instruction execution time

[Darrel Taylor]

Hi tj,

I don't think there is a list that shows execution times for each statement, because it depends on where the code is located in program memory, what bank the variables are in, and what size variable is being used. All the bank switching and code page changing takes time too.

However, it's pretty easy to actually measure the time it takes to execute any block of statements. This saves you from having to add them all up anyways.

Since a timer set to 1:1 prescaler uses FOSC/4, each tick represents the time it takes to execute 1 instruction cycle.

So you can simply zero the timer and then turn it on just before the statements you want to time. Then turn it off immediately after those statements. The value in the timer will indicate the number of cycles used. You can then convert that to uSeconds if needed fairly easily by multiplying * 1/(OSC*1000000/4).

Here's an example of measuring the time to do a 16/16 bit divide. But you can have any number of statements inbetween, as long as the time does not exceed 65535 instructions.

T1CON = 0                 ' Prescaler = 1:1,  Timer off
@  MOVE?CB  OSC, _PicOSC  ; Get OSC value, for Time calculation

W1        var word
W2        var word
Dummy  var word

W1 = 12345
W2 = 12

' -----  Measure Time for 16/16-bit variable divide -----
Gosub ClearTimer1:
@ bsf     T1CON, TMR1ON   ' Start timer
Dummy = W1/W2               ' The statement to measure
@ bcf     T1CON, TMR1ON   ' Stop timer
Hserout ["16/16 Var divide= "] : Gosub ShowTime

STOP
'----------------------------------------------------------------
PicOSC     Var Byte
Cycles     Var Word
Period     Var Word
Time       Var Word

ClearTimer1:
    TMR1H = 0
    TMR1L = 0
Return

ShowTime:
    Cycles.lowbyte  = TMR1L
    Cycles.highbyte = TMR1H
    Period = 1000 / PicOSC * 4 / 10   ' Time for 1 Instruction Cycle in 100ns
    Time = Cycles * Period
    Time = Div32 10
    Hserout [ Dec Cycles, "   ",Dec Time/10, ".", Dec Time//10," uS", 13,10]
Return

@ 20Mhz, the result for this is 382 instructions or 76.4uS

HTH,
   Darrel