On 2015-03-16 20:43, Clem Cole wrote: On Mon, Mar 16, 2015 at 3:13 PM, Johnny Billquist <bqt at softjar.se <mailto:bqt at softjar.se>> wrote: And one of the tricks were to reduce disk seek times during a backup. And that is done by actually not backing up files in the logical order of the blocks in the file, but in the order they appear on disk, which is why I said the format is slightly tricky. That was a common trade off - the reason being that backup was done way more often then restore. But the problem was that the a lot of the those formats had a big issues when you had a tape error (UNIX tp and the PDP-10 dumper formats were notorious for this problem). Tape errors gives you problems no matter which way you cut it. The BRU format allows you to deal with the rest of the file/tape properly anyway. But as you say a threaded format like that, you don't need a PhD to figure it out. You just need to be careful and recognize there is a bunch of housekeeping needed when you pull things back in. Today its a little easier, but on an 11/40 class (shared I/D space) that would be even more difficult, since you don't have a lot of room in active memory to keep those tables. Nitpick: The 11/40 "class" machines did not have split I/D space. Split I/D space was the 11/45,11/50,11/55 and 11/70, before we move into "modern" PDP-11s... The 11/40 is really a primitive model, where most things you even could have was optional. The basic CPU do not have an MMU, no EIS, no stack limit, not to mention that the 11/40 never even had the capability of an FPP. It had the FIS instead... (And no I/D space, nor supervisor mode, nor 22-bit addressing.) Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol