1 # IEEE Floating Point Adder (Single Precision)
2 # Copyright (C) Jonathan P Dawson 2013
5 from nmigen
import Module
, Signal
, Cat
, Const
6 from nmigen
.cli
import main
, verilog
10 """ Floating-point Number Class, variable-width TODO (currently 32-bit)
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)
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])
21 def __init__(self
, width
, m_width
=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
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))
38 """ decodes a latched value into sign / exponent / mantissa
40 bias is subtracted here, from the exponent. exponent
41 is extended to 10 bits so that subtract 127 is done on
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
50 def create(self
, s
, e
, m
):
51 """ creates a value from sign / exponent / mantissa
53 bias is added here, to the exponent
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
62 """ shifts a mantissa down by one. exponent is increased to compensate
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)
67 return [self
.e
.eq(self
.e
+ 1),
68 self
.m
.eq(Cat(self
.m
[0] | self
.m
[1], self
.m
[2:], 0))
72 return self
.create(s
, self
.P128
, 1<<22)
75 return self
.create(s
, self
.P128
, 0)
78 return self
.create(s
, self
.N127
, 0)
81 return (self
.e
== self
.P128
) & (self
.m
!= 0)
84 return (self
.e
== self
.P128
) & (self
.m
== 0)
87 return (self
.e
== self
.N127
) & (self
.m
== self
.mzero
)
89 def is_overflowed(self
):
90 return (self
.e
> self
.P127
)
92 def is_denormalised(self
):
93 return (self
.e
== self
.N126
) & (self
.m
[23] == 0)
97 def __init__(self
, width
):
100 self
.in_a
= Signal(width
)
101 self
.in_a_stb
= Signal()
102 self
.in_a_ack
= Signal()
104 self
.in_b
= Signal(width
)
105 self
.in_b_stb
= Signal()
106 self
.in_b_ack
= Signal()
108 self
.out_z
= Signal(width
)
109 self
.out_z_stb
= Signal()
110 self
.out_z_ack
= Signal()
112 def get_fragment(self
, platform
=None):
116 a
= FPNum(self
.width
)
117 b
= FPNum(self
.width
)
118 z
= FPNum(self
.width
, 24)
120 tot
= Signal(28) # sticky/round/guard bits, 23 result, 1 overflow
122 guard
= Signal() # tot[2]
123 round_bit
= Signal() # tot[1]
124 sticky
= Signal() # tot[0]
131 with m
.State("get_a"):
132 with m
.If((self
.in_a_ack
) & (self
.in_a_stb
)):
139 m
.d
.sync
+= self
.in_a_ack
.eq(1)
144 with m
.State("get_b"):
145 with m
.If((self
.in_b_ack
) & (self
.in_b_stb
)):
152 m
.d
.sync
+= self
.in_b_ack
.eq(1)
155 # unpacks operands into sign, mantissa and exponent
157 with m
.State("unpack"):
158 m
.next
= "special_cases"
159 m
.d
.sync
+= a
.decode()
160 m
.d
.sync
+= b
.decode()
163 # special cases: NaNs, infs, zeros, denormalised
165 with m
.State("special_cases"):
167 # if a is NaN or b is NaN return NaN
168 with m
.If(a
.is_nan() | b
.is_nan()):
172 # if a is inf return inf (or NaN)
173 with m
.Elif(a
.is_inf()):
175 m
.d
.sync
+= z
.inf(a
.s
)
176 # if a is inf and signs don't match return NaN
177 with m
.If((b
.e
== b
.P128
) & (a
.s
!= b
.s
)):
178 m
.d
.sync
+= z
.nan(b
.s
)
180 # if b is inf return inf
181 with m
.Elif(b
.is_inf()):
183 m
.d
.sync
+= z
.inf(b
.s
)
185 # if a is zero and b zero return signed-a/b
186 with m
.Elif(a
.is_zero() & b
.is_zero()):
188 m
.d
.sync
+= z
.create(a
.s
& b
.s
, b
.e
[0:8], b
.m
[3:-1])
190 # if a is zero return b
191 with m
.Elif(a
.is_zero()):
193 m
.d
.sync
+= z
.create(b
.s
, b
.e
[0:8], b
.m
[3:-1])
195 # if b is zero return a
196 with m
.Elif(b
.is_zero()):
198 m
.d
.sync
+= z
.create(a
.s
, a
.e
[0:8], a
.m
[3:-1])
200 # Denormalised Number checks
203 # denormalise a check
204 with m
.If(a
.e
== a
.N127
):
205 m
.d
.sync
+= a
.e
.eq(-126) # limit a exponent
207 m
.d
.sync
+= a
.m
[-1].eq(1) # set top mantissa bit
208 # denormalise b check
209 with m
.If(b
.e
== a
.N127
):
210 m
.d
.sync
+= b
.e
.eq(-126) # limit b exponent
212 m
.d
.sync
+= b
.m
[-1].eq(1) # set top mantissa bit
215 # align. NOTE: this does *not* do single-cycle multi-shifting,
216 # it *STAYS* in the align state until the exponents match
218 with m
.State("align"):
219 # exponent of a greater than b: increment b exp, shift b mant
220 with m
.If(a
.e
> b
.e
):
221 m
.d
.sync
+= b
.shift_down()
222 # exponent of b greater than a: increment a exp, shift a mant
223 with m
.Elif(a
.e
< b
.e
):
224 m
.d
.sync
+= a
.shift_down()
225 # exponents equal: move to next stage.
230 # First stage of add. covers same-sign (add) and subtract
231 # special-casing when mantissas are greater or equal, to
232 # give greatest accuracy.
234 with m
.State("add_0"):
236 m
.d
.sync
+= z
.e
.eq(a
.e
)
237 # same-sign (both negative or both positive) add mantissas
238 with m
.If(a
.s
== b
.s
):
243 # a mantissa greater than b, use a
244 with m
.Elif(a
.m
>= b
.m
):
249 # b mantissa greater than a, use b
257 # Second stage of add: preparation for normalisation.
258 # detects when tot sum is too big (tot[27] is kinda a carry bit)
260 with m
.State("add_1"):
261 m
.next
= "normalise_1"
262 # tot[27] gets set when the sum overflows. shift result down
267 round_bit
.eq(tot
[2]),
268 sticky
.eq(tot
[1] | tot
[0]),
276 round_bit
.eq(tot
[1]),
281 # First stage of normalisation.
282 # NOTE: just like "align", this one keeps going round every clock
283 # until the result's exponent is within acceptable "range"
284 # NOTE: the weirdness of reassigning guard and round is due to
285 # the extra mantissa bits coming from tot[0..2]
287 with m
.State("normalise_1"):
288 with m
.If((z
.m
[-1] == 0) & (z
.e
> z
.N126
)):
290 z
.e
.eq(z
.e
- 1), # DECREASE exponent
291 z
.m
.eq(z
.m
<< 1), # shift mantissa UP
292 z
.m
[0].eq(guard
), # steal guard bit (was tot[2])
293 guard
.eq(round_bit
), # steal round_bit (was tot[1])
296 m
.next
= "normalise_2"
299 # Second stage of normalisation.
300 # NOTE: just like "align", this one keeps going round every clock
301 # until the result's exponent is within acceptable "range"
302 # NOTE: the weirdness of reassigning guard and round is due to
303 # the extra mantissa bits coming from tot[0..2]
305 with m
.State("normalise_2"):
306 with m
.If(z
.e
< z
.N126
):
308 z
.e
.eq(z
.e
+ 1), # INCREASE exponent
309 z
.m
.eq(z
.m
>> 1), # shift mantissa DOWN
312 sticky
.eq(sticky | round_bit
)
320 with m
.State("round"):
321 m
.next
= "corrections"
322 with m
.If(guard
& (round_bit | sticky | z
.m
[0])):
323 m
.d
.sync
+= z
.m
.eq(z
.m
+ 1) # mantissa rounds up
324 with m
.If(z
.m
== z
.m1s
): # all 1s
325 m
.d
.sync
+= z
.e
.eq(z
.e
+ 1) # exponent rounds up
330 with m
.State("corrections"):
332 # denormalised, correct exponent to zero
333 with m
.If(z
.is_denormalised()):
334 m
.d
.sync
+= z
.m
.eq(-127)
335 # FIX SIGN BUG: -a + a = +0.
336 with m
.If((z
.e
== z
.N126
) & (z
.m
[0:] == 0)):
337 m
.d
.sync
+= z
.s
.eq(0)
342 with m
.State("pack"):
344 # if overflow occurs, return inf
345 with m
.If(z
.is_overflowed()):
348 m
.d
.sync
+= z
.create(z
.s
, z
.e
, z
.m
)
353 with m
.State("put_z"):
355 self
.out_z_stb
.eq(1),
358 with m
.If(self
.out_z_stb
& self
.out_z_ack
):
359 m
.d
.sync
+= self
.out_z_stb
.eq(0)
365 if __name__
== "__main__":
366 alu
= FPADD(width
=32)
368 alu
.in_a
, alu
.in_a_stb
, alu
.in_a_ack
,
369 alu
.in_b
, alu
.in_b_stb
, alu
.in_b_ack
,
370 alu
.out_z
, alu
.out_z_stb
, alu
.out_z_ack
,
374 # works... but don't use, just do "python fname.py convert -t v"
375 #print (verilog.convert(alu, ports=[
376 # alu.in_a, alu.in_a_stb, alu.in_a_ack,
377 # alu.in_b, alu.in_b_stb, alu.in_b_ack,
378 # alu.out_z, alu.out_z_stb, alu.out_z_ack,