mplab xc8 accessing a c bit structure from assembly

But if you go to another C compiler, there is nothing stopping the implementation from setting the MSB with B0. So this means that as long as you are using the XC8 compiler you are guarenteed that the first member will be the LSB. Subsequent bit-fields are allocated higher-order bits. D.10.4 The order of allocation of bit-fields within an int (6.5.2.1)The first bit-field defined in a structure is allocated the LSb position in the storage unit. The XC8 compiler (the implementation) says this. The alignment of the addressable storage unit is unspecified. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. From the C99 standards 6.7.2.1 Structureand union specifiers 10 - An implementation may allocate any addressable storage unit large enough to hold a bit-field. The order that the bits are put in memory is implementation defined. byte = AnyUnsignedCharVariable And, after that, I do a read of the flag, like this : if(testA) The result can be different ? I mean, testA could not represent the seventh bit of byte ?įirstly, if you only plan to use this code via the XC8 there is nothing wrong with using the implementation defined behaviour (if you are aware that it might not be able to work somewhere else). This means that you code will no longer be portable.Īre you saying that if I do an attribution, based on the code witten on post #14 flagA. Danbakana InvalidApple You can use a union with a bitfield, but you need to remember that if a value is entered using one field and read with the other field the results are implementation defined.

Mplab xc8 accessing a c bit structure from assembly