src/ieee754/fsgnj/fsgnj.py

   1 # IEEE Floating Point Conversion
   2 # Copyright (C) 2019 Luke Kenneth Casson Leighton <lkcl@lkcl.net>
   3
   4 from nmigen import Module, Signal, Cat
   5
   6 from nmutil.pipemodbase import PipeModBase
   7 from ieee754.fpcommon.basedata import FPBaseData
   8 from ieee754.fpcommon.packdata import FPPackData
   9
  10
  11 class FSGNJPipeMod(PipeModBase):
  12     """ FP Sign injection - replaces operand A's sign bit with one
  13         generated from operand B
  14
  15         self.ctx.i.op & 0x3 == 0x0 : Copy sign bit from operand B
  16         self.ctx.i.op & 0x3 == 0x1 : Copy inverted sign bit from operand B
  17         self.ctx.i.op & 0x3 == 0x2 : Sign bit is A's sign XOR B's sign
  18     """
  19     def __init__(self, in_pspec):
  20         self.in_pspec = in_pspec
  21         super().__init__(in_pspec, "fsgnj")
  22
  23     def ispec(self):
  24         return FPBaseData(self.in_pspec)
  25
  26     def ospec(self):
  27         return FPPackData(self.in_pspec)
  28
  29     def elaborate(self, platform):
  30         m = Module()
  31
  32         width = self.pspec.width
  33         comb = m.d.comb
  34
  35         z1 = self.o.z
  36         a = self.i.a
  37         b = self.i.b
  38
  39         opcode = self.i.ctx.op
  40
  41         sign = Signal()
  42
  43         with m.Switch(opcode):
  44             with m.Case(0b00):
  45                 comb += sign.eq(b[-1])
  46             with m.Case(0b01):
  47                 comb += sign.eq(~b[-1])
  48             with m.Case(0b10):
  49                 comb += sign.eq(a[-1] ^ b[-1])
  50             with m.Default():
  51                 comb += sign.eq(b[-1])
  52
  53         comb += z1.eq(Cat(a[0:width-1], sign))
  54
  55         # copy the context (muxid, operator)
  56         comb += self.o.ctx.eq(self.i.ctx)
  57
  58         return m