Subject: Re: Grrr: Small delays
To: David S. Miller <>
From: Erik E. Fair <>
List: port-sun3
Date: 08/16/1995 04:16:16
The subject of good timing (or lack thereof) in the MacOS came up in a
discussion I had with some colleagues on Monday, and we all bemoaned the
fact that the clock hardware in all Macs just plain sucks (you may consider
yourself lucky if the TOD clock doesn't gain a minute or two per day).

However, one of them remembered that some precision timing could be derived
from the serial interface chips (after all, if you have to transmit those
bits serially at some specific baud rate, you'd better have your clocking
right), and that this had been used in an application which required fine
resolution interval time (don't recall just what). I did not inquire about
the exact technique, but if it's not obvious from the chip documents, I can
ask for more detail. Lucky us, the Macs and the Suns (as I understand it)
use the same on-board serial chips.

I know it's slightly perverse, but you take your useful hacks where you can
find 'em. The alternative suggested here (timing loops) is fraught with
peril - different cycle time DRAM SIMMs can bite you, as well as what the
state of the CPU cache is, etc., etc. Quite aside from doing an accurate
system identification (e.g. is this a 3/50? a 3/60? etc. - same chip,
different MHz) in order to set the basic timing loop values.

        Erik E. Fair