[HECnet] How long has your 20 been up?

[Thomas DeBellis]

Greenwich Mean Daylight Savings Time?  I don't recall that I knew that; 
I had always thought of GMT being meridian based and other times being 
based off that, but GMT DST really would change the equation, 
particularly for those areas that don't do DST.

So UTC makes sense, although I do confess that I did chuckle over frog 
fulmination.  I spent some time in Italy studying for my second 
bachelors and while most Frenchmen struck me as straight stand up folk, 
there were the /others/...  My brother and I were in an overnight train 
from Naples to Trieste and had two Australians in the sleeper with us 
(it held six).  We were having a grand old time laughing and yacking 
until who knows how late when the Parisian simply reached up and turned 
out the light.  And so we went to bed...

I did the maintenance for the DN60 code, which was PDP-11 assembler, so 
I ought to know better, but I guess I have just gotten too large word 
width centric (36 or 64 bits).  You have enough bits so you just about 
never need to get clever for OS and relating systems code.  However, one 
of my favorite courses at WPI was Numerical Methods (we called it 
"Nums"), so I pay attention to fractions.

The carry on the PDP-11 is another thing I forgot about.  The Microsoft 
C library of the late 1980's handled 32 bit numbers as having the low 
order 16 bits in the accumulator (AX) and the high order in the data 
register (DX). ADD AX,/foo/ and ADC DX, 0 was an extremely common 
sequence and unremarkable sequence in 8086/80286 code.

What surprised me was that INC (add one) didn't set carry, so you 
couldn't do an INC AX - ADC DX, 0 sequence which would have been 
faster.  I never did figure out how that was great.

It's probably silly, but one of the things I really liked about PDP-11 
assembler were unnamed forward and backward relative jumps (I.E., $:).  
Of course, you could really get yourself into trouble when you started 
numbering them in hairy code.  At least I did.

> ------------------------------------------------------------------------
> On 1/20/22 4:04 PM, Johnny Billquist wrote:
>> ------------------------------------------------------------------------
>> On 2022-01-20 19:57, Paul Koning wrote:
>>
>>> On Jan 20, 2022, at 12:20 PM, Peter Lothberg <roll at stupi.com> wrote:
>>>
>>> GMT is solar time! (and not used...)
>>
>> Not quite.  The acronym actually says that it is not solar time, it's 
>> Greenwich MEAN Time.  UTC is merely the new name for GMT, obviously 
>> adopted to keep the French from fulminating.  (It's amusing that, 
>> just like "ISO", UTC isn't an acronym; it matches neither the English 
>> nor the French initials.)
> Well, again not quite. One big reason what GMT was replaced with UTC 
> is because the sortof ambiguity of GMT. What is GMT? Is that the time 
> it is right now at Greenwich, or the time we think it should be in 
> Greenwich if we didn't have daylight saving, or if any political 
> decision would change the time at Greenwich. Basically, having a 
> physical place defining normal time is problematic, because that might 
> lead you to want to check what the time actually is at that place, 
> which might not be what you would expect.
>
> The classical one being that Greenwich is at 1am in the summer, when 
> it is 0:00 GMT.
>
> UTC takes any such confusion away.
>> On Thomas DeBellis's observation that system programmers don't 
>> understand long arithmetic, that's obviously not true for PDP-11 
>> programmers.  (You can't deal with non-tiny disks if you don't 
>> understand this.)
>
> Very true. :-)
> Made even more painful because the PDP-11 don't have a proper add with 
> carry. (No, the ADC instruction isn't really a good alternative.)
>
>   Johnny
>
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