directly.   A typical use is  for a RAM disk,
which is a COHERENT file system kept in memory rather than on a floppy disk
or hard disk.

The  COHERENT RAM  device driver  has major  number 8.   You can  access it
either as  a block-special  device or  as a character-special  device.  The
high-order bit of the minor number gives the RAM device number (0 or 1); as
you can see, you can have no more than two RAM devices in memory at any one
time.   The low-order  seven  bits give  the device's  size in  64-kilobyte
chunks.

The  first call to  open() on  a RAM  device with nonzero  size (1  to 127)
allocates memory  for the device; open() fails if  sufficient memory is not
available.  Accessing a RAM device  with a minor number that specifies size
zero frees  the allocated  memory, provided  all earlier open()  calls have
been closed.

Initially, COHERENT  includes two block-special devices  for RAM disks: the
512-kilobyte device /dev/ram0  (8, 8) and the 192-kilobyte device /dev/ram1
(8,  131).   It  also  includes  the  devices  /dev/ram0close  (8,  0)  and
/dev/ram1close (8,  128).  You  should resize the  RAM devices to  suit the
amount of memory available on your system.

Examples

The  following  example  formats and  mounts  a  512-kilobyte  RAM disk  on
directory /fast.

    mkdir /fast
    /etc/mkfs /dev/ram0 1024
    /etc/mount /dev/ram0 /fast

When the RAM disk is no longer needed, its allocated memory can be freed as
follows:

    /etc/umount /dev/ram0
    cat /dev/null >/dev/rram0close

The next example replaces  the default /dev/ram0 with a one-megabyte device
that contains  a COHERENT file  system.  The minor number  16 specifies RAM
device 0 and a size of one megabyte (i.e., 16 chunks of 64 kilobytes each).
The new RAM device contains 2,048 blocks of 512 bytes each.

    rm /dev/ram0
    /etc/mknod /dev/ram0 b 8 16
    /etc/mknod /dev/rram0 c 8 16
    /etc/mkfs /dev/ram0 2048
    chmod ugo=rw /dev/ram0
    chmod ugo=rw /dev/rram0

The command chmod is necessary to make the new RAM drive accessible.

Files

/dev/ram*

See Also

compress,
device drivers,
fsck,
mkfs,
mount,
ramdisk,
umount,
uncompress,
zcat

Notes

Moving frequently used  commands or files to a RAM  disk can improve system
performance substantially.  However, the  contents of a RAM device are lost
if  the system  loses power,  reboots, or  crashes.  Therefore,  you should
frequently back up files from the RAM disk to a more permanent medium.

If a RAM  device uses most but not all  available system memory, its open()
call  will succeed  but subsequent commands  may fail  because insufficient
memory remains for the system.

The COHERENT installation program /etc/build uses RAM device /dev/ram1 as a
RAM  disk during  installation.  Commands  compress, uncompress,  zcat, and
fsck sometimes  use /dev/ram1 as a temporary  storage device.  Users should
avoid  using  /dev/ram1  as a  RAM  disk  because  of  these programs.   In
addition, users  of compress, uncompress,  and zcat may have  to change the
size of /dev/ram1 from the default  size of 192 to 512 kilobytes, to handle
files compressed to 16 bits.   The following script makes this change; note
that it must be run by the superuser root:

    cat /dev/null >/dev/rram1close
    rm /dev/ram1 /dev/rram1
    mknod /dev/ram1  b 8 136
    mknod /dev/rram1 c 8 136

© 2012 by Stephen A. Ness