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, Mux, 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         * loprop: propagates the low bit (LSB) on the shift
  19         * limclz: use this to limit the amount of shifting.
  20
  21         examples:
  22         exp = -30, mantissa = 0b00011 - output: -33, 0b11000
  23         exp =   2, mantissa = 0b01111 - output:   1, 0b11110
  24     """
  25     def __init__(self, m_width, e_width, limclz=False, loprop=False):
  26         self.m_width = m_width
  27         self.e_width = e_width
  28         self.loprop = loprop
  29         self.limclz = limclz and Signal((e_width, True), reset_less=True)
  30
  31         self.m_in = Signal(m_width, reset_less=True)
  32         self.e_in = Signal((e_width, True), reset_less=True)
  33         self.m_out = Signal(m_width, reset_less=True)
  34         self.e_out = Signal((e_width, True), reset_less=True)
  35
  36     def elaborate(self, platform):
  37         m = Module()
  38
  39         mwid = self.m_width
  40         pe = PriorityEncoder(mwid)
  41         m.submodules.pe = pe
  42
  43         # *sigh* not entirely obvious: count leading zeros (clz)
  44         # with a PriorityEncoder.  to find from the MSB
  45         # we reverse the order of the bits.  it would be better if PE
  46         # took a "reverse" argument.
  47
  48         clz = Signal((len(self.e_out), True), reset_less=True)
  49         temp = Signal(mwid, reset_less=True)
  50         if self.loprop:
  51             temp_r = Signal(mwid, reset_less=True)
  52             with m.If(self.m_in[0]):
  53                 # propagate low bit: do an ASL basically, except
  54                 # i can't work out how to do it in nmigen sigh
  55                 m.d.comb += temp_r.eq((self.m_in[0] << clz) - 1)
  56
  57         # limclz sets a limit (set by the exponent) on how far M can be shifted
  58         # this can be used to ensure that near-zero numbers don't then have
  59         # to be shifted *back* (e < -126 in the case of FP32 for example)
  60         if self.limclz is not False:
  61             limclz = Mux(self.limclz > pe.o, pe.o, self.limclz)
  62         else:
  63             limclz = pe.o
  64
  65         m.d.comb += [
  66             pe.i.eq(self.m_in[::-1]),     # inverted
  67             clz.eq(limclz),          # count zeros from MSB down
  68             temp.eq((self.m_in << clz)),  # shift mantissa UP
  69             self.e_out.eq(self.e_in - clz), # DECREASE exponent
  70         ]
  71         if self.loprop:
  72             m.d.comb += self.m_out.eq(temp | temp_r)
  73         else:
  74             m.d.comb += self.m_out.eq(temp),
  75
  76         return m