ram Device Driver ram
RAM device driver
The COHERENT ram devices allow the user to 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 diskette or hard disk.
The COHERENT RAM device driver has major number 8. It can be ac-
cessed either as a block-special device or as a character-special
device. The high-order bit of the minor number gives a RAM
device number (0 or 1), allowing the use of up to two RAM devices
simultaneously. The low-order seven bits specify the device size
in 64 KB (128 block) increments. The first open call on a RAM
device with nonzero size (1 to 127) allocates memory for the
device; the open call fails if sufficient memory is not avail-
able. Accessing a RAM device with a minor number specifying size
0 frees the allocated memory, provided all earlier open calls
have been closed.
Initially, COHERENT includes two RAM block devices, 512KB device
/dev/ram0 (8, 8) and 192KB device /dev/ram1 (8, 131). It also
includes /dev/ram0close (8, 0) and /dev/ram1close (8, 128). The
system administrator should change the RAM devices to sizes ap-
propriate for available system memory.
***** Note *****
The COHERENT installation program /etc/build uses RAM device
/dev/ram1 as a RAM disk during installation. Programs 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.
***** 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/ram0close
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ram Device Driver ram
The next example replaces the default /dev/ram0 with a one-
megabyte device containing a COHERENT file system. The new minor
number 16 specifies RAM device 0 and size 16 times 64 kilobytes
(i.e., one megabyte). The new RAM device contains 2,048 blocks
of 512 bytes each.
rm /dev/ram0
/etc/mknod /dev/ram0 b 8 16
/etc/mkfs /dev/ram0 2048
***** Files *****
/dev/ram*
***** See Also *****
compress, device drivers, fsck, mkfs, mount, umount, uncompress,
zcat
***** Notes *****
Moving frequently used commands or files to a RAM disk can im-
prove system performance substantially. However, the contents of
a RAM device are lost if the system loses power, reboots, or
crashes, so changes to files kept on a RAM disk should be stored
frequently to the hard disk or to diskette.
If a RAM device uses most but not all of available system memory,
its open call will succeed but subsequent commands may fail be-
cause insufficient memory remains for the system.
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