src/ieee754/fpmul/mul1.py

   1 """IEEE754 Floating Point Multiplier Pipeline
   2
   3 Copyright (C) 2019 Luke Kenneth Casson Leighton <lkcl@lkcl.net>
   4
   5 """
   6
   7 from nmigen import Module, Signal, Mux
   8 from nmigen.cli import main, verilog
   9
  10 from nmutil.pipemodbase import PipeModBase
  11 from ieee754.fpcommon.postcalc import FPPostCalcData
  12 from ieee754.fpmul.mul0 import FPMulStage0Data
  13
  14
  15 class FPMulStage1Mod(PipeModBase):
  16     """ Second stage of mul: preparation for normalisation.
  17     """
  18
  19     def __init__(self, pspec):
  20         super().__init__(pspec, "mul1")
  21
  22     def ispec(self):
  23         return FPMulStage0Data(self.pspec)
  24
  25     def ospec(self):
  26         return FPPostCalcData(self.pspec)
  27
  28     def elaborate(self, platform):
  29         m = Module()
  30         comb = m.d.comb
  31
  32         # copy sign as-is
  33         comb += self.o.z.s.eq(self.i.z.s)
  34
  35         # results are in the range 0.25 to 0.999999999999
  36         # sometimes the MSB will be zero, (0.5 * 0.5 = 0.25 which
  37         # in binary is 0b010000) so to compensate for that we have
  38         # to shift the mantissa up (and reduce the exponent by 1)
  39         p = Signal(len(self.i.product), reset_less=True)
  40         msb = Signal(reset_less=True)
  41         e = self.o.z.e
  42         comb += msb.eq(self.i.product[-1])
  43         comb += p.eq(Mux(msb, self.i.product, self.i.product<<1))
  44         comb += e.eq(Mux(msb, self.i.z.e, self.i.z.e-1))
  45
  46         # top bits are mantissa, then guard and round, and the rest of
  47         # the product is sticky
  48         mw = self.o.z.m_width
  49         comb += [
  50             self.o.z.m.eq(p[mw+2:]),            # mantissa
  51             self.o.of.m0.eq(p[mw+2]),           # copy of LSB
  52             self.o.of.guard.eq(p[mw+1]),        # guard
  53             self.o.of.round_bit.eq(p[mw]),      # round
  54             self.o.of.sticky.eq(p[0:mw].bool()) # sticky
  55         ]
  56
  57         # pass through context
  58         comb += self.o.out_do_z.eq(self.i.out_do_z)
  59         comb += self.o.oz.eq(self.i.oz)
  60         comb += self.o.ctx.eq(self.i.ctx)
  61
  62         return m