src/soc/fu/shift_rot/maskgen.py

   1 from nmigen import (Elaboratable, Signal, Module)
   2 import math
   3
   4 class MaskGen(Elaboratable):
   5     """MaskGen - create a diff mask
   6
   7     example: x=5 --> a=0b11111
   8              y=3 --> b=0b00111
   9              o:        0b11000
  10              x=2 --> a=0b00011
  11              y=4 --> b=0b01111
  12              o:        0b10011
  13     """
  14     def __init__(self, width):
  15         self.width = width
  16         self.shiftwidth = math.ceil(math.log2(width))
  17         self.mb = Signal(self.shiftwidth, reset_less=True)
  18         self.me = Signal(self.shiftwidth, reset_less=True)
  19
  20         self.o = Signal(width, reset_less=True)
  21
  22     def elaborate(self, platform):
  23         m = Module()
  24         comb = m.d.comb
  25
  26         x = Signal.like(self.mb)
  27         y = Signal.like(self.mb)
  28
  29         comb += x.eq(64 - self.mb)
  30         comb += y.eq(63 - self.me)
  31
  32         mask_a = Signal.like(self.o)
  33         mask_b = Signal.like(self.o)
  34
  35         comb += mask_a.eq((1<<x) - 1)
  36         comb += mask_b.eq((1<<y) - 1)
  37
  38         with m.If(x > y):
  39             comb += self.o.eq(mask_a ^ mask_b)
  40         with m.Else():
  41             comb += self.o.eq(mask_a ^ ~mask_b)
  42
  43
  44         return m
  45
  46     def ports(self):
  47         return [self.mb, self.me, self.o]