I suppose that there is no strict data structure and layout connected to BIOS based code .
If I am not worng BIOS has defined by its standard which clarify interface to BIOS and it is functions . Each BIOS should be specific to the motherboard it is placed on .
General standard for BIOS or at least link could be found from PC guide line .
PC Guide line is couple of standards ruling PC internal architecture . Go to the Microsoft site and begin search . But information is spread over different manufactures (Intel , IBM ...).