All, my mkfs.c $Revision: 1.17 $, $Date: 2008/05/04 14:20:12 $ now produces mountable filesystem images, and it's now checked into the svn repository. The problem was that the size of a directory is not the size of the blocks allocated to it, but is in fact the number of bytes of the in-use directory entries. For example, / was being allocated 1 block == 512 bytes. But 512 is not its size (as it would be in later UNIXes); in fact if / has entries 41 sdrwr- 7 root 70 Jan 1 00:00:00 . 41 sdrwr- 7 root 70 Jan 1 00:00:00 .. 43 sdrwr- 2 root 570 Jan 1 00:00:00 bin 42 sdrwr- 2 root 250 Jan 1 00:00:00 dev 99 sdrwr- 2 root 100 Jan 1 00:00:00 etc 108 sdrwr- 2 root 70 Jan 1 00:00:00 tmp 114 sdrwr- 2 root 20 Jan 1 00:00:00 usr then its size is 7 entries * 10 bytes each = 70 bytes. Importantly, this also means that we can now make bootable root filesystems without having to do a cold UNIX: % tools/mkfs /usr/local/src/V1 rf0.dsk rf # Make the / disk % tools/mkfs /usr/local/src/V1 rk0.dsk rk # Make the /usr disk, same stuff % ./simh.cfg PDP-11 simulator V3.7-3 ./simh.cfg> #!tools/pdp11 Unknown command Disabling CR Disabling XQ RF: buffering file in memory TC0: 16b format, buffering file in memory sim> g :login: root root # /tmp/a.out /dev/rk0# ls -l /usr/tmp total 8 112 sxrwrw 1 root 156 Jan 1 00:00:00 a.out 110 sxrwrw 1 root 1664 Jan 1 00:00:00 etma 113 sxrwrw 1 root 6 Jan 1 00:00:00 hello 109 sxrwrw 1 root 26 Jan 1 00:00:00 ttmp 111 sxrwrw 1 root 142 Jan 1 00:00:00 utmp # df 806+4602 # Number of free blocks on / and /usr I added /usr/tmp/hello just to verify that I was getting files that were not on the cold UNIX tape. Cheers, Warren