From 9601c69e1293dc74d598801ac298985472140d5a Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Mon, 4 Dec 2023 22:02:38 -0500
Subject: [PATCH] Implement blinking text.

---
 Components/6845/CRTC6845.hpp           | 22 ++++++++++++----------
 Machines/PCCompatible/PCCompatible.cpp |  6 +++++-
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/Components/6845/CRTC6845.hpp b/Components/6845/CRTC6845.hpp
index eb85dabf6..8c39cdbee 100644
--- a/Components/6845/CRTC6845.hpp
+++ b/Components/6845/CRTC6845.hpp
@@ -23,6 +23,10 @@ struct BusState {
 	bool cursor = false;
 	uint16_t refresh_address = 0;
 	uint16_t row_address = 0;
+
+	// Not strictly part of the bus state; provided because the partition between 6845 and bus handler
+	// doesn't quite hold up in some emulated systems where the two are integrated and share more state.
+	int field_count = 0;
 };
 
 class BusHandler {
@@ -262,13 +266,15 @@ template <class T, CursorType cursor_type> class CRTC6845 {
 				is_cursor_line_ |= bus_state_.row_address == (registers_[10] & 0x1f);
 
 				switch(cursor_type) {
-					// MDA-style blinking; timings are a bit of a guess for now.
+					// MDA-style blinking.
+					// https://retrocomputing.stackexchange.com/questions/27803/what-are-the-blinking-rates-of-the-caret-and-of-blinking-text-on-pc-graphics-car
+					// gives an 8/8 pattern for regular blinking though mode 11 is then just a guess.
 					case CursorType::MDA:
 						switch(registers_[10] >> 5) {
-							case 0b11: is_cursor_line_ &= (field_count_ & 15) < 5;	break;
-							case 0b00: is_cursor_line_ &= bool(field_count_ & 16);	break;
-							case 0b01: is_cursor_line_ = false;						break;
-							case 0b10: is_cursor_line_ = true;						break;
+							case 0b11: is_cursor_line_ &= (bus_state_.field_count & 8) < 3;	break;
+							case 0b00: is_cursor_line_ &= bool(bus_state_.field_count & 8);	break;
+							case 0b01: is_cursor_line_ = false;								break;
+							case 0b10: is_cursor_line_ = true;								break;
 							default: break;
 						}
 					break;
@@ -281,9 +287,7 @@ template <class T, CursorType cursor_type> class CRTC6845 {
 			line_is_visible_ = true;
 			line_address_ = uint16_t((registers_[12] << 8) | registers_[13]);
 			bus_state_.refresh_address = line_address_;
-			if constexpr (cursor_type != CursorType::None) {
-				++field_count_;
-			}
+			++bus_state_.field_count;
 		}
 
 		Personality personality_;
@@ -311,8 +315,6 @@ template <class T, CursorType cursor_type> class CRTC6845 {
 		unsigned int character_is_visible_shifter_ = 0;
 
 		bool is_cursor_line_ = false;
-
-		int field_count_ = 0;
 };
 
 }
diff --git a/Machines/PCCompatible/PCCompatible.cpp b/Machines/PCCompatible/PCCompatible.cpp
index 2012da767..7e8530bf6 100644
--- a/Machines/PCCompatible/PCCompatible.cpp
+++ b/Machines/PCCompatible/PCCompatible.cpp
@@ -595,7 +595,11 @@ class MDA {
 								break;
 							}
 
-							// TODO: blink.
+							// Apply blink if enabled.
+							if((control_ & 0x20) && (attributes & 0x80) && (state.field_count & 16)) {
+								row ^= 0xff;
+								blank = (blank == off) ? intensity : off;
+							}
 
 							if(((attributes & 7) == 1) && state.row_address == 13) {
 								// Draw as underline.