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From: jzamora@avellano.datsi.fi.upm.es (Juan Zamorano Flores)
Subject: Re: Timing Ada programs using the DEC UNIX microtimer kernel option
Date: 1998/04/27
Message-ID: <6i27ap$m85$1@panoramix.fi.upm.es>#1/1
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Distribution: world
References: <6hsab5$rh1$1@eplet.mira.net.au>
Organization: Departamento de Arquitectura. Facultad de Informatica.
 Universidad Politecnica de Madrid.
Newsgroups: comp.unix.osf.osf1,comp.sys.dec,comp.lang.ada
Date: 1998-04-27T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <6hsab5$rh1$1@eplet.mira.net.au>, dccoote@werple.mira.net.au (David Coote) writes:
|> 
|> "As for micro-timer resolution - the clock resolution remains the same - 
|> 1/1024; however, any call to clock_gettime(3) is guaranteed to be 
|> monotonically increasing with a granularity of 1 microsecond. (i.e. the clock 
|> is still ticking at 1/1024, but clock_gettime(3) calls return values that are 
|> unique and APPEAR [DEC's emphasis] to have 1 microsecond resolution. This is 
|> useful for critical timestamping.)"
|> 
|> Well, how do they do this monotonic increase? If the appropriate parts of the 
|> kernel are getting tickled every 1024Hz, how is the kernel returning finer 
|> resolution between ticks?
|> 
|> Anyone know anything about this?
|> 
|> David
|> 

     The hardware timer interrupts at 1024Hz. So the kernel has a time granularity
of 1/1024Hz. The clock_gettime call could use the hardware timer downcount
register to improve the resolution.

     I don't know DEC UNIX but in DOS the hardware timer interrupts at 18.2 Hz.
But you can have microseconds resolution if you read the hardware timer downcount
register.

      Juan