Convert a value into its signed counterpart given a particular bit width. Also shows creating simple test in python with asserts and exceptions.

bits2signed.py

def bits2signed( val, width ):
  '''Convert a value into its signed counterpart given a particular bit width'''
  shift=width-1
  max_val=(2**width)-1
  if val > max_val:
    raise ValueError('The value provided (%s) is greater than the bit width can represent(2^%s=%s)'%(val,width,max_val))
  sign_bit_mask=1<<shift
  magn_bit_mask=(1<<shift)-1
  return -(val&sign_bit_mask) | (val&magn_bit_mask)

# Test that the function works as expected
assert bits2signed(0xffd, 16)     == 4093
assert bits2signed(0xfffd, 16)    == -3
assert bits2signed(0xffff, 16)    == -1
assert bits2signed(0x8000, 16)    == -32768
assert bits2signed(0x7fff, 16)    == 32767
assert bits2signed(0x1, 16)       == 1
assert bits2signed(0xffcb, 16)    == -53
assert bits2signed(0xffffffd,32)  == 268435453
assert bits2signed(0xfffffffd,32) == -3
try:
  bits2signed(0xffffd, 16)   # Try to call the function with an intentional error
  raise Exception            # If the call was succesfull raise an Exception
except ValueError:           # If the call was not succesfull:
  pass                       # Then it did what was expected