From 8aa425c9d89b999016ce7f3e10febfec011a77c3 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Wed, 6 Nov 2019 23:25:36 -0500
Subject: [PATCH] Fixes medium resolution mode.

---
 Machines/AtariST/Video.cpp | 16 ++++++++++++----
 Machines/AtariST/Video.hpp |  5 ++++-
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/Machines/AtariST/Video.cpp b/Machines/AtariST/Video.cpp
index dfd930e1c..088f78f21 100644
--- a/Machines/AtariST/Video.cpp
+++ b/Machines/AtariST/Video.cpp
@@ -264,12 +264,20 @@ void Video::shift_out(int length) {
 						((output_shifter >> 63) & 1) |
 						((output_shifter >> 46) & 2)
 					];
-					output_shifter = (output_shifter << 1);// & 0xfeffffff;
+					// This ensures that the top two words shift one to the left;
+					// their least significant bits are fed from the most significant bits
+					// of the bottom two words, respectively.
+					shifter_halves[1] = (shifter_halves[1] << 1) & 0xfffefffe;
+					shifter_halves[1] |= (shifter_halves[0] & 0x80008000) >> 15;
+					shifter_halves[0] = (shifter_halves[0] << 1) & 0xfffefffe;
+
 					++pixel_buffer_.pixel_pointer;
 				}
 			} else {
 				while(length--) {
-					output_shifter = (output_shifter << 1);// & 0xfeffffff;
+					shifter_halves[1] = (shifter_halves[1] << 1) & 0xfffefffe;
+					shifter_halves[1] |= (shifter_halves[0] & 0x80008000) >> 15;
+					shifter_halves[0] = (shifter_halves[0] << 1) & 0xfffefffe;
 				}
 			}
 		break;
@@ -284,13 +292,13 @@ void Video::shift_out(int length) {
 						((output_shifter >> 29) & 4) |
 						((output_shifter >> 12) & 8)
 					];
-					output_shifter = (output_shifter << 1);// & 0xfefefefe;
+					output_shifter = (output_shifter << 1) & 0xfffefffefffefffe;
 					++pixel_buffer_.pixel_pointer;
 					length -= 2;
 				}
 			} else {
 				while(length) {
-					output_shifter = (output_shifter << 1);// & 0xfefefefe;
+					output_shifter = (output_shifter << 1) & 0xfffefffefffefffe;
 					length -= 2;
 				}
 			}
diff --git a/Machines/AtariST/Video.hpp b/Machines/AtariST/Video.hpp
index 413ade1d7..a2e4756c8 100644
--- a/Machines/AtariST/Video.hpp
+++ b/Machines/AtariST/Video.hpp
@@ -79,7 +79,10 @@ class Video {
 
 		int data_latch_position_ = 0;
 		uint16_t data_latch_[4];
-		uint64_t output_shifter;
+		union {
+			uint64_t output_shifter;
+			uint32_t shifter_halves[2];
+		};
 		void shift_out(int length);
 		void latch_word();