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Computing Elapsed Time
Computing Elapsed Time
The RmvTime procedure in the revised and extended Time Managers returns
any unused time in the tmCount field of the task record. This feature makes the
Time Manager extremely useful for computing elapsed times, which can, in
turn, provide performance measurements.
To compute the amount of time that a routine takes to execute, call
PrimeTime at the beginning of the interval to be measured and specify a
delay greater than the expected elapsed time. Then call RmvTime at the end of
the interval and subtract the unused time returned in tmCount from the
original delay passed to PrimeTime. To obtain the most accurate results, you
should do all timing in microseconds (in which case the tmCount field of the
task record has a range of about 35 minutes). To get an exact measurement,
you should compute the overhead associated with calling the Time Manager
and subtract it from the preliminary result. The following program illustrates
a technique for computing elapsed time.
;allocate and clear a TMTask record on the stack
;setting tmAddr := 0 means no task
moveq.l #(tmQSize/2)-1,d0 ;set up loop counter
; to clear TMTask
@clear clr.w -(sp) [TOKEN:15357]and clear TMTask record
dbra d0,@clear [TOKEN:15203]lear it a word at a time
move.l #60*1000*1000,d7 ;D7 := delay in microseconds
; (1 minute)
movea.l sp,a0 [TOKEN:15169]0 points to TMTask
_InsTime [TOKEN:15357]the task
move.l d7,d6 [TOKEN:15172]6 := copy of initial delay
move.l d7,d0 [TOKEN:15172]0 := delay time
neg.l d0 [TOKEN:15214]egate it for microseconds
_PrimeTime [TOKEN:15219]tart the timer
_RmvTime ;immediately stop it
;unused time will be returned in negated microseconds,
; so adding is really subtracting
add.l tmCount(a0),d7 ;D7 := initial delay -
; time remaining
;D7 now contains the overhead in microseconds of _PrimeTime and _RmvTime
movea.l sp,a0 [TOKEN:15169]0 points to TMTask record
_InsTime [TOKEN:15357]the task
move.l d6,d0 [TOKEN:15172]0 := delay time
neg.l d0 [TOKEN:15214]egate it for microseconds
_PrimeTime [TOKEN:15219]tart the timer
;beginning of code to be timed
; (in this example, a TimeDBRA loop)
move.w TimeDBRA,d0 ;number of DBRAs per millisecond
@dbraLoop dbra d0,@dbraLoop;waste a millisecond
;end of code to be timed
_RmvTime ;stop the timer
add.l tmCount(a0),d6 ;D6 := time used in microseconds
sub.l d7,d6 [TOKEN:15219]ubtract the Time Mgr overhead
adda.w #tmQSize,sp [TOKEN:15204]eallocate TMTask record
;register D6 now contains the number of microseconds
; used by the timed code
If you run this code, you might notice that on some models of the Macintosh,
register D6 is not very close to 1000 (one millisecond). This is not due to a
problem in the Time Manager. Rather, this occurs because TimeDBRA is the
number of DBRA instructions per millisecond when executing out of ROM, and
RAM accesses have different timing on some models.
Note: You should not run this sample code on a Macintosh Plus because
that computer's ROM does not support the TimeDBRA variable.
You can insert a task record into the Time Manager's queue by calling
InsTime or InsXTime. Use InsXTime only if you wish to use the
drift-free, fixed-frequency timing services of the extended Time Manager;
use InsTime in all other cases. After you have queued a task record, you can
activate it by calling PrimeTime. You can remove a task record from the
queue by calling RmvTime.
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