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Topic: Help with passing variables at global scope into inline assembly. (Read 5702 times) previous topic - next topic


This is all dependent (if I'm using assembly) in having the relevant port and bit passed into my assembly routine.
At one point (when I was walking along the oregon coastline) I thought about coding a huge bunch of #ifdefs with assembly inside them to correspond to every possible port / bit combo, but decided that was a non-starter.

Code's already been written (http://code.google.com/p/digitalwritefast/) for direct port IO with arbitrary defined pins, but you won't get a speed advantage unless the pins are constants and known to the compiler.  Maybe something like this might work:
Code: [Select]

  switch(pin) {
    case 0: digitalWriteFast(0, val);
    // ...

Well with my direct port manipulation I got a character out in 10.08 uS each whereas your assembly took 41.33 uS each. So you could still be better off tidying up the C version using port manipulation. And it will be easier to maintain later on.

According to Mr. Gammon's data, direct port manipulation is quite a bit faster than your assembly - it's also simpler, and doesn't have this passing-variable issue.


I'll be stuck with direct I/O, which is half as fast as assembly.

It wasn't half as fast in my test. Perhaps you should be concentrating on making the C code faster. I mean, ultimately, if you ask the compiler to do "sensible" things (eg. use bytes rather than ints) then the generated assembler is going to be similar to what you are trying to force it to do directly.

I'd like a user to be able to do:
(supposing I name my class IOMonster)
IOMonster io(4,5,6,7,8)     // cmd clock, i/o clock, i/o data, cmd_data, cmd_latch

Well this means the numbers have to be stored in variables, right? And ldi loads a number not a variable. So ultimately the compiler/assembler has to access the memory location where you stored 4 in, and get the 4 out of it. Rather than "ldi r24,4". If you want variables that is the price you pay.

You can probably get the syntax right to pull the variable contents in, I must admit examples were thin in the ground. But once again, the C compiler doesn't just sprinkle in "extra stuff to make it slower" for its amusement. If you code the same general idea that you were trying to do in assembler, in C (eg. direct port access) then it should run as fast.

Try the C version ... if it is still much slower than you expect post your code. Your loops may not be written optimally. For example in your original you had:

Code: [Select]
void shiftout(void)
  uint8_t mask=1;
  for (int i=0; i<8; i++) {
    if ((outbyte & mask)==0)
      digitalWrite(dpin, LOW);
      digitalWrite(dpin, HIGH);
    HL(dclk);  // Toggle clockpin.
  HL(dlat);    // Toggle latchpin

There's a few problems there. For one, I don't see mask changing. For another you are using an int in the loop where you could be using a byte. For a third you are calling another function (HL) when you could toggle the pin inline. Ditto for the latch. Plus of course you are using digitalWrite rather than direct port access. Tidy all that up and you should have a nice fast routine.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

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