PDP-11 stories (2)

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Bob Eager

Sep 17, 2020, 4:59:35 AM9/17/20
I thought I do another bit of reminiscing.

In my first yera as a CS postgrad, I had access to a couple of teaching
PDP-11s, one in particular. It ran DOS-11, which was slow but very good
on memory use; resident about 2kW as I remember, with a large core image
library (a .CIL file) containing numerous overlays. These were quite
small, and it took four overlays to run a program!

We hit one of the more stupid DEC decisions. They had decoded to store
file dates in 12 bit, with three spare bits in the directory entry (I
think they later expended it to 15). The last bit was used to indicate a
contiguous file (like a .CIL). Worse, they stored dates as
(year-1970)*1000 plus the day-in-year. So this ran out in early April
1974. We patched DOS (we had a badly photocopied source listing) to bias
all dates by two years, to give us a 'window'. We moved the window every

I also remember getting an RK05 disk (previously we had RF/RS). WE had
some weird behaviour, and after a lot of investigation I found something
like an arithmetic right shift where there should have been a logical
one, inside the disk driver. In general, the quality of DOS-11 was not
great rigth then. It improved later, no doubt because one of the major
players was a clever fellow postgrad of mine, who went there after he
left university.

DOS really wasn't all that nice, and it was slow due to all the overlays.
A friend have me a bootleg machine readable copy of the RT-11 source, for
interest. I ended up building and installing that. This was rather
involved; I used the DOS assembler and linker, and then wrote a program
to convert object formats. Also others to format an empty DECtape, put
RT-11 on it (including a boot block), and copy in all the files. It
worked (booting from DECtape), and then I built a disk version.

Unfortunately I had to put DOS back on it for others to use, but I made a
copy. Remember the standalone programs ROLLIN and ROLLOUT? I could save a
disk image on multiple DECtapes and put it back whenever I wanted.

I've recently done a similar thing. I am restoring a non-DEC operating
system. I have incomplete sources and no compiler/assembler. I've set up
cross compilation, and wrote many of the same tools that I used 45 years
ago. That system now boots and runs. I have ported a better editor to it.
It currently runs on an emulator, but it'll go on my PiDP-11 soon.

More later!
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