[HECnet] How long has your 20 been up?

[Tony Blews]

Do you mean BST? British Summer Time? GMT does not adjustand is the same 
as UTC (and I believe Zulu Time), but BST shifts an hour back and forth.

On 20/01/2022 21:42, Thomas DeBellis wrote:
>
> 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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