TLROM: use the smallest ROM implementation that works

The contents everywhere else are still zero.

src/main/scala/uncore/devices/Rom.scala

@@ -30,20 +30,22 @@ class TLROM(val base: BigInt, val size: Int, contentsDelayed: => Seq[Byte], exec
     }
 
     val contents = contentsDelayed
-    require (contents.size <= size)
+    val wrapSize = 1 << log2Ceil(contents.size)
+    require (wrapSize <= size)
 
     val in = io.in(0)
     val edge = node.edgesIn(0)
 
-    val words = (contents ++ Seq.fill(size-contents.size)(0.toByte)).grouped(beatBytes).toSeq
+    val words = (contents ++ Seq.fill(wrapSize-contents.size)(0.toByte)).grouped(beatBytes).toSeq
     val bigs = words.map(_.foldRight(BigInt(0)){ case (x,y) => (x.toInt & 0xff) | y << 8})
     val rom = Vec(bigs.map(x => UInt(x, width = 8*beatBytes)))
 
     in.d.valid := in.a.valid
     in.a.ready := in.d.ready
 
-    val index = in.a.bits.address(log2Ceil(size)-1,log2Ceil(beatBytes))
-    in.d.bits := edge.AccessAck(in.a.bits, UInt(0), rom(index))
+    val index = in.a.bits.address(log2Ceil(wrapSize)-1,log2Ceil(beatBytes))
+    val high = if (wrapSize == size) UInt(0) else in.a.bits.address(log2Ceil(size)-1, log2Ceil(wrapSize))
+    in.d.bits := edge.AccessAck(in.a.bits, UInt(0), Mux(high.orR, UInt(0), rom(index)))
 
     // Tie off unused channels
     in.b.valid := Bool(false)