Just looking for an explanation

Hi guys,

OK, encountered a weird result and was wondering if anyone could explain it. I am using a PIC12F683 and I wrote A LOT of lines of code. I actually filled the chip and had to make modifications to the code to consolidate space. Anyway, I found that when I wrote the code,

Code:

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    FOR fade = 0 to 255 STEP fadevalue

      PWM left, fade, 1
      PWM right, fade, 1

      gosub buttoncheck          ' check for a button press

    NEXT fade

    FOR fade = 255 to 0  STEP -fadevalue

      PWM left, fade, 1
      PWM right, fade, 1

      gosub buttoncheck          ' check for a button press

    NEXT fade

---

the chip compiled just fine, leaving VERY LITTLE space left. But when the fade result was a little too long for me and I rewrote the code to,

Code:

---

    FOR fade = 0 to 150  STEP fadevalue

      PWM left, fade, 1
      PWM right, fade, 1

      gosub buttoncheck          ' check for a button press

    NEXT fade

    FOR fade = 150 to 0  STEP -fadevalue

      PWM left, fade, 1
      PWM right, fade, 1

      gosub buttoncheck          ' check for a button press

    NEXT fade

---

I got an error message saying I was out of space. By just changing the value from 255 to 150, I ran out of space? Not sure why this was happening, but I wound up going through my code again and consolidating some more to make it fit on the chip.

I just want to know what's going on and why it was doing that. Anybody know why?

Thanks,
Tony

Probably because this compiler is intelligent ...

0 to 255 can be obtained without any value calculation ( yesss ... 255 + 1 = 0 for a byte ! ) , but 0 -150 need to compare actual value to 150 ( implies a calculation and resetting value to 0 when 150 reached ) @ each loop ...

just a possible explanation ...

Alain

Huh? Not really sure what you are talking about. Can you elaborate a little more please? Thanks.

I think what Alain means is when you go from 0 to 255, it simply adds 1 to the present number. When it reaches 255 it doesn't do any resetting. It simply adds 1 to 255 and it goes back to 0 itself. On 0 to 150 it has to constantly monitor the number and reset to 0 after 150. This uses more memory. I just learned a lesson from your post myself.

Comparing the generated Assembler code might give more clues.

Robert

Yes the generated assembler would be smaller.
An example of two BASIC IF...THEN comparisons in assembler

Code:

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@ movf                _Variable        ,W        'IF Variable = $A0 THEN...
@ sublw                0xA0                        'subtract comparison
@ btfss                status                ,Z        'test if result is zero
// condition is here

@ movf                _Variable        ,W        'IF Variable = $00 THEN...
@ btfss                status                ,Z        'test if result is zero
// condition is here

---

You are always testing for zero, helps if the comparison is zero!