20 Nov
2024
20 Nov
'24
10:27 p.m.
On Nov 17, 2024, at 6:59 PM, Thomas DeBellis
<tommytimesharing(a)gmail.com> wrote:
I would point out that sometimes things just 'happen' and it is difficult to
anticipate how long something is going to be used or even if that ever occurred to anyone.
So sometimes what got picked was very often 'good' for machines of limited
memory and CPU of the time.
One example that comes to my mind is the date format used on DECtapes before 1975, which
had a representation ((Y-1964)*12.+M-1)*31.+D-1, where "Y" is the year,
"M" is the month (1-12), and "D" is the day of the month (1-31).
However, the field itself used to store the date was 12 bits. Why 12 bits? One assumes
PDP-8, but I really can't recall why.
Yet it ran out in 1975 and had to be widened to 15 bits. Where to find the space? In
short, the 3 bits got scrounged up.
I remember that because we encountered that issue in college, and I found the cause and
created a patch that I then sent to DEC.
It isn't because of the PDP-8. The reason is that in DOS-11, where that format
originates, the sign bit meant "contiguous file" and the next 3 bits were for
the open count. At least that's what the documentation said, I don't know if DOS
actually used that. RSTS did not so it seemed safe to grab those 3 bits. Making it 16
bits was considered not so safe because that might make DOS think the file is contiguous,
which would make it read (or write!) the wrong data.
paul