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Plru

Introduction

• Pseudo least recently used combinatorial logic

• io.context.state need to be handled externally.

• When you want to specify a access to a entry, you can use the io.update interface to get the new state value.

• plru.io.evict.id tells you the id of the next block to be evicted

• plru.io.update.id lets you update what you recently used

PLRU Code

 val io = new Bundle{
  val context = new Bundle{
    //user -> plru, specify the current state
    val state = Plru.State(entries)
    //user -> plru, allow to specify prefered entries to remove. each bit set mean : "i would prefer that way to not to be selected by PLRU"
    val valids = withEntriesValid generate Bits(entries bits)
  }
  val evict = new Bundle{
    //PLRU -> user, Tells you the least recently used entry for the given context provided above
    val id =  UInt(log2Up(entries) bits)
  }
  val update = new Bundle{
    // user -> PLRU specify which entry the user want to mark as most recently used
    val id = UInt(log2Up(entries) bits)
   // PLRU -> user specfy what should then be the new value of the PLRU status
    val state = Plru.State(entries)
  }
}

Example usage in a cache

val plru = new Area {
   // Define a Mem, to track the state of each set
   val ram = Mem.fill(nSets)(Plru.State(wayCount))
   val write = ram.writePort
   val fromLoad, fromStore = cloneOf(write)
   write.valid := fromLoad.valid || fromStore.valid
   write.payload := fromLoad.valid.mux(fromLoad.payload, fromStore.payload)
}

Get the ID of the way to evict from

val replacedWay = plru.io.evict.id

Update recently used way

plru.update.id := refillWay