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How much more efficient is assembly code versus C in terms of clock cycles?

Catwell
My current PIC projects are getting more complicated to the point where I am thinking C might be needed. A side note: most jobs I've had, I ended up using assembly on all the projects due most to my adamant grandstanding of code efficiency of the base language. I was able to get away with this posture for a long time. However, I am now doing complex math on the PIC that I don't want to figure out a clever bit manipulation to solve. EX: Dividing a 16bit number by another using 8bit registers.

The PICs I am working on have limited (1Mhz to 4Mhz clock) cycles, and I am wondering if C is going to waste a substantial amount of time executing. Has anyone ever had experience with both and could share some thoughts on the comparison?

 

 

C

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  • 0

    A good C compiler will produce code nearly as good as a good assembly language programmer for a lot of common programming structures, but not all C compilers are that "good" (especially for architectures like the PIC that are not very C-friendly), and there are a few common assembler techniques that don't seem to translate well to C:

    1. Direct use of status bits.  In ASM on an 8-bit CPU, you can essentially do 9-bit math by paying attention to the Carry bit, which is very handy for things like, oh, calculating 8bit CRCs or pseudo-random-number generation.
    2. Mixed-size arithmetic, like adding a one-byte quantity to a 32bit sum.
    3. Reserved registers.  In ASM, it's pretty easy to say "these three registers will only be used by this interrupt service routine", making the ISR a lot quicker.  Frequently much more difficult in C.

    But the simpe truth is that most code doesn't need to be "as fast as possible", and being able to create it sooner, with less likelyhood of bugs, is a really good excuse for using a higher-level language.

  • 0 in reply to WestfW

    I completely agree with Bill,

    If a source program is well developed, I mean write flow-chart first, and then the real program, I believe assembly is the best way to write the source code.

    Writing a good flow-chart will help you in understanding what you need, Subs, functions, and tables.

    So, the first step in writing source code, will be to write own subs, functions, and tables.

    Then, you have only to follow the flow-chart.

     

    Alberto.

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  • 0 in reply to WestfW

    I completely agree with Bill,

    If a source program is well developed, I mean write flow-chart first, and then the real program, I believe assembly is the best way to write the source code.

    Writing a good flow-chart will help you in understanding what you need, Subs, functions, and tables.

    So, the first step in writing source code, will be to write own subs, functions, and tables.

    Then, you have only to follow the flow-chart.

     

    Alberto.

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