src/soc/experiment/plru.py

   1 # based on microwatt plru.vhdl
   2
   3 from nmigen import Elaboratable, Signal, Array, Module
   4
   5 class PLRU(Elaboratable):
   6
   7     def __init__(self, BITS=2):
   8         self.BITS = BITS
   9         self.acc = Signal(BITS)
  10         self.acc_en = Signal()
  11         self.lru_o = Signal(BITS)
  12
  13     def elaborate(self, platform):
  14         m = Module()
  15         comb, sync = m.d.comb, m.d.sync
  16
  17         tree = Array(Signal() for i in range(self.BITS))
  18
  19         # XXX Check if we can turn that into a little ROM instead that
  20         # takes the tree bit vector and returns the LRU. See if it's better
  21         # in term of FPGA resouces usage...
  22         node = Signal(self.BITS)
  23         for i in range(self.BITS):
  24             node_next = Signal(self.BITS)
  25             node2 = Signal(self.BITS)
  26             # report "GET: i:" & integer'image(i) & " node:" &
  27             # integer'image(node) & " val:" & Signal()'image(tree(node))
  28             comb += self.lru_o[self.BITS-1-i].eq(tree[node])
  29             if i != self.BITS-1:
  30                 comb += node2.eq(node << 1)
  31             else:
  32                 comb += node2.eq(node)
  33             with m.If(tree[node]):
  34                 comb += node_next.eq(node2 + 2)
  35             with m.Else():
  36                 comb += node_next.eq(node2 + 1)
  37             node = node_next
  38
  39         with m.If(self.acc_en):
  40             node = Signal(self.BITS)
  41             for i in range(self.BITS):
  42                 node_next = Signal(self.BITS)
  43                 node2 = Signal(self.BITS)
  44                 # report "GET: i:" & integer'image(i) & " node:" &
  45                 # integer'image(node) & " val:" & Signal()'image(tree(node))
  46                 abit = self.acc[self.BITS-1-i]
  47                 sync += tree[node].eq(~abit)
  48                 if i != self.BITS-1:
  49                     comb += node2.eq(node << 1)
  50                 else:
  51                     comb += node2.eq(node)
  52                 with m.If(abit):
  53                     comb += node_next.eq(node2 + 2)
  54                 with m.Else():
  55                     comb += node_next.eq(node2 + 1)
  56                 node = node_next
  57
  58         return m