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A way to perform two lines with one conditional statements?

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Ian.M
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RE: A way to perform two lines with one conditional statements? 2009/12/01 16:55:07 (permalink)
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Yes I copy/pasted the typo. Guilty as charged.
I'm afraid I was too deep into the exact SWAPF sequance and fighting the formatting in this editor to notice that all the code managed to do was load the Z flag from the old result. Stefan, thanks for spotting the ****up.  Hopefully  Bonedoc hasn't wasted too much time wondering why it didn't work

Ian
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Stefan Uhlemayr
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RE: A way to perform two lines with one conditional statements? 2009/12/02 03:23:25 (permalink)
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ORIGINAL: Ian.M

Yes I copy/pasted the typo.
wink No problem. That just happens, especially, if one of these members, which are known to have pretty good knowledge, is doing one of his very few mistakes - you don't expect it...Smile
ORIGINAL: Ian.M

I'm afraid I was too deep into the exact SWAPF sequance and fighting the formatting in this editor to notice that all the code managed to do was load the Z flag from the old result.
Especially, if I want to highlight some parts of a posted code, I do it this way: I write the code with any editor (formatted with spaces only), copy it the into the forum-editor, where I format it with the "courier new"-font. This code you can now format with color, bold, etc. (I've learned this from "vloki", quite useful sometimes...wink).
ORIGINAL: Ian.M

Assemble your 'Air Code' before posting to avoid embarrasment pink
I can't do this at home, as on my private pc I haven't installed mplab - and so you may find quite some "oops" in my history, too...[&:]

Greetings,
Stefan

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MBedder
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RE: A way to perform two lines with one conditional statements? 2009/12/02 04:28:11 (permalink)
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In addition to formatting - I use almost the same "technique" with an external editor configured to replace tabs with spaces (AkelPad in my case), and after pasting the text to a forum message composition window I select the text block and set the font face to Courier New (it's a monospace font) to keep the original formatting intact. A color can then be added without problems unlike a <code> tagged text.

How To Ask Questions The Smart Way
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1and0
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RE: A way to perform two lines with one conditional statements? 2009/12/02 10:50:05 (permalink)
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ORIGINAL: Ian.M

Its a litttle harder to deal with the 'False' value in another variable but it can be done.  The desired output in Myvar if the test fails is in  VarF, the one if it succeeds is in VarT :

  	movf	MainCnt,W
  	xorlw   .250		;condition->status, Zero is True
 swapf    VarF, F ;Swap nibbles in VarF in situ without wiping the STATUS
 swapf    VarF, W ;and back the right way round into W
 Swapf    VarF, F ;Unswap VarF
  	btfss    STATUS,Z	;deferred test of condition
  	movf    VarT,W;True value
 ; movf MyVar,F ;is *wrong* corrected below
 movwf    MyVar ;Set MyVar to the correct result without glitches

This takes one less instruction:

     movf    MainCnt,W
     xorlw   .250        ;condition->status, Zero is True
   
     swapf   VarF,W      ;swap nibbles in false value
     btfss   STATUS,Z    ;deferred test of condition
     swapf   VarT,W      ;swap nibbles in true value
   
     movwf   MyVar       ;set MyVar
     swapf   MyVar,F     ;unswap MyVar
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Ian.M
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RE: A way to perform two lines with one conditional statements? 2009/12/02 11:27:56 (permalink)
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 it avoids glitching VarF! grin
but it does glitch MyVar, which was the reason for discarding the preload MyVar with VarF then replace with VarT if the condition is true method.  sad
Its not appropriate if MyVar is volatile, but if you are just after determanism, its usefull

Thank you 1and0, I'm adding that to my collection.
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1and0
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RE: A way to perform two lines with one conditional statements? 2009/12/02 12:25:07 (permalink)
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This avoids glitching VarF and MyVar:

     movf    MainCnt,W
     sublw   .250        ;condition->status, Zero is True
   
     btfss   STATUS,Z
     bcf     STATUS,C    ;Carry = Zero bit
   
     movf    VarF,W
     btfss   STATUS,C
     movf    VarT,W
   
     movwf   MyVar

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Ian.M
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RE: A way to perform two lines with one conditional statements? 2009/12/02 14:20:32 (permalink)
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EDIT: I was wrong, see below.  IT IS CUTE, 1and1 is a genius.

I think you've just done
    IF MainCnt>=250
rather than
    IF MainCnt=250

Otherwise its cute.

Easily fixed by:
<UNNEEDED FIX REMOVED>

EDIT: Unless you have an absolute requirement not to use GOTO, at this point it would be easier and no slower to revert to using gotos and balance the execution paths with nops.  It is also far closer to a HLL IF ... THEN ... ELSE ... ENDIF which is no bad thing.:


              movf    MainCnt,W
                         sublw   .250        ;condition->status, Zero is True
              btfss   STATUS,Z
                         goto    skip67t ; true
              movf    VarF,W
              goto skip67x
 skip67t:    movf    VarT,W
              nop
 skip67x:    ...

post edited by Ian.M - 2009/12/02 16:02:45
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1and0
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RE: A way to perform two lines with one conditional statements? 2009/12/02 14:39:45 (permalink)
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It does

     IF MainCnt == 250
   
 SUBLW Result    Zero Bit    Carry Bit
 -------------------------------------
 Positive           0            1
 Zero               1            1
 Negative           0            0
   
     btfss   STATUS,Z
     bcf     STATUS,C    ;Carry = Zero bit
The above two instructions make the Carry bit equal to the Zero bit (positive case).

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Stefan Uhlemayr
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RE: A way to perform two lines with one conditional statements? 2009/12/02 15:27:50 (permalink)
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ORIGINAL: 1and0

This avoids glitching VarF and MyVar:

     movf    MainCnt,W
     sublw   .250        ;condition->status, Zero is True
   
     btfss   STATUS,Z
     bcf     STATUS,C    ;Carry = Zero bit
   
     movf    VarF,W
     btfss   STATUS,C
     movf    VarT,W
   
     movwf   MyVar

Well done!Smile I've added this to the "neat asm-tricks" in the Software-Gallery.

Anyone who knows a better title then "Conditional (content of a register) register-copying:"?

Thanks,
Stefan

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DarioG
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RE: A way to perform two lines with one conditional statements? 2009/12/02 17:07:02 (permalink)
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Yup! sounds cool, will have to find an application for that Smile

thx to everybody

GENOVA :D :D ! GODO
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1and0
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RE: A way to perform two lines with one conditional statements? 2009/12/02 21:18:15 (permalink)
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ORIGINAL: Stefan Uhlemayr

Well done!Smile I've added this to the "neat asm-tricks" in the Software-Gallery.

Thanks. gringringrin
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1and0
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RE: A way to perform two lines with one conditional statements? 2009/12/03 13:18:27 (permalink)
+2 (1)
Here are improvements that use one less instruction:

     movf    MainCnt,w   ;do comparison test
     sublw   .250        ;condition in Zero bit
   
     addlw   0xFF        ;Carry = ~Zero bit
   
     movf    VarEQ,w     ;get value assuming condition is equal
     btfsc   STATUS,C    ;equal condition? yes, skip, else
     movf    VarNE,w     ;get value for condition is not equal
   
     movwf   MyVar       ;store value
and

     movf    MainCnt,w   ;do comparison test
     sublw   .250        ;condition in Zero bit
   
     sublw   0           ;Carry = Zero bit
   
     movf    VarEQ,w     ;get value assuming condition is equal
     btfss   STATUS,C    ;equal condition? yes, skip, else
     movf    VarNE,w     ;get value for condition is not equal
   
     movwf   MyVar       ;store value

EDIT: Added "SUBLW 0" method.
post edited by 1and0 - 2009/12/06 14:43:20
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Stefan Uhlemayr
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RE: A way to perform two lines with one conditional statements? 2009/12/03 14:23:13 (permalink)
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I changed the link in the gallery to this improved one. In fact, both variants are interesting, because of the method to use the "C"-bit as zero-bit. As a lot of instructions aren't changing the "C"-bit, you may save an instruction here and there using this trick.

Thanks again,Smile
Stefan

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