From cb0dc7b434db6c73837d2c4aaac91f771fa78c53 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Mon, 31 Jul 2017 22:01:54 -0400
Subject: [PATCH] I'm sure it's not going to be this easy, but this is a
 genuine attempt at full horizontal and vertical timing.

---
 Components/6845/CRTC6845.hpp | 87 +++++++++++++++++++++++++++++++++++-
 1 file changed, 85 insertions(+), 2 deletions(-)

diff --git a/Components/6845/CRTC6845.hpp b/Components/6845/CRTC6845.hpp
index 15e6b9201..289f25b1c 100644
--- a/Components/6845/CRTC6845.hpp
+++ b/Components/6845/CRTC6845.hpp
@@ -33,8 +33,82 @@ template <class T> class CRTC6845 {
 		void run_for(Cycles cycles) {
 			int cyles_remaining = cycles.as_int();
 			while(cyles_remaining--) {
-				// TODO: update state (!)
+				// check for end of horizontal sync
+				if(hsync_down_counter_) {
+					hsync_down_counter_--;
+					if(!hsync_down_counter_) {
+						bus_state_.hsync = false;
+					}
+				}
 
+				// advance horizontal counter
+				character_counter_++;
+
+				// check for start of horizontal sync
+				if(character_counter_ == registers_[2]) {
+					hsync_down_counter_ = registers_[3] & 15;
+					if(hsync_down_counter_) bus_state_.hsync = true;
+				}
+
+				// check for end of visible characters
+				if(character_counter_ == registers_[1]) {
+					character_is_visible_ = false;
+				}
+
+				// check for end-of-line
+				if(character_counter_ == registers_[0]+1) {
+					character_counter_ = 0;
+					character_is_visible_ = true;
+
+					// check for end of vertical sync
+					if(vsync_down_counter_) {
+						vsync_down_counter_--;
+						if(!vsync_down_counter_) {
+							bus_state_.vsync = false;
+						}
+					}
+
+					if(is_in_adjustment_period_) {
+						line_counter_++;
+						if(line_counter_ == registers_[5]) {
+							line_is_visible_ = true;
+							line_counter_ = 0;
+							is_in_adjustment_period_ = false;
+							line_address_ = (registers_[12] << 8) | registers_[13];
+							bus_state_.refresh_address = line_address_;
+						}
+					} else {
+						// advance vertical counter
+						bus_state_.row_address++;
+						if(bus_state_.row_address == registers_[9]) {
+							line_address_ = bus_state_.refresh_address;
+							bus_state_.row_address = 0;
+							line_counter_++;
+
+							// check for end of visible lines
+							if(line_counter_ == registers_[6]) {
+								line_is_visible_ = false;
+							}
+
+							// check for start of vertical sync
+							if(line_counter_ == registers_[7]) {
+								bus_state_.vsync = true;
+								vsync_down_counter_ = 16;	// TODO
+							}
+
+							// check for entry into the overflow area
+							if(line_counter_ == registers_[4]) {
+								is_in_adjustment_period_ = true;
+								bus_state_.row_address = 0;
+								line_counter_ = 0;
+							}
+						} else {
+							bus_state_.refresh_address = line_address_;
+						}
+					}
+				}
+
+				bus_state_.display_enable = line_is_visible_ && line_is_visible_;
 				bus_handler_.perform_bus_cycle(bus_state_);
 			}
 		}
@@ -53,7 +127,6 @@ template <class T> class CRTC6845 {
 
 		void set_register(uint8_t value) {
 			registers_[selected_register_] = value;
-			if(!selected_register_) printf("Horizontal total: %d\n", value);
 		}
 
 	private:
@@ -62,6 +135,16 @@ template <class T> class CRTC6845 {
 
 		uint8_t registers_[16];
 		int selected_register_;
+
+		uint8_t character_counter_;
+		uint8_t line_counter_;
+
+		bool character_is_visible_, line_is_visible_;
+
+		int hsync_down_counter_;
+		int vsync_down_counter_;
+		bool is_in_adjustment_period_;
+		uint16_t line_address_;
 };
 
 }