From 348c42bdeaa5a146c40ccabea11f7a62918268db Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Wed, 18 Jan 2023 22:23:19 -0500
Subject: [PATCH] Start trying to bluff my way through extended status.

---
 Components/9918/Implementation/9918.cpp     | 30 +++++++++++++++++++--
 Components/9918/Implementation/9918Base.hpp | 19 ++++++++++++-
 2 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/Components/9918/Implementation/9918.cpp b/Components/9918/Implementation/9918.cpp
index 9d4d8841c..d99995c8e 100644
--- a/Components/9918/Implementation/9918.cpp
+++ b/Components/9918/Implementation/9918.cpp
@@ -649,6 +649,12 @@ void Base<personality>::write_register(uint8_t value) {
 				}
 			break;
 
+			case 15:
+				if constexpr (is_yamaha_vdp(personality)) {
+					Storage<personality>::selected_status_ = this->low_write_ & 0xf;
+				}
+			break;
+
 			default:
 				LOG("Unknown TMS write: " << int(this->low_write_) << " to " << int(value));
 			break;
@@ -699,6 +705,26 @@ uint8_t Base<personality>::read_vram() {
 
 template <Personality personality>
 uint8_t Base<personality>::read_register() {
+	if constexpr (is_yamaha_vdp(personality)) {
+		switch(Storage<personality>::selected_status_) {
+			case 0: break;
+
+			case 2:
+				// b7 = transfer ready flag (i.e. VDP ready for next transfer)
+				// b6 = 1 during vblank
+				// b5 = 1 during hblank
+				// b4 = set if colour detected during search command
+				// b1 = display field odd/even
+				// b0 = command ongoing
+				return
+					queued_access_ == MemoryAccess::None ? 0x80 : 0x00 |
+					0x40 |
+					0x20;
+
+			break;
+		}
+	}
+
 	// Gets the status register.
 	const uint8_t result = this->status_;
 	this->status_ &= ~(StatusInterrupt | StatusSpriteOverflow | StatusSpriteCollision);
@@ -868,8 +894,8 @@ void TMS9918<personality>::latch_horizontal_counter() {
 
 template class TI::TMS::TMS9918<Personality::TMS9918A>;
 template class TI::TMS::TMS9918<Personality::V9938>;
-template class TI::TMS::TMS9918<Personality::V9958>;
+//template class TI::TMS::TMS9918<Personality::V9958>;
 template class TI::TMS::TMS9918<Personality::SMSVDP>;
 template class TI::TMS::TMS9918<Personality::SMS2VDP>;
 template class TI::TMS::TMS9918<Personality::GGVDP>;
-template class TI::TMS::TMS9918<Personality::MDVDP>;
+//template class TI::TMS::TMS9918<Personality::MDVDP>;
diff --git a/Components/9918/Implementation/9918Base.hpp b/Components/9918/Implementation/9918Base.hpp
index 06deb5b0d..204c8cb03 100644
--- a/Components/9918/Implementation/9918Base.hpp
+++ b/Components/9918/Implementation/9918Base.hpp
@@ -118,7 +118,24 @@ constexpr uint8_t StatusSpriteOverflow = 0x40;
 constexpr int StatusSpriteCollisionShift = 5;
 constexpr uint8_t StatusSpriteCollision = 0x20;
 
-template <Personality personality> struct Base {
+/// A container for personality-specific storage; see specific instances below.
+template <Personality personality, typename Enable = void> struct Storage {
+};
+
+template <> struct Storage<Personality::TMS9918A> {
+};
+
+// Yamaha-specific storage.
+template <Personality personality> struct Storage<personality, std::enable_if_t<is_yamaha_vdp(personality)>> {
+	int selected_status_ = 0;
+};
+
+// Master System-specific storage.
+template <Personality personality> struct Storage<personality, std::enable_if_t<is_sega_vdp(personality)>> {
+	// TODO: relocate contents of master_system_ struct;
+};
+
+template <Personality personality> struct Base: public Storage<personality> {
 	Base();
 
 	static constexpr int output_lag = 11;	// i.e. pixel output will occur 11 cycles