diff --git a/Machines/PCCompatible/PCCompatible.cpp b/Machines/PCCompatible/PCCompatible.cpp
index 4bb5c17c1..95f7a3f72 100644
--- a/Machines/PCCompatible/PCCompatible.cpp
+++ b/Machines/PCCompatible/PCCompatible.cpp
@@ -295,6 +295,7 @@ class IO {
 				case 0x0064:	case 0x0065:	case 0x0066:	case 0x0067:
 				case 0x0068:	case 0x0069:	case 0x006a:	case 0x006b:
 				case 0x006c:	case 0x006d:	case 0x006e:	case 0x006f:
+					// Likely to be helpful: https://github.com/tmk/tmk_keyboard/wiki/IBM-PC-XT-Keyboard-Protocol
 					printf("TODO: PPI write of %02x at %04x\n", value, port);
 				break;