Can't create small bitfields

I'm trying to build up bitfields that are smaller than the word size of
the machine, only 1 or 2 bytes.  Unfortunately, the smallest Structure
I can get is 4 bytes.  Any ideas on what I'm doing wrong?  OS is
x86_64 Ubuntu, Python is 2.7.4 or 3.3.1 with equal results.  Sample code
below.

---

from __future__ import print_function
from ctypes import *

class Struct(Structure):
    _pack_ = 1
    
Struct._fields_ = [
        ('low', c_uint, 4),
        ('high', c_uint, 4),
    ]
    
x = Struct(low = 1, high = 2)
    
print('Structure Size:', sizeof(Struct))
print('Instance Size:', sizeof(x))
print('Structure Fields:', Struct.low, Struct.high)
print('Instance Fields:', x.low, x.high)

arr = (c_uint8 * sizeof(x)).from_buffer(x)
print('Bytes:', ','.join('{0:02X}'.format(b) for b in arr))

---

$python_vme_mods$ python test_bitfield.py 
Structure Size: 4
Instance Size: 4
Structure Fields: <Field type=c_uint, ofs=0:0, bits=4> <Field
type=c_uint, ofs=0:4, bits=4> Instance Fields: 1 2
Bytes: 21,00,00,00



-- 
Rob Gaddi, Highland Technology
18 Otis St.
San Francisco, CA 94103
415-551-1700

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