src/nmutil/byterev.py

   1 # SPDX-License-Identifier: LGPL-3-or-later
   2 """
   3     This work is funded through NLnet under Grant 2019-02-012
   4
   5     License: LGPLv3+
   6 """
   7
   8 from nmigen import Signal, Cat
   9
  10 # TODO: turn this into a module?
  11
  12
  13 def byte_reverse(m, name, data, length):
  14     """byte_reverse: unlike nmigen word_select this takes a dynamic length
  15
  16     nmigen Signal.word_select may only take a fixed length.  we need
  17     bigendian byte-reverse, half-word reverse, word and dword reverse.
  18
  19     This only outputs the first `length` bytes, higher bytes are zeroed.
  20     """
  21     comb = m.d.comb
  22     data_r = Signal.like(data, name=name)
  23
  24     # if length is a static integer, we do not require a Case statement
  25     if isinstance(length, int):
  26         j = length
  27         rev = []
  28         for i in range(j):
  29             dest = data_r.word_select(i, 8)
  30             rev.append(data.word_select(j-1-i, 8))
  31         comb += data_r.eq(Cat(*rev))
  32         return data_r
  33
  34     # Switch statement needed: dynamic length had better be = 1,2,4 or 8
  35     with m.Switch(length):
  36         for j in [1, 2, 4, 8]:
  37             with m.Case(j):
  38                 rev = []
  39                 for i in range(j):
  40                     rev.append(data.word_select(j-1-i, 8))
  41                 comb += data_r.eq(Cat(*rev))
  42     return data_r