> The data section must be nearly out of space.
A second, less noticeable, but astonishing peculiarity is
the space allocation: temporary storage is allocated that
deliberately overwrites the beginning of the program,
smashing its initialization code to save space.
PDP-11 Unix did eventually support overlays, but I don't think they were widely used. The ‘Unix way’ would be to have (as this C compiler does) separate programs run consecutively with intermediate state in temporary files.
And this is "a poor man's" overlay of the data buffer over the piece of the code, specified explicitly and manually at the time of writing the program (the "bigger" overlay mechanisms I know of are also specified at the time of writing the program, not at the time of build or link). As the "scratch" area was overlaid, nothing has to be loaded, that's the only difference to the typical overlay where more often some code would also be overwritten over some other code. The other difference to the "classical," "bigger" overlay is that this was done exactly once over the life of the program.
It's creating space in the code section right after ospace. If you look at ospace, it's being used as a buffer. So we have a buffer in the code section instead of the data section. So the question is why? Remember that this is a PDP-11/20 and the maximum memory is only 56 KB (maybe the one at AT&T had less). The data section must be nearly out of space.
There is more: notice that the variables are always allocated at the end of each file, and that "extern" is being used within each function as a forward reference. I think the purpose of this is to keep the symbol table usage within the compiler low- at the end of each function you get the symbol table space used by the externs back. The only symbols which remain are the function names themselves until the end of the file.
I never used UNIX on a PDP-11 (except simh), but I did use DEC's RSX-11 (on an 11/34). In that operating system a lot of effort was dedicated toward making a good overlay linker. Overlays were in the form of a tree: You had to carefully structure your code to maximize the efficiency of this, so that the most commonly referenced things were closer to root. UNIX didn't have any of this..