From 28550c0227fd68890bf6ebfdb27a37bad10c5954 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 26 Aug 2017 13:56:23 -0400
Subject: [PATCH] Breaks the 6845 bus cycle into a phase 1 and a phase 2 per
 the belief that sync line changes, which are observable, happen at the end of
 the first phase rather than at the beginning of the next. This may have
 interrupt timing effects, as machines often derive an interrupt from sync.

---
 Components/6845/CRTC6845.hpp | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/Components/6845/CRTC6845.hpp b/Components/6845/CRTC6845.hpp
index 2f4a76dad..f8a60e17c 100644
--- a/Components/6845/CRTC6845.hpp
+++ b/Components/6845/CRTC6845.hpp
@@ -28,7 +28,19 @@ struct BusState {
 
 class BusHandler {
 	public:
-		void perform_bus_cycle(const BusState &) {}
+		/*!
+			Performs the first phase of a 6845 bus cycle; this is the phase in which it is intended that
+			systems using the 6845 respect the bus state and produce pixels, sync or whatever they require.
+		*/
+		void perform_bus_cycle_phase1(const BusState &) {}
+
+		/*!
+			Performs the second phase of a 6845 bus cycle. Some bus state — including sync — is updated
+			directly after phase 1 and hence is visible to an observer during phase 2. Handlers may therefore
+			implement @c perform_bus_cycle_phase2 to be notified of the availability of that state without
+			having to wait until the next cycle has begun.
+		*/
+		void perform_bus_cycle_phase2(const BusState &) {}
 };
 
 enum Personality {
@@ -94,8 +106,8 @@ template <class T> class CRTC6845 {
 					end_of_line_address_ = bus_state_.refresh_address;
 				}
 
-				perform_bus_cycle();
-				bus_state_.refresh_address++;
+				perform_bus_cycle_phase1();
+				bus_state_.refresh_address = (bus_state_.refresh_address + 1) & 0x3fff;
 
 				// check for end-of-line
 				if(character_counter_ == registers_[0]) {
@@ -106,6 +118,8 @@ template <class T> class CRTC6845 {
 					// increment counter
 					character_counter_++;
 				}
+
+				perform_bus_cycle_phase2();
 			}
 		}
 
@@ -114,10 +128,14 @@ template <class T> class CRTC6845 {
 		}
 
 	private:
-		inline void perform_bus_cycle() {
+		inline void perform_bus_cycle_phase1() {
 			bus_state_.display_enable = character_is_visible_ && line_is_visible_;
-			bus_state_.refresh_address &= 0x3fff;
-			bus_handler_.perform_bus_cycle(bus_state_);
+			bus_handler_.perform_bus_cycle_phase1(bus_state_);
+		}
+
+		inline void perform_bus_cycle_phase2() {
+			bus_state_.display_enable = character_is_visible_ && line_is_visible_;
+			bus_handler_.perform_bus_cycle_phase2(bus_state_);
 		}
 
 		inline void do_end_of_line() {