src/soc/fu/common_output_stage.py

   1 # This stage is intended to handle the gating of carry out,
   2 # and updating the condition register
   3 from nmigen import (Module, Signal, Cat)
   4 from nmutil.pipemodbase import PipeModBase
   5 from ieee754.part.partsig import PartitionedSignal
   6 from soc.decoder.power_enums import InternalOp
   7
   8
   9 class CommonOutputStage(PipeModBase):
  10     def __init__(self, pspec):
  11         super().__init__(pspec, "output")
  12
  13     def elaborate(self, platform):
  14         m = Module()
  15         comb = m.d.comb
  16         op = self.i.ctx.op
  17
  18         # op requests inversion of the output...
  19         o = Signal.like(self.i.o)
  20         if hasattr(op, "invert_out"): # ... optionally
  21             with m.If(op.invert_out):
  22                 comb += o.eq(~self.i.o.data)
  23             with m.Else():
  24                 comb += o.eq(self.i.o.data)
  25         else:
  26             comb += o.eq(self.i.o.data) # ... no inversion
  27
  28         # target register if 32-bit is only the 32 LSBs
  29         # XXX ah.  right.  this needs to be done only if the *mode* is 32-bit
  30         # see https://bugs.libre-soc.org/show_bug.cgi?id=424
  31         target = Signal(64, reset_less=True)
  32         #with m.If(op.is_32bit):
  33         #    comb += target.eq(o[:32])
  34         #with m.Else():
  35         #    comb += target.eq(o)
  36         comb += target.eq(o)
  37
  38         # carry-out only if actually present in this input spec
  39         # (note: MUL and DIV do not have it, but ALU and Logical do)
  40         if hasattr(self.i, "xer_ca"):
  41             # Handle carry_out
  42             comb += self.o.xer_ca.data.eq(self.i.xer_ca.data)
  43             comb += self.o.xer_ca.ok.eq(op.output_carry)
  44
  45         # create condition register cr0 and sticky-overflow
  46         is_nzero = Signal(reset_less=True)
  47         is_positive = Signal(reset_less=True)
  48         is_negative = Signal(reset_less=True)
  49         msb_test = Signal(reset_less=True) # set equal to MSB, invert if OP=CMP
  50         is_cmp = Signal(reset_less=True)     # true if OP=CMP
  51         is_cmpeqb = Signal(reset_less=True)  # true if OP=CMPEQB
  52         self.so = Signal(1, reset_less=True)
  53         cr0 = Signal(4, reset_less=True)
  54
  55         # TODO: if o[63] is XORed with "operand == OP_CMP"
  56         # that can be used as a test of whether to invert the +ve/-ve test
  57         # see https://bugs.libre-soc.org/show_bug.cgi?id=305#c60
  58
  59         comb += is_cmp.eq(op.insn_type == InternalOp.OP_CMP)
  60         comb += is_cmpeqb.eq(op.insn_type == InternalOp.OP_CMPEQB)
  61         # nope - if *processor* mode is 32-bit
  62         #with m.If(op.is_32bit):
  63         #    comb += msb_test.eq(target[-1] ^ is_cmp) # 64-bit MSB
  64         #with m.Else():
  65         #    comb += msb_test.eq(target[31] ^ is_cmp) # 32-bit MSB
  66         comb += msb_test.eq(target[-1]) # 64-bit MSB
  67         comb += is_nzero.eq(target.bool())
  68         with m.If(is_cmp): # invert pos/neg tests
  69             comb += is_positive.eq(msb_test)
  70             comb += is_negative.eq(is_nzero & ~msb_test)
  71         with m.Else():
  72             comb += is_negative.eq(msb_test)
  73             comb += is_positive.eq(is_nzero & ~msb_test)
  74
  75         with m.If(is_cmpeqb):
  76             comb += cr0.eq(self.i.cr0.data)
  77         with m.Else():
  78             comb += cr0.eq(Cat(self.so, ~is_nzero, is_positive, is_negative))
  79
  80         # copy out [inverted?] output, cr0, and context out
  81         comb += self.o.o.data.eq(o)
  82         comb += self.o.o.ok.eq(self.i.o.ok)
  83         # CR0 to be set
  84         comb += self.o.cr0.data.eq(cr0)
  85         comb += self.o.cr0.ok.eq(op.write_cr0)
  86         # context
  87         comb += self.o.ctx.eq(self.i.ctx)
  88
  89         return m