C instruction time

jksor1234 · Joined: 18 Jan 2007 Posts: 9

Is there a document which outlines how much time it takes to execute every 'C' instruction.

In the datasheet for the PIC16F913, it states that the Instruction Cycle Time equals four divided by the oscillator frequency --> Tcy = 4/Fosc

I want to know how many instructions an IF statement uses, how many a WHILE loop uses, how many an Interrupt Routine uses, etc. In the CCS Manual, it gives a table for the amount of time math functions take.

Is there a document which says how many instruction cycles each instruction takes or do I need to just go through the LST file and figure it out?

Ttelmah · Guest

Simple answer. No.
It will even vary according to the data involved, and where the instructions occur.
There is a table in the manual, giving 'typical' times for the maths operations, but the problem is that a single C instruction, may involve anything from one assembler instruction to thousands. The compiler optimiser, is smart enough so that (for instance), if a page switch is necessary in one location, but not in another, it is removed in the latter case, so exactly the same access instruction, can have different numbers of machine instructions in different locations. As another example, taking the 'modulus' (remainder after division), of two numbers. This according to the timing 'table', takes typically 116 instructions for two 8 bit numbers. However the latter optimsers are smart enough, that if the second number is a 'binary' value (2, 4, 8 etc.), and a constant, the '&' operand is used instead, taking just a couple of instructions.
The only way to find out, is to either manually count instructions, or use a tool like the MPLAB simulator, and sopwatch function, and physically time the operation for real. Even here you need to be aware that timings for arithmetic operations in particular, will change with different values...

Best Wishes

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

Use the LST file.

The time to complete an if or while will vary with the argument list. Is your argument list a simple variable comparison (i<3) or a more complex multi-level with pointers? Also things like chip archetecture can affect the time.

This has been suggested MANY times and I'll say it again. Learn a little assembly first. You don't have to be proficient, but at least be able to read it and recognize the basics. Then when you are working with the CCS compiler and you just KNOW your C code is correct but yet the wrong thing happens, you have a chance. Look at the LST file, see what the compiler produced. It does occasionally generate "crap" (usually though it is a mater of GIGO).
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

jksor1234 · Joined: 18 Jan 2007 Posts: 9

Where in the LST file can you see the instructions required to get into an interrupt and the instructions required to get out of an interrupt? I want to calculate the amount of time my interrupt routines are taking.

Ttelmah · Guest

Comment out the line '#nolist' in the include file for the processor you are using. This allows the 'hidden' bits of housekeeping code to be seen.
Then just look at the code starting at the hardware interrupt address (depends on whether this is a PIC16, or a PIC18). Expect perhaps 30 to 40 instruction times, before the actual jump to the handler code, and a similar number on the return.

Best Wishes

jksor1234 · Joined: 18 Jan 2007 Posts: 9

I am using a PIC16F913. Where is the hardware interrupt address?

From the LST file:

Ttelmah · Guest

The hardware address for a 16 chip, is 0004.
Look at the data sheet...
You will find elsewhere in the assembly, the 'global' handler. This saves all the registers, then jumps to the individual hanlder (in your case at 0073). Then the code at here, jumps back when it finishes, which restores the registers, and returns to the caller.
The typical 'global' handler, for a 16 chip, is: