On the x86 CPU, you can access any sized data object at any address (albeit some accesses are more efficient than others). On non-x86 CPUs, you can't—as a general rule, you can access only N-byte objects on an N-byte boundary.
For example, to access a 4-byte long integer, it must be aligned on a 4-byte address (e.g., 0x7FBBE008). An address like 0x7FBBE009 will cause the CPU to generate a fault. (An x86 processor happily generates multiple bus cycles and gets the data anyway.)
Generally, this will not be a problem with structures defined in the header files for QNX Neutrino, as we've taken care to ensure that the members are aligned properly. The major place that this occurs is with hardware devices that can map a window into the address space (for configuration registers, etc.), and protocols where the protocol itself presents data in an unaligned manner (e.g., CIFS/SMB protocol).