beginning of a linked list in hardware

bs/TagEngine.bs

@@ -9,9 +9,15 @@ import Util
 
 data Tag numTags
     = Next UIntLog2N(numTags)
+    | Freed
     | Tail
     deriving (Bits, Eq, FShow)
 
+data TagPtr numTags
+    = SomeTagPtr UIntLog2N(numTags)
+    | None
+    deriving (Bits, Eq, FShow)
+
 interface (TagEngine :: # -> *) numTags =
     requestTag :: ActionValue UIntLog2N(numTags)
     retireTag  :: UIntLog2N(numTags) -> Action
@@ -32,10 +38,10 @@ initTagTail = do
     return t
 
 initTagVec :: Module(Vector numTags (Reg (Tag numTags)))
-initTagVec = 
+initTagVec =
     let
         lastIdx :: Integer = (fromInteger |> valueOf numTags) - 1
-        initByIdx currIdx = 
+        initByIdx currIdx =
             if (currIdx < lastIdx)
                 then initTagNext (currIdx + 1)
                 else initTagTail
@@ -48,6 +54,9 @@ mkTagEngine =
         tagVec :: Vector numTags (Reg (Tag numTags))
         tagVec <- initTagVec
 
+        head :: Reg(TagPtr(numTags)) <- mkReg(SomeTagPtr(0))
+        tail :: Reg(TagPtr(numTags)) <- mkReg(SomeTagPtr(lastIdx))
+
         debugOnce <- mkReg True
 
         addRules $
@@ -61,11 +70,28 @@ mkTagEngine =
         return $
           interface TagEngine
             requestTag :: ActionValue UIntLog2N(numTags)
-            requestTag = do
-                -- placeholder
-                counter := counter + 1
-                return counter
+            requestTag =
+                do
+                    let currHeadPtr :: UIntLog2N(numTags) =
+                            case head of
+                                SomeTagPtr ptr -> ptr
+                                -- we technically will never hit this arm
+                                -- due to when guard `SomeTagPtr ptr <- head`
+                                None -> 0
+                    let currHead :: (Reg (Tag numTags)) = (select tagVec currHeadPtr)
+                    let nextHeadPtr :: UIntLog2N(numTags) =
+                            case currHead of
+                                Next ptr -> ptr
+                                -- TODO : handle tail correctly
+                                Tail -> 0
+                    currHead := Freed
+                    head := SomeTagPtr(nextHeadPtr)
+                    return nextHeadPtr
+                when
+                    SomeTagPtr ptr <- head
             retireTag :: UIntLog2N(numTags) -> Action
             retireTag tag = do
                 -- placeholder
                 counter := 0
+    where
+        lastIdx :: UIntLog2N(numTags) = (fromInteger |> valueOf numTags) - 1