src/soc/fu/alu/output_stage.py

   1 # This stage is intended to handle the gating of carry and overflow
   2 # out, summary overflow generation, and updating the condition
   3 # register
   4 from nmigen import (Module, Signal, Cat, Repl)
   5 from nmutil.pipemodbase import PipeModBase
   6 from soc.fu.alu.pipe_data import ALUInputData, ALUOutputData
   7 from ieee754.part.partsig import PartitionedSignal
   8 from soc.decoder.power_enums import InternalOp
   9
  10
  11 class ALUOutputStage(PipeModBase):
  12     def __init__(self, pspec):
  13         super().__init__(pspec, "output")
  14
  15     def ispec(self):
  16         return ALUOutputData(self.pspec) # TODO: ALUIntermediateData
  17
  18     def ospec(self):
  19         return ALUOutputData(self.pspec)
  20
  21     def elaborate(self, platform):
  22         m = Module()
  23         comb = m.d.comb
  24         op = self.i.ctx.op
  25
  26         # op requests inversion of the output
  27         o = Signal.like(self.i.o)
  28         with m.If(op.invert_out):
  29             comb += o.eq(~self.i.o)
  30         with m.Else():
  31             comb += o.eq(self.i.o)
  32
  33         # target register if 32-bit is only the 32 LSBs
  34         target = Signal(64, reset_less=True)
  35         with m.If(op.is_32bit):
  36             comb += target.eq(o[:32])
  37         with m.Else():
  38             comb += target.eq(o)
  39
  40         # Handle carry_out
  41         with m.If(self.i.ctx.op.output_carry):
  42             comb += self.o.carry_out.eq(self.i.carry_out)
  43
  44         # create condition register cr0 and sticky-overflow
  45         is_zero = Signal(reset_less=True)
  46         is_positive = Signal(reset_less=True)
  47         is_negative = Signal(reset_less=True)
  48         msb_test = Signal(reset_less=True) # set equal to MSB, invert if OP=CMP
  49         is_cmp = Signal(reset_less=True)   # true if OP=CMP
  50         so = Signal(reset_less=True)
  51
  52         # TODO: if o[63] is XORed with "operand == OP_CMP"
  53         # that can be used as a test
  54         # see https://bugs.libre-soc.org/show_bug.cgi?id=305#c60
  55
  56         comb += is_cmp.eq(op.insn_type == InternalOp.OP_CMP)
  57         comb += msb_test.eq(target[-1] ^ is_cmp)
  58         comb += is_zero.eq(target == 0)
  59         comb += is_positive.eq(~is_zero & ~msb_test)
  60         comb += is_negative.eq(~is_zero & msb_test)
  61         comb += so.eq(self.i.so | self.i.ov)
  62
  63         with m.If(op.insn_type != InternalOp.OP_CMPEQB):
  64             comb += self.o.cr0.eq(Cat(so, is_zero, is_positive, is_negative))
  65         with m.Else():
  66             comb += self.o.cr0.eq(self.i.cr0)
  67
  68         # copy [inverted] output, sticky-overflow and context out
  69         comb += self.o.o.eq(o)
  70         comb += self.o.so.eq(so)
  71         comb += self.o.ctx.eq(self.i.ctx)
  72
  73         return m