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

Richard B. Gilbert rgilbert88 at comcast.net
Wed Jul 19 08:34:22 UTC 2006


Terje Mathisen wrote:

> Richard B. Gilbert wrote:
> 
>> Ulrich Windl wrote:
>>
>>> Talking about 128bit time formats or so: With today's computing power 
>>> (just
>>> think of MP3 decoders), a switch to BCD coded time in 128 bit should be
>>> sufficient:
>>> Byte#: 1 2  3  4  5  6  7  8  910 111213 141516
>>> Value:2006-07-18 09:26:13.12 1234 567890 123456
>>>
>>> So you'd have significant sub-picosecond resolution and peace until year
>>> "9999".
>>>
>>> Regards,
>>> Ulrich
>>
>>
>> Do YOU want to write the code to manipulate that format?  It looks 
>> like a nightmare to me!
> 
> 
> Not at all! It is in effect identical to standard ISO format, plus the 
> fractional part, except for the two-to-one compression achieved by 
> storing in BCD instead of ASCII, but that particular conversion is of 
> course trivial.
> 
> BTW, I have written code to convert both ways, between ISO and Unix/NTP 
> seconds, and it is the seconds-to-ISO operations which is the tough one: 
> The naive code takes several hundred clock cycles to do this, while my 
> version did it close to an order of magnitude faster. :-)
> 
> For the ascii/bcd fractional part I have also invented a way to convert 
> a 32-bit unsigned number from binary to ascii (the reverse is easy of 
> course) in something like 30-50 clock cycles. AMD stole/borrowed this 
> code (or about 95% of it), without attribution, for their optimization 
> guide.
> 
> Terje
> 

I was thinking of the cost of doing all the arithmetic in BCD.  The NTP 
timestamps are exchanged between systems for the purpose of 
synchronizing to UTC; there is no advantage to exchanging them in BCD. 
They do not have to be "people readable"!




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