src/soc/fu/shift_rot/main_stage.py

   1 # This stage is intended to do most of the work of executing shift
   2 # instructions, as well as carry and overflow generation. This module
   3 # however should not gate the carry or overflow, that's up to the
   4 # output stage
   5 from nmigen import (Module, Signal, Cat, Repl, Mux, Const)
   6 from nmutil.pipemodbase import PipeModBase
   7 from soc.fu.logical.pipe_data import LogicalOutputData
   8 from soc.fu.shift_rot.pipe_data import ShiftRotInputData
   9 from ieee754.part.partsig import PartitionedSignal
  10 from soc.decoder.power_enums import MicrOp
  11 from soc.fu.shift_rot.rotator import Rotator
  12
  13 from soc.decoder.power_fields import DecodeFields
  14 from soc.decoder.power_fieldsn import SignalBitRange
  15
  16
  17 class ShiftRotMainStage(PipeModBase):
  18     def __init__(self, pspec):
  19         super().__init__(pspec, "main")
  20         self.fields = DecodeFields(SignalBitRange, [self.i.ctx.op.insn])
  21         self.fields.create_specs()
  22
  23     def ispec(self):
  24         return ShiftRotInputData(self.pspec)
  25
  26     def ospec(self):
  27         return LogicalOutputData(self.pspec)
  28
  29     def elaborate(self, platform):
  30         m = Module()
  31         comb = m.d.comb
  32         op = self.i.ctx.op
  33         o = self.o.o
  34
  35         # NOTE: the sh field immediate is read in by PowerDecode2
  36         # (actually DecodeRB), whereupon by way of rb "immediate" mode
  37         # it ends up in self.i.rb.
  38
  39         # obtain me and mb fields from instruction.
  40         m_fields = self.fields.instrs['M']
  41         md_fields = self.fields.instrs['MD']
  42         mb = Signal(m_fields['MB'][0:-1].shape())
  43         me = Signal(m_fields['ME'][0:-1].shape())
  44         mb_extra = Signal(1, reset_less=True)
  45         comb += mb.eq(m_fields['MB'][0:-1])
  46         comb += me.eq(m_fields['ME'][0:-1])
  47         comb += mb_extra.eq(md_fields['mb'][0:-1][0])
  48
  49         # set up microwatt rotator module
  50         m.submodules.rotator = rotator = Rotator()
  51         comb += [
  52             rotator.me.eq(me),
  53             rotator.mb.eq(mb),
  54             rotator.mb_extra.eq(mb_extra),
  55             rotator.rs.eq(self.i.rs),
  56             rotator.ra.eq(self.i.a),
  57             rotator.shift.eq(self.i.rb), # can also be sh (in immediate mode)
  58             rotator.is_32bit.eq(op.is_32bit),
  59             rotator.arith.eq(op.is_signed),
  60         ]
  61
  62         comb += o.ok.eq(1) # defaults to enabled
  63
  64         # instruction rotate type
  65         mode = Signal(4, reset_less=True)
  66         with m.Switch(op.insn_type):
  67             with m.Case(MicrOp.OP_SHL):  comb += mode.eq(0b0000) # L-shift
  68             with m.Case(MicrOp.OP_SHR):  comb += mode.eq(0b0001) # R-shift
  69             with m.Case(MicrOp.OP_RLC):  comb += mode.eq(0b0110) # clear LR
  70             with m.Case(MicrOp.OP_RLCL): comb += mode.eq(0b0010) # clear L
  71             with m.Case(MicrOp.OP_RLCR): comb += mode.eq(0b0100) # clear R
  72             with m.Case(MicrOp.OP_EXTSWSLI): comb += mode.eq(0b1000) # L-ext
  73             with m.Default():
  74                 comb += o.ok.eq(0) # otherwise disable
  75
  76         comb += Cat(rotator.right_shift,
  77                     rotator.clear_left,
  78                     rotator.clear_right,
  79                     rotator.sign_ext_rs).eq(mode)
  80
  81         # outputs from the microwatt rotator module
  82         comb += [o.data.eq(rotator.result_o),
  83                  self.o.xer_ca.data.eq(Repl(rotator.carry_out_o, 2))]
  84
  85         ###### sticky overflow and context, both pass-through #####
  86
  87         comb += self.o.ctx.eq(self.i.ctx)
  88
  89         return m