ramdisk man page on Xenix

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     RAMDISK(HW)	      XENIX System V		   RAMDISK(HW)

     Name
	  ramdisk - Memory block device

     Description
	  The ramdisk device driver provides a block interface to
	  memory. A ramdisk can be used like any other block device,
	  including making it into a file systems using mkfs(C).
	  There are eight ramdisks available.

	  The characteristics of a ramdisk file are determined by its
	  minor device number.	The bits in the minor device number
	  encode its size, longevity, and which of the eight possible
	  ramdisks it is.

	  The three low-order bits of the minor device number
	  determine which of the eight ramdisks is being accessed.

	  The next four bits of the minor device number determine the
	  size of the ramdisk.	The size of a ramdisk must be a power
	  of 2, and must be at least 16K.  Since 4 bits are available,
	  there are 16 possible sizes, starting at 16K and doubling
	  every time the size indicator is incremented, to produce
	  possible sizes of 16K, 32K, 64K, and up.

	  The high-order bit is a longevity indicator.	If set, memory
	  is permanently allocated to that ramdisk, and can be
	  deallocated only by rebooting the system.  Permanent
	  ramdisks can only be allocated by the superuser.  However,
	  once a permanent ramdisk is allocated (by opening it), it
	  can be read and written by anyone having the appropriate
	  permissions on the ramdisk inode.

	  If clear, the ramdisk is deallocated when no processes have
	  it open.  To create an easily removable, but semi-permanent
	  ramdisk, use a separate process to keep the device open for
	  as long as necessary.

     Page 1					      (printed 8/7/87)

     RAMDISK(HW)	      XENIX System V		   RAMDISK(HW)

	  Since a complete set of ramdisks (8) would consume 256
	  inodes, only one example 16K ramdisk (/dev/ram00) is created
	  when the system is installed.	 The system administrator can
	  use this example to determine the major device numbers for
	  any other required ramdisks.	The following table shows how
	  the minor device number is constructed:

	  _______________________________________________________________
	 |	     Example Minor Device Number Construction		|
	 |______________________________________________________________|
	 |   Description     Longe-	Size (see	 Ram	  Minor |
	 |		      vity     next table)    Disk No.	  Device|
	 |							  Number|
	 |________________|________|___|___|___|___|___|___|___|________|
	 | 16K (#1)	  |    0   |  0|  0|  0|  0|  0|  0|  1|     1	|
	 |  (Temporary)	  |	   |   |   |   |   |   |   |   |	|
	 | 16K (#1)	  |    1   |  0|  0|  0|  0|  0|  0|  1|   129	|
	 |  (Permanent)	  |	   |   |   |   |   |   |   |   |	|
	 | 64K (#0)	  |    0   |  0|  0|  1|  0|  0|  0|  0|    16	|
	 |  (Temporary)	  |	   |   |   |   |   |   |   |   |	|
	 | 512K (#7)	  |    1   |  0|  1|  0|  1|  1|  1|  1|   175	|
	 |  (Permanent)	  |	   |   |   |   |   |   |   |   |	|
	 |________________|________|___|___|___|___|___|___|___|________|

	  The contents of the size field and the corresponding ramdisk
	  size is shown in the next table.

			 ______________________________
			    Size Bits  |  Ramdisk Size
			 ______________________________
			  0   0	  0   0	       16K
			 ______________________________
			  0   0	  0   1	       32K
			 ______________________________
			  0   0	  1   0	       64K
			 ______________________________
			  0   0	  1   1	      128K
			 ______________________________
			  0   1	  0   0	      256K
			 ______________________________
			  0   1	  0   1	      512K
			 ______________________________
			  0   1	  1   0		1M
			 ______________________________
			  0   1	  1   1		2M
			 ______________________________
			  1   0	  0   0		4M
			 ______________________________

     Page 2					      (printed 8/7/87)

     RAMDISK(HW)	      XENIX System V		   RAMDISK(HW)

			  1   0	  0   1		8M
			 ______________________________
			  1   0	  1   0	       16M
			 ______________________________
			  1   0	  1   1	       32M
			 ______________________________
			  1   1	  0   0	       64M
			 ______________________________
			  1   1	  0   1	      128M
			 ______________________________
			  1   1	  1   0	      256M
			 ______________________________
			  1   1	  1   1	      512M
			 ______________________________
			|  |   |   |   |	      |
	  Once you determine the major and minor device numbers of a
	  new ramdisk, make|the|appropriate device node using
	  mknod(C).	|  |   |   |   |	      |
			|  |   |   |   |	      |
	  To make a file|system|on a non-permanent ramdisk, the device
	  file must be held|open between the mkfs and the mount(C)
	  operations.  Otherwise, the ramdisk is allocated at the
	  start of the mkfs|command, and deallocated at its end.  Once
	  the ramdisk is|mounted, it is|open until it is unmounted.
			|  |   |   |   |	      |
	  The following shell fragment shows one way to use mkfs on a
	  non-permanent 512K ramdisk, then mount it:  |
			|  |   |   |   |	      |
	       (     /etc/mkfs /dev/ram40 512	      |
		 /etc/mount|/dev/ram40 /dev/mnt	      |
	       ) > /dev/ram40  |   |   |	      |
			|  |   |   |   |	      |
     Notes		   |   |   |   |
	  ramdisks must occupy contiguous memory.  If free memory is
	  fragmented, opening a ramdisk may fail even though there is
	  enough total memory available.  Ideally, all ramdisks should
	  be allocated at system startup.  This helps prevent the
	  ramdisks themselves from fragmenting memory.

	  ramdisks are geared towards use in specialized applications.
	  In many cases, you will notice a decrease in system
	  performance when ramdisks are used, because  can generally
	  put the memory to better use elsewhere.

     Files
	  /dev/ram00

     See Also
	  mkfs(C), mount(C), mknod(C)

     Page 3					      (printed 8/7/87)

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