Merge pull request #249 from MahirAbbas/master

Add Plru Docs

Author: Dolu1990

source/SpinalHDL/Libraries/Misc/Plru.rst

@@ -0,0 +1,68 @@
+.. role:: raw-html-m2r(raw)
+   :format: html
+
+Plru
+==========================
+
+Introduction
+--------------------
+- Pseudo least recently used combinatorial logic
+- io.context.state need to be handled externaly.
+- 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
+
+.. code-block:: scala
+
+   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 
+
+.. code-block:: scala
+
+   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
+
+.. code-block:: scala
+
+   val replacedWay = plru.io.evict.id
+
+Update recently used way
+
+.. code-block:: scala
+
+   plru.update.id := refillWay
+
+
+
+