Path: utzoo!attcan!uunet!husc6!rutgers!ucsd!ucbvax!dewey.soe.berkeley.edu!oster
From: oster@dewey.soe.berkeley.edu (David Phillip Oster)
Newsgroups: comp.sys.mac.programmer
Subject: Re: A few questions about Ticks
Message-ID: <25509@ucbvax.BERKELEY.EDU>
Date: 7 Aug 88 01:27:35 GMT
References: <3417@polya.Stanford.EDU> <460@poseidon.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster)
Organization: School of Education, UC-Berkeley
Lines: 35

In article <460@poseidon.UUCP> ech@poseidon.UUCP (Edward C Horvath) writes:
>> 2)  Why use the TickCount() trap when the Global variable Ticks is available?
>But it's just a copy of a real timer elsewhere.  Under A/UX, or any future Mac
>OS that memory-maps the low-memory globals, the TickCount trap can be
>supported by checking the hardware clock; but keeping the Ticks global
>up-to-date is fairly wastful.

This is a misunderstanding of what is really going on. The Mac _does_ have
a hardware timer, but it is implemented using a fairly slow LSI chip, so
it is rarely used. 

On system start-up, the Mac copies time information from the real timer
into the global variable Time. From then on, in normal operation, both
Time and Ticks are update by an ordinary VBL task, not by the hardware
timer.  As has been discussed earlier, if the system is busy, it is
allowed to skip executing VBL tasks. As a result, the variables Ticks and
Time drift.  My Menu Clock (shareware $5.00) resynchronizes to the
hardware timer once every five minutes or so. The drift is negligable over
that period, and the extra CPU it costs to update the clock variables form
the slow hardware is also negligable, though I wouldn't want to try doing
it 60 times a second.

The TickCount() system call usually just returns the value of Ticks. If
you have "profiling" on, (See I. M. Vol 1 (the Event Manager) for more
deatils about this use of the word profiling.) then a Guided Tour or a
macro can slow down or speed up the passing of time, from the point of
view of an application program.

An interesting bug in most versions of the imagewriter driver is that it
assumes that registers A0, and A1 will be preserved across a call to
TickCount(), which isn't true if profiling is on.

--- David Phillip Oster            --When you asked me to live in sin with you
Arpa: oster@dewey.soe.berkeley.edu --I didn't know you meant sloth.
Uucp: {uwvax,decvax,ihnp4}!ucbvax!oster%dewey.soe.berkeley.edu