src/soc/fu/alu/main_stage.py

   1 # This stage is intended to do most of the work of executing the Arithmetic
   2 # instructions. This would be like the additions, compares, and sign-extension
   3 # as well as carry and overflow generation. This module
   4 # however should not gate the carry or overflow, that's up to the
   5 # output stage
   6 from nmigen import (Module, Signal, Cat, Repl, Mux, Const)
   7 from nmutil.pipemodbase import PipeModBase
   8 from nmutil.extend import exts
   9 from soc.fu.alu.pipe_data import ALUInputData, ALUOutputData
  10 from ieee754.part.partsig import PartitionedSignal
  11 from soc.decoder.power_enums import MicrOp
  12
  13
  14 # microwatt calc_ov function.
  15 def calc_ov(msb_a, msb_b, ca, msb_r):
  16     return (ca ^ msb_r) & ~(msb_a ^ msb_b)
  17
  18
  19 class ALUMainStage(PipeModBase):
  20     def __init__(self, pspec):
  21         super().__init__(pspec, "main")
  22
  23     def ispec(self):
  24         return ALUInputData(self.pspec) # defines pipeline stage input format
  25
  26     def ospec(self):
  27         return ALUOutputData(self.pspec) # defines pipeline stage output format
  28
  29     def elaborate(self, platform):
  30         m = Module()
  31         comb = m.d.comb
  32
  33         # convenience variables
  34         cry_o, o, cr0 = self.o.xer_ca, self.o.o, self.o.cr0
  35         ov_o = self.o.xer_ov
  36         a, b, cry_i, op = self.i.a, self.i.b, self.i.xer_ca, self.i.ctx.op
  37
  38         # check if op is 32-bit, and get sign bit from operand a
  39         is_32bit = Signal(reset_less=True)
  40         sign_bit = Signal(reset_less=True)
  41         comb += is_32bit.eq(op.is_32bit)
  42         comb += sign_bit.eq(Mux(is_32bit, a[31], a[63]))
  43
  44         # little trick: do the add using only one add (not 2)
  45         # LSB: carry-in [0].  op/result: [1:-1].  MSB: carry-out [-1]
  46         add_a = Signal(a.width + 2, reset_less=True)
  47         add_b = Signal(a.width + 2, reset_less=True)
  48         add_o = Signal(a.width + 2, reset_less=True)
  49         with m.If((op.insn_type == MicrOp.OP_ADD) |
  50                   (op.insn_type == MicrOp.OP_CMP)):
  51             # in bit 0, 1+carry_in creates carry into bit 1 and above
  52             comb += add_a.eq(Cat(cry_i[0], a, Const(0, 1)))
  53             comb += add_b.eq(Cat(Const(1, 1), b, Const(0, 1)))
  54             comb += add_o.eq(add_a + add_b)
  55
  56         ##########################
  57         # main switch-statement for handling arithmetic operations
  58
  59         with m.Switch(op.insn_type):
  60             #### CMP, CMPL ####
  61             with m.Case(MicrOp.OP_CMP):
  62                 # this is supposed to be inverted (b-a, not a-b)
  63                 # however we have a trick: instead of adding either 2x 64-bit
  64                 # MUXes to invert a and b, or messing with a 64-bit output,
  65                 # swap +ve and -ve test in the *output* stage using an XOR gate
  66                 comb += o.data.eq(add_o[1:-1])
  67                 comb += o.ok.eq(0) # use o.data but do *not* actually output
  68
  69             #### add ####
  70             with m.Case(MicrOp.OP_ADD):
  71                 # bit 0 is not part of the result, top bit is the carry-out
  72                 comb += o.data.eq(add_o[1:-1])
  73                 comb += o.ok.eq(1) # output register
  74
  75                 # see microwatt OP_ADD code
  76                 # https://bugs.libre-soc.org/show_bug.cgi?id=319#c5
  77                 ca = Signal(2, reset_less=True)
  78                 comb += ca[0].eq(add_o[-1])                   # XER.CA
  79                 comb += ca[1].eq(add_o[33] ^ (a[32] ^ b[32])) # XER.CA32
  80                 comb += cry_o.data.eq(ca)
  81                 comb += cry_o.ok.eq(1)
  82                 # 32-bit (ov[1]) and 64-bit (ov[0]) overflow
  83                 ov = Signal(2, reset_less=True)
  84                 comb += ov[0].eq(calc_ov(a[-1], b[-1], ca[0], add_o[-2]))
  85                 comb += ov[1].eq(calc_ov(a[31], b[31], ca[1], add_o[32]))
  86                 comb += ov_o.data.eq(ov)
  87                 comb += ov_o.ok.eq(1)
  88
  89             #### exts (sign-extend) ####
  90             with m.Case(MicrOp.OP_EXTS):
  91                 with m.If(op.data_len == 1):
  92                     comb += o.data.eq(exts(a, 8, 64))
  93                 with m.If(op.data_len == 2):
  94                     comb += o.data.eq(exts(a, 16, 64))
  95                 with m.If(op.data_len == 4):
  96                     comb += o.data.eq(exts(a, 32, 64))
  97                 comb += o.ok.eq(1) # output register
  98
  99             #### cmpeqb ####
 100             with m.Case(MicrOp.OP_CMPEQB):
 101                 eqs = Signal(8, reset_less=True)
 102                 src1 = Signal(8, reset_less=True)
 103                 comb += src1.eq(a[0:8])
 104                 for i in range(8):
 105                     comb += eqs[i].eq(src1 == b[8*i:8*(i+1)])
 106                 comb += o.data[0].eq(eqs.any())
 107                 comb += o.ok.eq(0) # use o.data but do *not* actually output
 108                 comb += cr0.data.eq(Cat(Const(0, 2), eqs.any(), Const(0, 1)))
 109                 comb += cr0.ok.eq(1)
 110
 111         ###### sticky overflow and context, both pass-through #####
 112
 113         comb += self.o.xer_so.data.eq(self.i.xer_so)
 114         comb += self.o.ctx.eq(self.i.ctx)
 115
 116         return m