From 4f5a38b5c52e4146e5e122b802be5118fe834de4 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sun, 5 May 2019 23:05:24 -0400
Subject: [PATCH] Adds support for the alternate video buffer.

---
 Machines/Apple/Macintosh/Macintosh.cpp | 20 ++++++++++++--------
 Machines/Apple/Macintosh/Video.cpp     | 11 +++++++----
 Machines/Apple/Macintosh/Video.hpp     |  2 ++
 3 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/Machines/Apple/Macintosh/Macintosh.cpp b/Machines/Apple/Macintosh/Macintosh.cpp
index e498fadef..09e0ee0dd 100644
--- a/Machines/Apple/Macintosh/Macintosh.cpp
+++ b/Machines/Apple/Macintosh/Macintosh.cpp
@@ -189,6 +189,10 @@ class ConcreteMachine:
 			ROM_is_overlay_ = rom_is_overlay;
 		}
 
+		void set_use_alternate_screen_buffer(bool use_alternate_screen_buffer) {
+			video_.set_use_alternate_screen_buffer(use_alternate_screen_buffer);
+		}
+
 	private:
 		class VIAPortHandler: public MOS::MOS6522::PortHandler {
 			public:
@@ -213,8 +217,9 @@ class ConcreteMachine:
 									b3:	0 = use alternate sound buffer, 1 = use ordinary sound buffer
 									b2–b0:	audio output volume
 							*/
-							printf(" w A: %02x", value);
+							printf("6522 A: %02x\n", value);
 							machine_.set_rom_is_overlay(!!(value & 0x10));
+							machine_.set_use_alternate_screen_buffer(!(value & 0x40));
 						break;
 
 						case Port::B:
@@ -229,7 +234,7 @@ class ConcreteMachine:
 									b1:	clock's data-clock line
 									b0:	clock's serial data line
 							*/
-							printf(" w B: %02x", value);
+							printf("6522 B: %02x\n", value);
 						break;
 					}
 				}
@@ -237,18 +242,17 @@ class ConcreteMachine:
 				uint8_t get_port_input(Port port) {
 					switch(port) {
 						case Port::A:
-							printf(" r A");
-						break;
+							printf("6522 r A\n");
+						return 0xff;
 
 						case Port::B:
-							printf(" r B");
-						break;
+							printf("6522 r B\n");
+						return 0x00;
 					}
-					return 0xff;
 				}
 
 				void set_control_line_output(Port port, Line line, bool value) {
-					printf(" l %c%d: %c", port ? 'B' : 'A', int(line), value ? 't' : 'f');
+					printf("6522 line %c%d: %c\n", port ? 'B' : 'A', int(line), value ? 't' : 'f');
 				}
 
 			private:
diff --git a/Machines/Apple/Macintosh/Video.cpp b/Machines/Apple/Macintosh/Video.cpp
index ab72ad956..78ad4294f 100644
--- a/Machines/Apple/Macintosh/Video.cpp
+++ b/Machines/Apple/Macintosh/Video.cpp
@@ -137,11 +137,14 @@ void Video::run_for(HalfCycles duration) {
 		frame_position_ = frame_position_ + cycles_left_in_line;
 		if(frame_position_ == frame_length) {
 			frame_position_ = HalfCycles(0);
-			video_address_ = 0x1a700 >> 1;
+			/*
+				Video: $1A700 and the alternate buffer starts at $12700; for a 512K Macintosh, add $60000 to these numbers.
+			*/
+			video_address_ = use_alternate_screen_buffer_ ? (0x12700 >> 1) : (0x1a700 >> 1);
 		}
 	}
 }
 
-/*
-	Video: $1A700 and the alternate buffer starts at $12700; for a 512K Macintosh, add $60000 to these numbers.
-*/
+void Video::set_use_alternate_screen_buffer(bool use_alternate_screen_buffer) {
+	use_alternate_screen_buffer_ = use_alternate_screen_buffer;
+}
diff --git a/Machines/Apple/Macintosh/Video.hpp b/Machines/Apple/Macintosh/Video.hpp
index 27fb3e005..b37479101 100644
--- a/Machines/Apple/Macintosh/Video.hpp
+++ b/Machines/Apple/Macintosh/Video.hpp
@@ -20,6 +20,7 @@ class Video {
 		Video(uint16_t *ram);
 		void set_scan_target(Outputs::Display::ScanTarget *scan_target);
 		void run_for(HalfCycles duration);
+		void set_use_alternate_screen_buffer(bool use_alternate_screen_buffer);
 
 		// TODO: feedback on blanks and syncs.
 
@@ -30,6 +31,7 @@ class Video {
 		size_t video_address_;
 		uint16_t *ram_;
 		uint8_t *pixel_buffer_;
+		bool use_alternate_screen_buffer_ = false;
 };
 
 }