src/add/nmigen_add_experiment.py

   1 # IEEE Floating Point Adder (Single Precision)
   2 # Copyright (C) Jonathan P Dawson 2013
   3 # 2013-12-12
   4
   5 from nmigen import Module, Signal, Cat, Const
   6 from nmigen.cli import main, verilog
   7
   8
   9 class FPNum:
  10     """ Floating-point Number Class, variable-width TODO (currently 32-bit)
  11
  12         Contains signals for an incoming copy of the value, decoded into
  13         sign / exponent / mantissa.
  14         Also contains encoding functions, creation and recognition of
  15         zero, NaN and inf (all signed)
  16
  17         Four extra bits are included in the mantissa: the top bit
  18         (m[-1]) is effectively a carry-overflow.  The other three are
  19         guard (m[2]), round (m[1]), and sticky (m[0])
  20     """
  21     def __init__(self, width, m_width=None):
  22         self.width = width
  23         if m_width is None:
  24             m_width = width - 5 # mantissa extra bits (top,guard,round)
  25         self.v = Signal(width)      # Latched copy of value
  26         self.m = Signal(m_width)    # Mantissa
  27         self.e = Signal((10, True)) # Exponent: 10 bits, signed
  28         self.s = Signal()           # Sign bit
  29
  30         self.mzero = Const(0, (m_width, False))
  31         self.m1s = Const(-1, (m_width, False))
  32         self.P128 = Const(128, (10, True))
  33         self.P127 = Const(127, (10, True))
  34         self.N127 = Const(-127, (10, True))
  35         self.N126 = Const(-126, (10, True))
  36
  37     def decode(self):
  38         """ decodes a latched value into sign / exponent / mantissa
  39
  40             bias is subtracted here, from the exponent.  exponent
  41             is extended to 10 bits so that subtract 127 is done on
  42             a 10-bit number
  43         """
  44         v = self.v
  45         return [self.m.eq(Cat(0, 0, 0, v[0:23])), # mantissa
  46                 self.e.eq(v[23:31] - self.P127), # exp (minus bias)
  47                 self.s.eq(v[31]),                 # sign
  48                 ]
  49
  50     def create(self, s, e, m):
  51         """ creates a value from sign / exponent / mantissa
  52
  53             bias is added here, to the exponent
  54         """
  55         return [
  56           self.v[31].eq(s),          # sign
  57           self.v[23:31].eq(e + self.P127), # exp (add on bias)
  58           self.v[0:23].eq(m)         # mantissa
  59         ]
  60
  61     def shift_down(self):
  62         """ shifts a mantissa down by one. exponent is increased to compensate
  63
  64             accuracy is lost as a result in the mantissa however there are 3
  65             guard bits (the latter of which is the "sticky" bit)
  66         """
  67         return [self.e.eq(self.e + 1),
  68                 self.m.eq(Cat(self.m[0] | self.m[1], self.m[2:], 0))
  69                ]
  70
  71     def nan(self, s):
  72         return self.create(s, self.P128, 1<<22)
  73
  74     def inf(self, s):
  75         return self.create(s, self.P128, 0)
  76
  77     def zero(self, s):
  78         return self.create(s, self.N127, 0)
  79
  80     def is_nan(self):
  81         return (self.e == self.P128) & (self.m != 0)
  82
  83     def is_inf(self):
  84         return (self.e == self.P128) & (self.m == 0)
  85
  86     def is_zero(self):
  87         return (self.e == self.N127) & (self.m == self.mzero)
  88
  89     def is_overflowed(self):
  90         return (self.e > self.P127)
  91
  92     def is_denormalised(self):
  93         return (self.e == self.N126) & (self.m[23] == 0)
  94
  95 class FPOp:
  96     def __init__(self, width):
  97         self.width = width
  98
  99         self.v     = Signal(width)
 100         self.stb = Signal()
 101         self.ack = Signal()
 102
 103     def ports(self):
 104         return [self.v, self.stb, self.ack]
 105
 106
 107 class FPADD:
 108     def __init__(self, width):
 109         self.width = width
 110
 111         self.in_a     = FPOp(width)
 112         self.in_b     = FPOp(width)
 113         self.out_z     = FPOp(width)
 114
 115     def get_fragment(self, platform=None):
 116         m = Module()
 117
 118         # Latches
 119         a = FPNum(self.width)
 120         b = FPNum(self.width)
 121         z = FPNum(self.width, 24)
 122
 123         tot = Signal(28)     # sticky/round/guard bits, 23 result, 1 overflow
 124
 125         guard = Signal()     # tot[2]
 126         round_bit = Signal() # tot[1]
 127         sticky = Signal()    # tot[0]
 128
 129         with m.FSM() as fsm:
 130
 131             # ******
 132             # gets operand a
 133
 134             with m.State("get_a"):
 135                 with m.If((self.in_a.ack) & (self.in_a.stb)):
 136                     m.next = "get_b"
 137                     m.d.sync += [
 138                         a.v.eq(self.in_a.v),
 139                         self.in_a.ack.eq(0)
 140                     ]
 141                 with m.Else():
 142                     m.d.sync += self.in_a.ack.eq(1)
 143
 144             # ******
 145             # gets operand b
 146
 147             with m.State("get_b"):
 148                 with m.If((self.in_b.ack) & (self.in_b.stb)):
 149                     m.next = "unpack"
 150                     m.d.sync += [
 151                         b.v.eq(self.in_b.v),
 152                         self.in_b.ack.eq(0)
 153                     ]
 154                 with m.Else():
 155                     m.d.sync += self.in_b.ack.eq(1)
 156
 157             # ******
 158             # unpacks operands into sign, mantissa and exponent
 159
 160             with m.State("unpack"):
 161                 m.next = "special_cases"
 162                 m.d.sync += a.decode()
 163                 m.d.sync += b.decode()
 164
 165             # ******
 166             # special cases: NaNs, infs, zeros, denormalised
 167
 168             with m.State("special_cases"):
 169
 170                 # if a is NaN or b is NaN return NaN
 171                 with m.If(a.is_nan() | b.is_nan()):
 172                     m.next = "put_z"
 173                     m.d.sync += z.nan(1)
 174
 175                 # if a is inf return inf (or NaN)
 176                 with m.Elif(a.is_inf()):
 177                     m.next = "put_z"
 178                     m.d.sync += z.inf(a.s)
 179                     # if a is inf and signs don't match return NaN
 180                     with m.If((b.e == b.P128) & (a.s != b.s)):
 181                         m.d.sync += z.nan(b.s)
 182
 183                 # if b is inf return inf
 184                 with m.Elif(b.is_inf()):
 185                     m.next = "put_z"
 186                     m.d.sync += z.inf(b.s)
 187
 188                 # if a is zero and b zero return signed-a/b
 189                 with m.Elif(a.is_zero() & b.is_zero()):
 190                     m.next = "put_z"
 191                     m.d.sync += z.create(a.s & b.s, b.e[0:8], b.m[3:-1])
 192
 193                 # if a is zero return b
 194                 with m.Elif(a.is_zero()):
 195                     m.next = "put_z"
 196                     m.d.sync += z.create(b.s, b.e[0:8], b.m[3:-1])
 197
 198                 # if b is zero return a
 199                 with m.Elif(b.is_zero()):
 200                     m.next = "put_z"
 201                     m.d.sync += z.create(a.s, a.e[0:8], a.m[3:-1])
 202
 203                 # Denormalised Number checks
 204                 with m.Else():
 205                     m.next = "align"
 206                     # denormalise a check
 207                     with m.If(a.e == a.N127):
 208                         m.d.sync += a.e.eq(-126) # limit a exponent
 209                     with m.Else():
 210                         m.d.sync += a.m[-1].eq(1) # set top mantissa bit
 211                     # denormalise b check
 212                     with m.If(b.e == a.N127):
 213                         m.d.sync += b.e.eq(-126) # limit b exponent
 214                     with m.Else():
 215                         m.d.sync += b.m[-1].eq(1) # set top mantissa bit
 216
 217             # ******
 218             # align.  NOTE: this does *not* do single-cycle multi-shifting,
 219             #         it *STAYS* in the align state until the exponents match
 220
 221             with m.State("align"):
 222                 # exponent of a greater than b: increment b exp, shift b mant
 223                 with m.If(a.e > b.e):
 224                     m.d.sync += b.shift_down()
 225                 # exponent of b greater than a: increment a exp, shift a mant
 226                 with m.Elif(a.e < b.e):
 227                     m.d.sync += a.shift_down()
 228                 # exponents equal: move to next stage.
 229                 with m.Else():
 230                     m.next = "add_0"
 231
 232             # ******
 233             # First stage of add.  covers same-sign (add) and subtract
 234             # special-casing when mantissas are greater or equal, to
 235             # give greatest accuracy.
 236
 237             with m.State("add_0"):
 238                 m.next = "add_1"
 239                 m.d.sync += z.e.eq(a.e)
 240                 # same-sign (both negative or both positive) add mantissas
 241                 with m.If(a.s == b.s):
 242                     m.d.sync += [
 243                         tot.eq(a.m + b.m),
 244                         z.s.eq(a.s)
 245                     ]
 246                 # a mantissa greater than b, use a
 247                 with m.Elif(a.m >= b.m):
 248                     m.d.sync += [
 249                         tot.eq(a.m - b.m),
 250                         z.s.eq(a.s)
 251                     ]
 252                 # b mantissa greater than a, use b
 253                 with m.Else():
 254                     m.d.sync += [
 255                         tot.eq(b.m - a.m),
 256                         z.s.eq(b.s)
 257                 ]
 258
 259             # ******
 260             # Second stage of add: preparation for normalisation.
 261             # detects when tot sum is too big (tot[27] is kinda a carry bit)
 262
 263             with m.State("add_1"):
 264                 m.next = "normalise_1"
 265                 # tot[27] gets set when the sum overflows. shift result down
 266                 with m.If(tot[27]):
 267                     m.d.sync += [
 268                         z.m.eq(tot[4:28]),
 269                         guard.eq(tot[3]),
 270                         round_bit.eq(tot[2]),
 271                         sticky.eq(tot[1] | tot[0]),
 272                         z.e.eq(z.e + 1)
 273                 ]
 274                 # tot[27] zero case
 275                 with m.Else():
 276                     m.d.sync += [
 277                         z.m.eq(tot[3:27]),
 278                         guard.eq(tot[2]),
 279                         round_bit.eq(tot[1]),
 280                         sticky.eq(tot[0])
 281                 ]
 282
 283             # ******
 284             # First stage of normalisation.
 285             # NOTE: just like "align", this one keeps going round every clock
 286             #       until the result's exponent is within acceptable "range"
 287             # NOTE: the weirdness of reassigning guard and round is due to
 288             #       the extra mantissa bits coming from tot[0..2]
 289
 290             with m.State("normalise_1"):
 291                 with m.If((z.m[-1] == 0) & (z.e > z.N126)):
 292                     m.d.sync +=[
 293                         z.e.eq(z.e - 1),  # DECREASE exponent
 294                         z.m.eq(z.m << 1), # shift mantissa UP
 295                         z.m[0].eq(guard), # steal guard bit (was tot[2])
 296                         guard.eq(round_bit), # steal round_bit (was tot[1])
 297                     ]
 298                 with m.Else():
 299                     m.next = "normalise_2"
 300
 301             # ******
 302             # Second stage of normalisation.
 303             # NOTE: just like "align", this one keeps going round every clock
 304             #       until the result's exponent is within acceptable "range"
 305             # NOTE: the weirdness of reassigning guard and round is due to
 306             #       the extra mantissa bits coming from tot[0..2]
 307
 308             with m.State("normalise_2"):
 309                 with m.If(z.e < z.N126):
 310                     m.d.sync +=[
 311                         z.e.eq(z.e + 1),  # INCREASE exponent
 312                         z.m.eq(z.m >> 1), # shift mantissa DOWN
 313                         guard.eq(z.m[0]),
 314                         round_bit.eq(guard),
 315                         sticky.eq(sticky | round_bit)
 316                     ]
 317                 with m.Else():
 318                     m.next = "round"
 319
 320             # ******
 321             # rounding stage
 322
 323             with m.State("round"):
 324                 m.next = "corrections"
 325                 with m.If(guard & (round_bit | sticky | z.m[0])):
 326                     m.d.sync += z.m.eq(z.m + 1) # mantissa rounds up
 327                     with m.If(z.m == z.m1s): # all 1s
 328                         m.d.sync += z.e.eq(z.e + 1) # exponent rounds up
 329
 330             # ******
 331             # correction stage
 332
 333             with m.State("corrections"):
 334                 m.next = "pack"
 335                 # denormalised, correct exponent to zero
 336                 with m.If(z.is_denormalised()):
 337                     m.d.sync += z.m.eq(-127)
 338                 # FIX SIGN BUG: -a + a = +0.
 339                 with m.If((z.e == z.N126) & (z.m[0:] == 0)):
 340                     m.d.sync += z.s.eq(0)
 341
 342             # ******
 343             # pack stage
 344
 345             with m.State("pack"):
 346                 m.next = "put_z"
 347                 # if overflow occurs, return inf
 348                 with m.If(z.is_overflowed()):
 349                     m.d.sync += z.inf(0)
 350                 with m.Else():
 351                     m.d.sync += z.create(z.s, z.e, z.m)
 352
 353             # ******
 354             # put_z stage
 355
 356             with m.State("put_z"):
 357               m.d.sync += [
 358                   self.out_z.stb.eq(1),
 359                   self.out_z.v.eq(z.v)
 360               ]
 361               with m.If(self.out_z.stb & self.out_z.ack):
 362                   m.d.sync += self.out_z.stb.eq(0)
 363                   m.next = "get_a"
 364
 365         return m
 366
 367
 368 if __name__ == "__main__":
 369     alu = FPADD(width=32)
 370     main(alu, ports=alu.in_a.ports() + alu.in_b.ports() + alu.out_z.ports())
 371
 372
 373     # works... but don't use, just do "python fname.py convert -t v"
 374     #print (verilog.convert(alu, ports=[
 375     #                        ports=alu.in_a.ports() + \
 376     #                              alu.in_b.ports() + \
 377     #                              alu.out_z.ports())