[ntp:questions] Re: NTP4 has 3 different time formats! Namly(32, 64, 128) bits wide

David L. Mills mills at udel.edu
Mon Jul 17 03:58:16 UTC 2006


Danny,

Careful what you say about the 704; I learned machine programming on it 
in 1959. It had not 8K, but 32K of 36-bit words divided into four fields 
in bigendian order, 3-bit opcode, 15-bit decrement (used in loop 
instructions) known by Lispers as CDR, 3-bit tag (index register select) 
and 15-bit address known by Lispers as CAR. It would be absolutely 
stupid to divide the word in two 18-bit fields and very expensive in 
programming. So far as I remember, the time was kept in two 36-bit 
words, but then that was specific to the U Michigan operating system. In 
that era there were lots of operating systems, all different and all 
incompatible.

In 1957 your reference might be the 1820 or even the 650, which was an 
absolutely stupid machine that used tables to do arithmetic add. The 
1820 I remember fell of the back of a truck and was destroyed. The last 
704 I remember was buried in the Negev desert because the Israelis 
wouldn't pay the rent beyond one year. The Universities got them for 
free, at least until the Feds told IBM they couldn't write them off for 
tax purposes.

The successor to the 704 was the 709, also using vacuum tubes, and that 
succeeded by the 709T (for transistorized), popularly called the 7090, 
and then the 7094. The 70x breed was dead by 1968, killed by the System/360.

Dave

Danny Mayer wrote:
> David J Taylor wrote:
> 
>>Danny Mayer wrote:
>>
>>>Richard B. Gilbert wrote:
>>>
>>>>NTP could do worse than to adopt the VMS 64 bit time format.  IIRC it
>>>>was a count of 100 nanosecond "ticks" since some date in (I think)
>>>>November 1857.
>>>
>>>18 November, 1857 to be exact! See, I still remember!
>>
>>I often wondered why that base was chosen - something to do with the 
>>Smithsonian?
>>
> 
> 
> Oops, I'm off by a year, it's 1858. From this URL if you want all of the
> details: http://vms.tuwien.ac.at/info/humour/vms-base-time-origin.txt
> 
> "So why 1858? The Julian Day 2,400,000 just happens to be November 17,
> 1858.
> [...]
> 
> The Modified Julian Day was adopted by the Smithsonian Astrophysical
> Observatory (SAO) in 1957 for satellite tracking. SAO started tracking
> satellites with an 8K (non-virtual) 36-bit IBM 704 computer in 1957,
> when Sputnik was launched. The Julian day was 2,435,839 on January 1,
> 1957. This is 11,225,377 in octal notation, which was too big to fit
> into an 18-bit field (half of its standard 36-bit word). And, with only
> 8K of memory, no one wanted to waste the 14 bits left over by keeping
> the Julian Day in its own 36-bit word. However, they also needed to
> track hours and minutes, for which 18 bits gave enough accuracy. So,
> they decided to keep the number of days in the left 18 bits and the
> hours and minutes in the right 18 bits of a word."
> 
> Danny
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