src/ieee754/fpcommon/msbhigh.py

   1 """ module for adjusting a mantissa and exponent so that the MSB is always 1
   2 """
   3
   4 from nmigen import Module, Signal, Elaboratable
   5 from nmigen.lib.coding import PriorityEncoder
   6
   7
   8 class FPMSBHigh(Elaboratable):
   9     """ makes the top mantissa bit hi (i.e. shifts until it is)
  10
  11         NOTE: this does NOT do any kind of checks.  do not pass in
  12         zero (empty) stuff, and it's best to check if the MSB is
  13         already 1 before calling it.  although it'll probably work
  14         in both cases...
  15
  16         * exponent is signed
  17         * mantissa is unsigned.
  18
  19         examples:
  20         exp = -30, mantissa = 0b00011 - output: -33, 0b11000
  21         exp =   2, mantissa = 0b01111 - output:   1, 0b11110
  22     """
  23     def __init__(self, m_width, e_width):
  24         self.m_width = m_width
  25         self.e_width = e_width
  26
  27         self.m_in = Signal(m_width, reset_less=True)
  28         self.e_in = Signal((e_width, True), reset_less=True)
  29         self.m_out = Signal(m_width, reset_less=True)
  30         self.e_out = Signal((e_width, True), reset_less=True)
  31
  32     def elaborate(self, platform):
  33         m = Module()
  34
  35         mwid = self.m_width
  36         pe = PriorityEncoder(mwid)
  37         m.submodules.pe = pe
  38
  39         # *sigh* not entirely obvious: count leading zeros (clz)
  40         # with a PriorityEncoder: to find from the MSB
  41         # we reverse the order of the bits.
  42         temp = Signal(mwid, reset_less=True)
  43         clz = Signal((len(self.e_out), True), reset_less=True)
  44         m.d.comb += [
  45             pe.i.eq(insel.m[::-1]),       # inverted
  46             clz.eq(pe.o),                 # count zeros from MSB down
  47             temp.eq((self.m_in << clz)),  # shift mantissa UP
  48             self.e_out.eq(insel.e - clz), # DECREASE exponent
  49             self.m_out.eq(temp),
  50         ]
  51