de - A Minix Disk Editor

	      Terrence W. Holm, Jan. 1989


INTRODUCTION

    The de(1) disk editor allows a system administrator to
    look at and modify a Minix file system device. Commands
    allow movement throughout a file system device, displaying
    information in a couple of formats, writing blocks from
    the device onto another file, and rewriting words on the
    disk.

    A few changes to the Minix file system aid recovering files.
    I-node numbers are retained in directory entries now (they
    get moved to the end). And all the i-node information is not
    zeroed-out when a file is unlinked. So, after a file is
    accidently rm(1)'ed, you can find the old i-node, and then
    manually (or automatically) go to each of the freed blocks
    and write them to a new file.


USES FOR THE DISK EDITOR

    1)	EDUCATION. Students can look at a file system in
        a painless manner. For example you don't have to
	use od(1) to look at the zone numbers in i-nodes.

	A simple assignment is to change the size of an un-mounted
	floppy disk file system from 360 to 300 blocks. (A more
	difficult assignment is to explain why this works, even
	though fsck(1) and df(1) do not report the correct number
	of free blocks. :-)

    2)  ADMINISTRATION. You can visually check inconsistencies
	reported by fsck(1) before letting fsck(1) fix them.
	You can change any word on the disk, this greatly simplifies
	editing file system information. For example, changing the
	size of a block special device is actually fun, no more
	"blind" writing to your partitions.

	Bit maps can be displayed with 2048 "bits" per screen,
	(on the IBM/PC console), see how your zones are allocated!


INSTALLATION

	- Install de.1 in /usr/man/cat1.

	- Install the files: Makefile, README, de.h, de.c, de_stdin.c,
	  de_stdout.c, de_diskio.c and de_recover.c in commands/de.
	  Add -F and -T. to the Makefile, if necessary.

	- "make" de(1). If a header file is not found, don't worry:
	  You probably have it somewhere, just link it to what de(1)
	  is looking for. This program also requires the subroutine
	  tolower(3), see EFTH MINIX report #50, if you don't have it.


USING DE(1) FOR THE FIRST TIME

    De(1) starts up in "word" mode at block 0 of the specified
    device. Hit the PGDN (or space bar) a few times, observing
    all the information on the screen. Each PGUP/PGDN moves to
    the next 1024 byte block, (de(1) only knows about 1 block per
    zone file systems). Note that "word" mode only displays 32
    bytes at a time, so you are only observing the first 32 bytes
    in the first few blocks when you skip using PGDN.

    Now go back to block 3, (zone bit map), using "g 3 ENTER".
    Change to "map" mode "v m", and then use the down arrow key
    to check each 2 Megs in the zone bit map.

    Now change to "block" mode using "v b". And go to some data
    block, eg. "g 1000 ENTER". Use PGUP/PGDN to see what data
    is in each nearby block.

    Remember 'h' gives you a help page.

    Try some more commands, for example: 'END', 'I', '/'.
    (Note: searching through a whole disk under Minix takes a
    long time: 30-60 seconds per megabyte, depending on your
    machine, drive and controller, [Minix is embarrassingly slow].)

    Don't worry about looking at a mounted device, you must specify
    the "-w" option before the 's' command is operational, and
    this command is the only one which will try to modify the
    contents of the device.


MINIX-ST

    Please contact me if you are interesting in attempting a port
    to MINIX-ST.


MINIX-vmd

    After repairing too many type errors this program runs under Minix-vmd,
    but file recovery is forcefully removed, because a on a real system deleted
    blocks are reused before you know it.  (Proper support for undelete is to
    be added, alas I don't care enough to do it soon myself.)  By the way, the
    search times above should read: "1-2 seconds per megabyte" nowadays.
    					Kees J. Bot  (kjb@cs.vu.nl)


$PchId: README,v 1.3 1995/11/27 22:17:36 philip Exp $