From de4e5c40aa85e0aae85e6e6eff3820946cc25c52 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Wed, 3 Oct 2018 23:28:33 -0400
Subject: [PATCH] Implements horizontal scrolling lock.

---
 Components/9918/9918.cpp                    | 26 +++++++++++----------
 Components/9918/Implementation/9918Base.hpp |  3 ++-
 2 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/Components/9918/9918.cpp b/Components/9918/9918.cpp
index baa98a251..b4815fd29 100644
--- a/Components/9918/9918.cpp
+++ b/Components/9918/9918.cpp
@@ -647,7 +647,7 @@ void Base::draw_tms_text(int start, int end) {
 }
 
 void Base::draw_sms(int start, int end) {
-	if(!start) {
+	if(!start && (row_ >= 16 || !master_system_.horizontal_scroll_lock)) {
 		pixel_target_ += master_system_.horizontal_scroll & 7;
 	}
 
@@ -655,7 +655,9 @@ void Base::draw_sms(int start, int end) {
 	int byte_column = start >> 3;
 	int pixels_left = end - start;
 	int length = std::min(pixels_left, 8 - shift);
+
 	uint32_t pattern = *reinterpret_cast<uint32_t *>(master_system_.tile_graphics[byte_column]);
+	uint8_t *const pattern_index = reinterpret_cast<uint8_t *>(&pattern);
 
 	if(master_system_.names[byte_column].flags&2)
 		pattern >>= shift;
@@ -664,25 +666,25 @@ void Base::draw_sms(int start, int end) {
 
 	while(true) {
 		pixels_left -= length;
-		const uint32_t palette_offset = static_cast<uint32_t>((master_system_.names[byte_column].flags&0x08) << 1);
+		const int palette_offset = (master_system_.names[byte_column].flags&0x08) << 1;
 		if(master_system_.names[byte_column].flags&2) {
 			for(int c = 0; c < length; ++c) {
-				const uint32_t value =
-					((pattern & 0x00000001) >> 0) |
-					((pattern & 0x00000100) >> 6) |
-					((pattern & 0x00010000) >> 15) |
-					((pattern & 0x01000000) >> 24) |
+				const int value =
+					((pattern_index[3] & 0x01) << 3) |
+					((pattern_index[2] & 0x01) << 2) |
+					((pattern_index[1] & 0x01) << 1) |
+					((pattern_index[0] & 0x01) << 0) |
 					palette_offset;
 				pixel_target_[c] = master_system_.colour_ram[value];
 				pattern >>= 1;
 			}
 		} else {
 			for(int c = 0; c < length; ++c) {
-				const uint32_t value =
-					((pattern & 0x00000080) >> 7) |
-					((pattern & 0x00008000) >> 14) |
-					((pattern & 0x00800000) >> 21) |
-					((pattern & 0x80000000) >> 28) |
+				const int value =
+					((pattern_index[3] & 0x80) >> 4) |
+					((pattern_index[2] & 0x80) >> 5) |
+					((pattern_index[1] & 0x80) >> 6) |
+					((pattern_index[0] & 0x80) >> 7) |
 					palette_offset;
 				pixel_target_[c] = master_system_.colour_ram[value];
 				pattern <<= 1;
diff --git a/Components/9918/Implementation/9918Base.hpp b/Components/9918/Implementation/9918Base.hpp
index 13177ecf3..de34e51b8 100644
--- a/Components/9918/Implementation/9918Base.hpp
+++ b/Components/9918/Implementation/9918Base.hpp
@@ -597,7 +597,7 @@ class Base {
 */
 
 #define fetch_tile_name(column)	{\
-		const size_t scrolled_column = (column - (master_system_.horizontal_scroll >> 3)) & 0x1f;\
+		const size_t scrolled_column = (column - horizontal_offset) & 0x1f;\
 		const size_t address = pattern_address_base + (scrolled_column << 1);	\
 		master_system_.names[column].flags = ram_[address+1];	\
 		master_system_.names[column].offset = static_cast<size_t>(	\
@@ -634,6 +634,7 @@ class Base {
 			const int scrolled_row = (row_ + master_system_.vertical_scroll) % 224;
 			const size_t pattern_address_base = (pattern_name_address_ | size_t(0x3ff)) & static_cast<size_t>(((scrolled_row & ~7) << 3) | 0x3800);
 			const size_t sub_row[2] = {static_cast<size_t>((scrolled_row & 7) << 2), 28 ^ static_cast<size_t>((scrolled_row & 7) << 2)};
+			const int horizontal_offset = (row_ >= 16 || !master_system_.horizontal_scroll_lock) ? (master_system_.horizontal_scroll >> 3) : 0;
 
 			/*
 				To add, relative to the times below: