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A couple of years ago, Netrino engineer Dan Smith was writing stepper motor control firmware that interfaced to lots of registers with binary fields and sub-fields.
Here are some examples of the usage of these macros: If I ever needed to change XXX to , just change a single 0 to a 1 in the source code, and everything magically changes. No error-prone conversion to hexadecimal necessary, no figuring out which bits belong to which nibbles, etc. What is that old saying? This entry was posted on Wednesday, September 30th, at 7: You can follow any responses to this entry through the RSS 2.
You can leave a response , or trackback from your own site. Great post or perhaps steal: I've always wanted a way to express binary numbers in C, and now I have one! I took the code and experimented a bit with it. However, the lowest level of optimization fixed that problem.
The MISRA compliance checker did of course have an absolute fit over the use of these macros, generating a large number of complaints. I think this is yet another example where one has to make an intelligent trade off. MISRA compliance allows exceptions for exactly this kind of thingThis is making your code safer and not less safe: The macros encapsulate individually unsafe constructs but package them up with a very safe interface.
I'm trying to learn the finer points of C and how to be creative with it. I don't understand something the binary macros. Its with this constant that I get lost. I understand how the macro is supposed to work. My problem, I guess is with the syntax. Can someone shed some light? By way of example, lets look at B8 It does this via the stringize operator.
This is a pretty obscure part of the preprocessor — you'll find more information online. It consists of 8 lines, each of which effectively tests one bit position. This evidently evaluates to true and so the ternary operator returns 1. This evidently evaluates to true and so the ternary operator returns 2, which is added to the previous result, giving a total of decimal 3.
This evidently evaluates to false and so the ternary operator returns 0, which is added to the previous result, keeping our total at decimal 3. This evidently evaluates to true and so the ternary operator returns 8, which is added to the previous result, giving a total of decimal 11 — and our final result. The B16 and B32 macros simply build upon this technique by using appropriate casts and shifts.
If not post again and I'll try and expand upon the explanation. There are no trigraphs in this code. It would have fitted very nicely into […]. I never quite understood the need of binary literals.
I think you should be able to safely assume that every programmer can read hex. In a real application, those bit masks would of course have meaningful names instead.
B32 , , , , B32 , , , However, sometimes a bit pattern is exactly that: Whenever a value represents some visual pattern, binary literals are an invaluable help. When working in Ada and other languages with binary numeric literals, I can hardly remember either using them or seeing them used. You can put disable warnings about the macro definitions and make the expansions MISRA type compliant, but you would need to ensure that The GHS compiler would evaluate the binary expresssions as the equivalent numeric literal at any optimisation level, which after all is the point of them.
Mail will not be published required. Michael Barr is an expert on the design of software-powered medical devices and other embedded computer systems. Binary Literals in C Wednesday, September 30th, by Michael Barr A couple of years ago, Netrino engineer Dan Smith was writing stepper motor control firmware that interfaced to lots of registers with binary fields and sub-fields.
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Barr Code Michael Barr Michael Barr is an expert on the design of software-powered medical devices and other embedded computer systems.