Bit manipulations good practices

Question

As a beginner C programmer, I am wondering, what would be the best easy-to-read and easy-to-understand solution for setting control bits in a device. Are there any standards

Answer 1

You could use bit-fields, despite what all the fear-mongers here have been saying. You would just need to know how the compiler(s) and system ABI(s) you intend your code to work with define the "implementation defined" aspects of bit-fields. Don't be scared off by pedants putting words like "implementation defined" in bold.

However what others so far seem to have missed out on are the various aspects of how memory-mapped hardware devices might behave that can be counter-intuitive when dealing with a higher-level language like C and the optimization features such languages offer. For example every read or write of a hardware register may have side-effects sometimes even if bits are not changed on the write. Meanwhile the optimizer may make it difficult to tell when the generated code is actually reading or writing to the address of the register, and even when the C object describing the register is carefully qualified as volatile, great care is required to control when I/O occurs.

Perhaps you will need to use some specific technique defined by your compiler and system in order to properly manipulate memory-mapped hardware devices. This is the case for many embedded systems. In some cases compiler and system vendors will indeed use bit-fields, just as Linux does in some cases. I would suggest reading your compiler manual first.

The bit description table you quote appears to be for the control register of the the Intel Avalon DMA controller core. The "read/write/clear" column gives a hint as to how a particular bit behaves when it is read or written. The status register for that device has an example of a bit where writing a zero will clear a bit value, but it may not read back the same value as was written -- i.e. writing the register may have a side-effect in the device, depending on the value of the DONE bit. Interestingly they document the SOFTWARERESET bit as "RW", but then describe the procedure as writing a 1 to it twice to trigger the reset, and then they also warn Executing a DMA software reset when a DMA transfer is active may result in permanent bus lockup (until the next system reset). The SOFTWARERESET bit should therefore not be written except as a last resort. Managing a reset in C would take some careful coding no matter how you describe the register.

As for standards, well ISO/IEC have produced a "technical report" known as "ISO/IEC TR 18037", with the subtitle "Extensions to support embedded processors". It discusses a number of the issues related to using C to manage hardware addressing and device I/O, and specifically for the kinds of bit-mapped registers you mention in your question it documents a number of macros and techniques available through an include file they call . If your compiler provides such a header file, then you might be able to use these macros.

There are draft copies of TR 18037 available, the latest being TR 18037(2007), though it provides for rather dry reading. However it does contain an example implementation of .

Perhaps a good example of a real-world implementation is in QNX. The QNX documentation offers a decent overview (and an example, though I would strongly suggest using enums for integer values, never macros): QNX