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Using instruction timing in Assembler

 
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RLScott



Joined: 10 Jul 2007
Posts: 465

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Using instruction timing in Assembler
PostPosted: Tue Jul 10, 2007 8:44 am     Reply with quote
When using in-line assembler to generate precise timing with instruction timing, it is often useful to do something like this to optionally add in one more instruction cycle:

Code:

    btfsc   STATUS,C
    goto    $+1         ; one cycle or two, depending on STATUS,C


Problem 1: "$+1" is not recognized by the CCS assember (I am using an older version - perhaps the newer version does recognize it?) Of course this problem has an easy solution. Just use:

Code:

     btfsc   STATUS,C
     goto   L123
L123:


Problem 2: With #OPT set any higher than 0, the goto L123 is entirely optimized away as unnecessary, which of course breaks the intention of my code. The only work-around I can find for this is to use #OPT 0, which is global and precludes the use of any optimizations elsewhere.

Any comments?

Robert Scott
Real-Time Specialties
Ypsilanti, Michigan
Ttelmah
Guest
PostPosted: Tue Jul 10, 2007 8:56 am     Reply with quote
Add 'asis' to the start of your assembler block.
The 'assembler' syntax, is C like in form, and hence the 'idea' of '$' for the current PC, doesn't exist.
#asm asis
btfsc STATUS,C
goto L123
L123:
#endasm

Best Wishes
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