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Building tiny systems with embedded NetBSD (page 2)
Back to page one Creating the filesystem imageNow that our binaries are all wrapped up, the next thing to do is to populate the filesystem. This involves linking all the programs to the crunched binary and copying or creating the necessary data files. This is easily done with the following commands. Don't forget to set the permissions to root:wheel!$ mkdir files/bin files/sbin files/dev $ cp /dev/MAKEDEV files/dev $ cd files/dev $ ./MAKEDEV floppy ramdisk wscons $ cd ../.. $ cp work/mytiny files/sbin/init $ ln files/sbin/init files/bin/sh $ ln files/sbin/init files/bin/ls $ ln files/sbin/init files/sbin/reboot $ su root # chown -R root:wheel files # exitWith the filesystem populated, we can wrap it up into an image file that can be embedded into a kernel. This is done with the makefs command. This tool lets you take a directory and bundle it up into a single file. Make sure that your size is not larger than the size you specified in your kernel configuration. Once the image is created, link it to the kernel with the mdsetimage command. Your kernel is now ready to go! You can compress your kernel if you like. The standard NetBSD bootloader knows how to decompress gzipped kernels. Remember, your spartan OS includes 4 megabytes of mostly empty space in the filesystem. I compressed my kernel down to 825,544 bytes. # makefs -s 4m -t ffs crunch.image files # mdsetimage netbsd.ramdisk crunch.image # gzip -c netbsd.ramdisk > netbsd # ls -l netbsd -rw-r--rw- 1 brose users 825544 Aug 24 22:28 netbsd When you boot this kernel, you will see the normal output and then it will present you with a shell prompt. At that time, you can do whatever it is you need to do. My example will let you move around the file structure, list the contents with ls, and reboot. Not very useful, but it is a good starting point. The minimum multi user systemA multi user system is built in the same manner, but you need a few extra programs and data files. I also use the standard init program. Notice that I commented out the special init line, so that I use the stock init for the multiuser configuration. You may want to go back to your init source and make a fresh init, just in case the .o files are from your single user build. $ cd /usr/src/sbin/init $ make clean; makeHere's my crunchgen configuration. srcdirs /usr/src/bin /usr/src/sbin /usr/src/usr.bin /usr/src/usr.sbin /usr/src/libexec progs init mount newfs mount_ffs sh ttyflags getty pwd_mkdb passwd login reboot ls ln sh -sh ln newfs mount_mfs # special init objpaths /usr/src/sbin/init/init.smallprog.o # libraries used by the programs # ---------------- Minimum single user files # init : -lutil -lcrypt # mount : # newfs : -lutil # mount_ffs : # sh : -ll -ledit -ltermcap # ---------------- Minimum multiuser files # ttyflags : # getty : -lutil -ltermcap # pwd_mkdb : -lutil # passwd : -lrpcsvc -lcrypt -lutil -lkrb5 -lcrypto -lasn1 -lcom_err -lroken # login : -lutil -lcrypt -lskey -lkrb5 -lasn1 -lkrb -lcrypto -lroken -lcom_err # ---------------- Useful utilities # ls : # reboot : -lutil # umount : # libs -lutil -ll -ledit -ltermcap -lcrypt -lrpcsvc -lkrb5 -lkrb -lcrypto -lasn1 -lcom_err -lroken -lskeyAnd my shell commands for populating the filesystem. mkdir files/bin files/sbin files/usr files/etc files/var files/dev files/tmp files/root files/home mkdir files/usr/bin files/usr/sbin files/usr/libexec mkdir files/var/run files/var/db files/var/crash mkdir files/usr/share mkdir files/usr/share/misc cp /dev/MAKEDEV files/dev cd files/dev ./MAKEDEV floppy ramdisk wscons cd ../.. echo "/dev/md0a / ffs rw 1 1" > files/etc/fstab echo "echo Initializing system..." > files/etc/rc echo "export PATH=/sbin:/bin:/usr/sbin:/usr/bin" >> files/etc/rc echo "mount -ua" >> files/etc/rc echo "ttyflags -a" >> files/etc/rc cp work/mytiny files/sbin/init ln files/sbin/init files/bin/ls ln files/sbin/init files/sbin/mount ln files/sbin/init files/sbin/mount_ffs ln files/sbin/init files/sbin/mount_mfs ln files/sbin/init files/sbin/umount ln files/sbin/init files/bin/sh ln files/sbin/init files/usr/libexec/getty ln files/sbin/init files/sbin/ttyflags ln files/sbin/init files/sbin/pwd_mkdb ln files/sbin/init files/usr/bin/passwd ln files/sbin/init files/usr/bin/login ln files/sbin/init files/sbin/reboot ln files/sbin/init files/sbin/newfs cp /etc/ttys files/etc cp /etc/master.passwd files/etc cp /etc/pwd.db files/etc cp /etc/spwd.db files/etc cp /etc/passwd files/etc cp termcap.mini files/usr/share/misc/termcap cp /etc/gettytab files/etc The termcap.mini file is simply a hand trimmed version of the termcap file. This is used by the getty program when initializing the console. Since it is very large (over 500k) and most of it is useless, you can trim out the terminal types that you don't use. I trimmed mine down to about 8k. Once you have done this, simply place this filesystem into a kernel and boot. As you can see, a multi user setup is a bit larger. But most of the extra space is from incorporating the extra libraries for the login and getty programs. As you add more code to the system, your code growth should not be as dramatic.
# makefs -s 4m -t ffs crunch.image files # mdsetimage netbsd.ramdisk crunch.image # gzip -c netbsd.ramdisk > netbsd # ls -l netbsd -rw-r--rw- 1 brose users 1066057 Aug 24 22:28 netbsd
Brian Rose is an electrical engineer who has developed embedded software for the telecom and video distribution industries. He is currently on involuntary hiatus (layoff) and pondering the benefits of being a scuba instructor at Mexican resorts.
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Very Nice. - Ian Harding
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