src/ieee754/fclass/fclass.py

   1 # IEEE754 FCLASS Module
   2 # Copyright (C) 2019 Luke Kenneth Casson Leighon <lkcl@lkcl.net>
   3
   4 from nmigen import Module, Signal, Cat
   5
   6 from nmutil.pipemodbase import PipeModBase
   7 from ieee754.fpcommon.getop import FPADDBaseData
   8 from ieee754.fpcommon.pack import FPPackData
   9 from ieee754.fpcommon.fpbase import FPNumDecode, FPNumBaseRecord
  10
  11
  12 class FPClassMod(PipeModBase):
  13     """ obtains floating point information (zero, nan, inf etc.)
  14     """
  15     def __init__(self, in_pspec, out_pspec):
  16         self.in_pspec = in_pspec
  17         self.out_pspec = out_pspec
  18         super().__init__(in_pspec, "fclass")
  19
  20     def ispec(self):
  21         return FPADDBaseData(self.in_pspec)
  22
  23     def ospec(self):
  24         return FPPackData(self.out_pspec)
  25
  26     def elaborate(self, platform):
  27         m = Module()
  28         comb = m.d.comb
  29
  30         # decode incoming FP number
  31         print("in_width out", self.in_pspec.width,
  32               self.out_pspec.width)
  33         a1 = FPNumBaseRecord(self.in_pspec.width, False)
  34         print("a1", a1.width, a1.rmw, a1.e_width, a1.e_start, a1.e_end)
  35         m.submodules.sc_decode_a = a1 = FPNumDecode(None, a1)
  36         comb += a1.v.eq(self.i.a)
  37
  38         # FCLASS: work out the "type" of the FP number
  39
  40         finite_nzero = Signal(reset_less=True)
  41         msbzero = Signal(reset_less=True)
  42         is_sig_nan = Signal(reset_less=True)
  43         # XXX use *REAL* mantissa width to detect msb.
  44         # XXX do NOT use a1.m_msbzero because it has extra bitspace
  45         comb += msbzero.eq(a1.m[a1.rmw-1] == 0) # sigh, 1 extra msb bit
  46         comb += finite_nzero.eq(~a1.is_nan & ~a1.is_inf & ~a1.is_zero)
  47         comb += is_sig_nan.eq(a1.exp_128 & (msbzero) & (~a1.m_zero))
  48         subnormal = a1.exp_n127
  49
  50         # this is hardware-optimal but very hard to understand.
  51         # see unit test test_fclass_pipe.py fclass() for what's
  52         # going on.
  53         comb += self.o.z.eq(Cat(
  54                     a1.s   & a1.is_inf,                 # | −inf.
  55                     a1.s   & finite_nzero & ~subnormal, # | -normal number.
  56                     a1.s   & finite_nzero &  subnormal, # | -subnormal number.
  57                     a1.s & a1.is_zero,                  # | −0.
  58                     ~a1.s & a1.is_zero,                 # | +0.
  59                     ~a1.s & finite_nzero &  subnormal,  # | +subnormal number.
  60                     ~a1.s & finite_nzero & ~subnormal,  # | +normal number.
  61                     ~a1.s & a1.is_inf,                  # | +inf.
  62                     is_sig_nan,                         # | a signaling NaN.
  63                     a1.is_nan & ~is_sig_nan))           # | a quiet NaN
  64
  65         comb += self.o.ctx.eq(self.i.ctx)
  66
  67         return m