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Yes I can. This seems strange, but it is correct. The little endian case look like:
 Little endian
 ---------------------------------------
 | unsigned short | unsigned short |
 ---------------------------------------
 | bbbbaaaa ....cccc dddddddd ....dddd |
 ---------------------------------------
 | 00010010 00110100 01010110 01111000 |
 ---------------------------------------
where the 'd' bits pack from left to right, so '1000 01010110'.
The big endian case look like:
 Big endian
 ---------------------------------------
 | unsigned short | unsigned short |
 ---------------------------------------
 | aaaabbbb cccc.... dddddddd dddd.... |
 ---------------------------------------
 | 00010010 00110100 01010110 01111000 |
 ---------------------------------------
where the 'd' bits pack from right to left, so '01010110 0111'.
The native case (Structure) can typically be verified using your host C compiler. For example, the above code can be represented in C as:
#include <stdio.h>
struct T
{
 unsigned char a : 4;
 unsigned char b : 4;
 unsigned short c : 4;
 unsigned short d : 12;
};
int main (int argc, char **argv)
{
 unsigned char bytes[] = {0x12, 0x34, 0x56, 0x78};
 struct T *t = (struct T*)&bytes;
 printf ("%X\n", t->a);
 printf ("%X\n", t->b);
 printf ("%X\n", t->c);
 printf ("%X\n", t->d);
}
With respect to structure layout, ctypes typically behaves the same way as the native compiler used to build the interpreter.