src/soc/fu/shift_rot/maskgen.py

   1 from nmigen import (Elaboratable, Signal, Module)
   2 import math
   3 # Copyright (C) 2020 Michael Nolan <mtnolan2640@gmail.com>
   4
   5 class MaskGen(Elaboratable):
   6     """MaskGen - create a diff mask
   7
   8     example: x=5 --> a=0b11111
   9              y=3 --> b=0b00111
  10              o:        0b11000
  11              x=2 --> a=0b00011
  12              y=4 --> b=0b01111
  13              o:        0b10011
  14     """
  15     def __init__(self, width):
  16         self.width = width
  17         self.shiftwidth = math.ceil(math.log2(width))
  18         self.mb = Signal(self.shiftwidth, reset_less=True)
  19         self.me = Signal(self.shiftwidth, reset_less=True)
  20
  21         self.o = Signal(width, reset_less=True)
  22
  23     def elaborate(self, platform):
  24         m = Module()
  25         comb = m.d.comb
  26
  27         x = Signal.like(self.mb)
  28         y = Signal.like(self.mb)
  29
  30         comb += x.eq(64 - self.mb)
  31         comb += y.eq(63 - self.me)
  32
  33         mask_a = Signal.like(self.o)
  34         mask_b = Signal.like(self.o)
  35
  36         comb += mask_a.eq((1<<x) - 1)
  37         comb += mask_b.eq((1<<y) - 1)
  38
  39         with m.If(x > y):
  40             comb += self.o.eq(mask_a ^ mask_b)
  41         with m.Else():
  42             comb += self.o.eq(mask_a ^ ~mask_b)
  43
  44
  45         return m
  46
  47     def ports(self):
  48         return [self.mb, self.me, self.o]