From be8e7a41448315b46f1a63a225d363d124399c4f Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 10 Aug 2017 11:22:30 -0400
Subject: [PATCH] Eliminated false register aliasing, restricted register sizes
 and locked out reading and writing where appropriate.

---
 Components/6845/CRTC6845.hpp | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/Components/6845/CRTC6845.hpp b/Components/6845/CRTC6845.hpp
index 15a50e6ba..0f3d1e768 100644
--- a/Components/6845/CRTC6845.hpp
+++ b/Components/6845/CRTC6845.hpp
@@ -138,7 +138,7 @@ template <class T> class CRTC6845 {
 		}
 
 		void select_register(uint8_t r) {
-			selected_register_ = (int)r & 15;
+			selected_register_ = r;
 		}
 
 		uint8_t get_status() {
@@ -146,18 +146,25 @@ template <class T> class CRTC6845 {
 		}
 
 		uint8_t get_register() {
+			if(selected_register_ < 12 || selected_register_ > 17) return 0xff;
 			return registers_[selected_register_];
 		}
 
 		void set_register(uint8_t value) {
-			registers_[selected_register_] = value;
+			static uint8_t masks[] = {
+				0xff, 0xff, 0xff, 0xff, 0x7f, 0x1f, 0x7f, 0x7f,
+				0xff, 0x1f, 0x7f, 0x1f, 0x3f, 0xff, 0x3f, 0xff
+			};
+
+			if(selected_register_ < 16)
+				registers_[selected_register_] = value & masks[selected_register_];
 		}
 
 	private:
 		T &bus_handler_;
 		BusState bus_state_;
 
-		uint8_t registers_[16];
+		uint8_t registers_[18];
 		int selected_register_;
 
 		uint8_t character_counter_;