From 9cb6ca3440ff220874d5cbb174dba1149a221e1b Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sun, 26 Nov 2017 20:01:11 -0500
Subject: [PATCH] Adds elementary decoding of VDP accesses.

---
 Components/9918/9918.cpp | 89 +++++++++++++++++++++++++++++++++++++++-
 Components/9918/9918.hpp | 26 ++++++++++++
 Machines/MSX/MSX.cpp     |  1 -
 3 files changed, 114 insertions(+), 2 deletions(-)

diff --git a/Components/9918/9918.cpp b/Components/9918/9918.cpp
index 37aa9a0b6..e7d3da4fc 100644
--- a/Components/9918/9918.cpp
+++ b/Components/9918/9918.cpp
@@ -27,8 +27,95 @@ void TMS9918::run_for(const Cycles cycles) {
 }
 
 void TMS9918::set_register(int address, uint8_t value) {
+	// Writes to address 0 are writes to the video RAM. Store
+	// the value and return.
+	if(!(address & 1)) {
+		write_phase_ = false;
+		read_ahead_buffer_ = value;
+		ram_[ram_pointer_ & 16383] = value;
+		ram_pointer_++;
+		return;
+	}
+
+	// Writes to address 1 are performed in pairs; if this is the
+	// low byte of a value, store it and wait for the high byte.
+	if(!write_phase_) {
+		low_write_ = value;
+		write_phase_ = true;
+		return;
+	}
+
+	write_phase_ = false;
+	if(value & 0x80) {
+		// This is a write to a register.
+		switch(value & 7) {
+			case 0:
+				screen_mode_ = (screen_mode_ & 6) | ((low_write_ & 2) >> 1);
+			break;
+
+			case 1:
+				blank_screen_ = !!(low_write_ & 0x40);
+				generate_interrupts_ = !!(low_write_ & 0x20);
+				screen_mode_ = (screen_mode_ & 1) | ((low_write_ & 0x18) >> 3);
+				sprites_16x16_ = !!(low_write_ & 0x02);
+				sprites_magnified_ = !!(low_write_ & 0x01);
+				reevaluate_interrupts();
+			break;
+
+			case 2:
+				pattern_name_address_ = (low_write_ & 0xf) << 10;
+			break;
+
+			case 3:
+				colour_table_address_ = low_write_ << 6;
+			break;
+
+			case 4:
+				pattern_generator_table_address_ = (low_write_ & 0x07) << 11;
+			break;
+
+			case 5:
+				sprite_attribute_table_address_ = (low_write_ & 0x7f) << 7;
+			break;
+
+			case 6:
+				sprite_generator_table_address_ = (low_write_ & 0x07) << 11;
+			break;
+
+			case 7:
+				text_colour_ = low_write_ >> 4;
+				background_colour_ = low_write_ & 0xf;
+			break;
+		}
+	} else {
+		// This is a write to the RAM pointer.
+		ram_pointer_ = static_cast<uint16_t>(low_write_ | (value << 8));
+		if(!(value & 0x40)) {
+			// Officially a 'read' set, so perform lookahead.
+			read_ahead_buffer_ = ram_[ram_pointer_ & 16383];
+			ram_pointer_++;
+		}
+	}
 }
 
 uint8_t TMS9918::get_register(int address) {
-	return 0;
+	write_phase_ = false;
+
+	// Reads from address 0 read video RAM, via the read-ahead buffer.
+	if(!(address & 1)) {
+		uint8_t result = read_ahead_buffer_;
+		read_ahead_buffer_ = ram_[ram_pointer_ & 16383];
+		ram_pointer_++;
+		return result;
+	}
+
+	// Reads from address 1 get the status register;
+	uint8_t result = status_;
+	status_ &= ~(0x80 | 0x20);
+	reevaluate_interrupts();
+	return result;
+}
+
+void TMS9918::reevaluate_interrupts() {
+	
 }
diff --git a/Components/9918/9918.hpp b/Components/9918/9918.hpp
index ec03a1d1c..6f13f519b 100644
--- a/Components/9918/9918.hpp
+++ b/Components/9918/9918.hpp
@@ -41,6 +41,32 @@ class TMS9918 {
 
 	private:
 		std::shared_ptr<Outputs::CRT::CRT> crt_;
+	
+		uint16_t ram_[16384];
+
+		uint16_t ram_pointer_ = 0;
+		uint8_t read_ahead_buffer_ = 0;
+
+		uint8_t status_ = 0;
+
+		bool write_phase_ = false;
+		uint8_t low_write_ = 0;
+
+		// The various register flags.
+		int screen_mode_ = 0;
+		bool blank_screen_ = true;
+		bool sprites_16x16_ = false;
+		bool sprites_magnified_ = false;
+		bool generate_interrupts_ = false;
+		uint16_t pattern_name_address_ = 0;
+		uint16_t colour_table_address_ = 0;
+		uint16_t pattern_generator_table_address_ = 0;
+		uint16_t sprite_attribute_table_address_ = 0;
+		uint16_t sprite_generator_table_address_ = 0;
+		uint8_t text_colour_ = 0;
+		uint8_t background_colour_ = 0;
+
+		void reevaluate_interrupts();
 };
 
 };
diff --git a/Machines/MSX/MSX.cpp b/Machines/MSX/MSX.cpp
index ddd488264..a4ec1dede 100644
--- a/Machines/MSX/MSX.cpp
+++ b/Machines/MSX/MSX.cpp
@@ -102,7 +102,6 @@ class ConcreteMachine:
 				case CPU::Z80::PartialMachineCycle::Output:
 					switch(address & 0xff) {
 						case 0x98:	case 0x99:
-							printf("VDP %d %02x\n", address&1, *cycle.value);
 							vdp_->set_register(address, *cycle.value);
 						break;