So I seem to have stumbled on something interesting here (or maybe it's not, and the history is just unclear - well, unclear to me at least, I'm sure someone knows). Looking at "Unix Program Description" (January 1976), it's clearly marked as "Published by the UNIX Support Group". (I have an actual hardcopy, which I don't recall how I came by, but for those who wish to follow along this document is available in the TUHS archive, at: http://www.tuhs.org/Archive/Distributions/USDL/unix_program_description_jan… and in other TUHS mirrors). So, given the credit, I _assume_ that it documents some version of the USG system. So I started looking at that, and the PWB version that's in the archive: http://www.tuhs.org/Archive/Distributions/USDL/spencer_pwb.tar.gz to see how they compare, and it turns out (somewhat to my surprise) that the USG document describes what seems to be an older version of the system. For example, in text.c, it doesn't cover xlock()/xunlock()/xexpand(), all in the PWB system - just xalloc()/xccdec()/xfree()/xswap(). Even more telling, in sys1.c, the USG document describes the older version of exec(), where arguments are collected in a disk buffer, not (as in the PWB system) in swap space. (I had thought that this change was mentioned in the PWB paper in the BSTJ issue, but on checking, it appears my memory was incorrect. Many of the PWB changes appear to be to things like the shell, not the OS.) So it seems the USG document describes a system very close to the 'classic' V6 - not what I had expected. This may also make it hard to recognize USG source (at least, the early versions). More generally, it would be good to try and elucidate the relationship among all these early Bell/AT+T versions: Research, USG, PWB, etc. Clearly the two latter (from what we know now) are descended from V6 - but was there any interchange between USG and PWB? And did either of them feed back into V7? Or, perhaps more likely, were the improvements to text.c, exec() etc _Research_ improvements that got fed into PWB? More questions than answers, sadly... I'm not at all familiar with V7, I'll have to go read it at some point, and compare it to PWB. Not that I expect it will answer many (any?) of these questions, but maybe we'll get lucky and there will e.g. be stuff in this PWB which isn't in V7. Speaking of which, I seem to recall there's more than one PWB version. I wonder which one we have (above). Although there's another 'PWB' tape in the archive: http://www.tuhs.org/Archive/Distributions/USDL/bostic_pwb.tar.gz (much larger than the other one), when I poked around a bit through that, seeing what's there, and comparing it to the other one, the system sources I looked at there all seemed to be the same as the one on the Spencer tape. So, my memory was in error here; the text.c matches the one from the PWB tape, _not_ the USG document. In general, the parts of the MIT system seem to be a close match to what's on the PWB tape, with the exception that the MIT one seems to be slightly earlier (no 'register' argument types). Without a better understanding of what was really in /TS, this is totally opaque. And for once, I think I was right. The MIT system _does_ closely match the one on the 'PWB' tapes - whatever that was! Noel