More keys. Implemented Border. Completes LOAD readiness sequence

Core/Io/IO_Bus.cs

@@ -5,8 +5,10 @@ namespace Core.Io
 {
     public class IO_Bus
 {
-    // 8 rows representing the Spectrum keyboard matrix. Default to 0xFF (unpressed).
-    public byte[] KeyboardRows = new byte[8] { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
+        public byte BorderColorIndex { get; private set; } = 7; // 7 is White
+
+        // 8 rows representing the Spectrum keyboard matrix. Default to 0xFF (unpressed).
+        public byte[] KeyboardRows = new byte[8] { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
 
     public byte ReadPort(ushort portAddress)
     {
@@ -35,9 +37,17 @@ namespace Core.Io
         return 0xFF; 
     }
 
-    public void WritePort(ushort portAddress, byte portValue)
+        public void WritePort(ushort portAddress, byte portValue)
         {
+            // The ULA intercepts any write to an even port address
+            if ((portAddress & 0x01) == 0)
+            {
+                // The bottom 3 bits (0-2) define the border color!
+                BorderColorIndex = (byte)(portValue & 0x07);
 
+                // (Bits 3 and 4 handle the cassette MIC output and the internal speaker, 
+                // which we will need when we start playing audio!)
+            }
         }
-}
+    }
 }