Thread: Advantages of using macro for BIT positions

Well, I don't see any DISADVANTAGE with it - it's easier to read "BIT3" than figure out that 8 is bit number three. And any decent compiler should be able to translate 1 << 3 into 8 at compile-time.

More importantly, you may not want to call it "BIT3", but "EnableSoundBit" or some such, that indicates the purpose of setting this bit.

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Mats

Originally Posted by donmorr

I want to use a set of macros for generic bit setting, hence the use of "BIT3". Would I be better off with a "BIT(x)" macro?

Don

But why would you want to generically set bit X? Normally when you want to set individual bits in a some integer type, it's because they mean something. Or are you planning to do something like a bitmap to keep track of (say) which squares on a chessboard have pieces on them?

Yes, you can have BIT(x) which translates to (1 << (x)) [don't forget those parenthesis, as << is one of the nasty ones for precedence, so you want to make sure you get what you expect].

Of coruse, you can then use
#define EnableSoundBit BIT(3)


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Mats

_PARTICULARLY_ in the embedded world, bits usually have particular meaning for each position (or set of positions, e.g. a set of three consecutive bits are used to indicate some 0..7 value).

Having a macro to define bits (and sets of bits?) would be handy, but you also should seriously consider naming bits once you get to defining the hardware device or software that uses those bit positions, so that you can easily figure out what the code intends to do.

x |= 8 or x |= 1 << 3 or x |= BIT(3) are all pretty much equally unreadable compared to x |= EnableSoundBit.

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Mats

Originally Posted by donmorr

Yes I agree, in a situation where I would want to control a particular bit in a hardware register I would always give it a meaningfull name:
e.g.

Code:

#define ADF7020_SREAD               (PORTAbits.RA5)

But in the case of manipulating data (extracting information, etc), I would still want to use the BIT macros.


Don

I still think that you don't use BIT(3) when extracting data - your bit actually means something. Sometimes you want BIT(x), where x is variable, for example with some compression algorithms. But that is a slightly different matter, and in this case, I personally prefer (1 << x) than a BIT(x) macro.

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Mats