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I have to store instructions, commands that I will be receiving via serial. The commands will be 8 bits long.
I need to preserve transparency between command name, and its value. So as to avoid having to translate an 8-bit number received in serial into any type.
I'd like to use Enumerations to deal with them in my code. Only a enumeration corresponds to a on this platform a 16 bit integer.
The platform is AVR ATmega169V microcontroller, on the Butterfly demo board. It is a 8bit system with some limited support for 16bit operations. It is not a fast system and has about 1KB of RAM. It doesn't have any luxuries like file I/O, or an operating systems.
So any suggestions as to what type I should be using to store 8-bit commands?
There has got to be something better than a massive header of #defines.
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On an 8-bit processor, enums can be 16-bits wide. On a 32-bit processor they can be 32-bits wide or more or less. The GCC C compiler will allocate enough memory for an enum to hold any of the values that you have declared. So, if your code only uses values below 256, your enum should be 8 bits wide.
The size is four bytes because the enum is stored as an int . With only 12 values, you really only need 4 bits, but 32 bit machines process 32 bit quantities more efficiently than smaller quantities.
For C++ and relaxed C89/C99/C11, the compiler allows enumeration constants up to the largest integral type (64 bits).
In C language, an enum is guaranteed to be of size of an int . There is a compile time option ( -fshort-enums ) to make it as short (This is mainly useful in case the values are not more than 64K). There is no compile time option to increase its size to 64 bit.
gcc's -fshort-enums might be useful:
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the "enum" type will be equivalent to the smallest integer type which has enough room.
In fact, here's a page with a lot of relevant information. I hope you come across many GCC switches you never knew existed. ;)
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You are trying to solve a problem that does not exist.
Your question is tagged C. In C language enum types in value context are fully compatible with integral types and behave just like other integral types. When used in expressions, they are subjected to exactly the same integral promotions as other integral types. Once you take that into account, you should realize that if you want to store values described by enumeration constants in a 8-bit integral type, all you have to do is to choose a suitable generic 8-bit integral type (say int8_t) and use it instead of enum type. You'll lose absolutely nothing by storing your enum constant values in an object of type int8_t (as opposed to an object explicitly declared with enum type).
The issue you describe would exist in C++, where enum types are separated much farther from other integral types. In C++ using an integral type in place of enum type for the purpose of saving memory is more difficult (although possible). But not in C, where it requires no additional effort whatsoever.
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