Introduces horizontal counter latching and reading.

Then makes a new guess at frame IRQ position. But gets it wrong. Hmmm.

Components/9918/9918.cpp

@@ -526,8 +526,9 @@ uint8_t TMS9918::get_current_line() {
 	int source_row = (column_ < mode_timing_.line_interrupt_position) ? (row_ + mode_timing_.total_lines - 1)%mode_timing_.total_lines : row_;
 	// This assumes NTSC 192-line. TODO: other modes.
 	if(source_row >= 0xdb) source_row -= 6;
+
+//	printf("Current row: %d -> %d\n", row_, source_row);
 	return static_cast<uint8_t>(source_row);
-}
 
 /*
 	TODO: Full proper sequence of current lines:
@@ -539,6 +540,21 @@ uint8_t TMS9918::get_current_line() {
 	PAL 256x224	00-FF, 00-02, CA-FF
 	PAL 256x240	00-FF, 00-0A, D2-FF
 */
+}
+
+uint8_t TMS9918::get_latched_horizontal_counter() {
+	// Translate from internal numbering, which puts pixel output
+	// in the final 256 pixels of 342, to the public numbering,
+	// which makes the 256 pixels the first 256 spots, but starts
+	// counting at -48, and returns only the top 8 bits of the number.
+	int public_counter = latched_column_ - 86;
+	if(public_counter < -48) public_counter += 342;
+	return uint8_t(public_counter >> 1);
+}
+
+void TMS9918::latch_horizontal_counter() {
+	latched_column_ = column_;
+}
 
 uint8_t TMS9918::get_register(int address) {
 	write_phase_ = false;

Components/9918/9918.hpp

@@ -66,6 +66,10 @@ class TMS9918: public Base {
 		/*! Gets the current scan line; provided by the Master System only. */
 		uint8_t get_current_line();
 
+		uint8_t get_latched_horizontal_counter();
+
+		void latch_horizontal_counter();
+
 		/*!
 			Returns the amount of time until get_interrupt_line would next return true if
 			there are no interceding calls to set_register or get_register.

Components/9918/Implementation/9918Base.hpp

@@ -106,7 +106,7 @@ class Base {
 		uint8_t background_colour_ = 0;
 
 		// Internal mechanisms for position tracking.
-		int column_ = 0, row_ = 0;
+		int column_ = 0, row_ = 0, latched_column_ = 0;
 		int cycles_error_ = 0;
 		HalfCycles half_cycles_before_internal_cycles(int internal_cycles);
 
@@ -156,8 +156,8 @@ class Base {
 			// Set the position, in cycles, of the two interrupts,
 			// within a line.
 			struct {
-				int column = 342;
-				int row = 191;
+				int column = 4;
+				int row = 193;
 			} end_of_frame_interrupt_position;
 			int line_interrupt_position = -1;
 

Machines/MasterSystem/MasterSystem.cpp

@@ -184,10 +184,13 @@ class ConcreteMachine:
 								printf("TODO: [input] I/O port control\n");
 								*cycle.value = 0xff;
 							break;
-							case 0x40: case 0x41:
+							case 0x40:
 								update_video();
 								*cycle.value = vdp_->get_current_line();
 							break;
+							case 0x41:
+								*cycle.value = vdp_->get_latched_horizontal_counter();
+							break;
 							case 0x80: case 0x81:
 								update_video();
 								*cycle.value = vdp_->get_register(address);
@@ -201,7 +204,11 @@ class ConcreteMachine:
 							} break;
 							case 0xc1: {
 								Joystick *const joypad2 = static_cast<Joystick *>(joysticks_[1].get());
-								*cycle.value = (joypad2->get_state() >> 2) | 0xf;
+
+								*cycle.value =
+									(joypad2->get_state() >> 2) |
+									0x30 |
+									get_th_values();
 							} break;
 
 							default:
@@ -219,9 +226,20 @@ class ConcreteMachine:
 									page_cartridge();
 								}
 							break;
-							case 0x01:
-								printf("TODO: [output] I/O port control\n");
-							break;
+							case 0x01: {
+								// A programmer can force the TH lines to 0 here,
+								// causing a phoney lightgun latch, so check for any
+								// discontinuity in TH inputs.
+								const auto previous_ths = get_th_values();
+								io_port_control_ = *cycle.value;
+								const auto new_ths = get_th_values();
+
+								// Latch if either TH has newly gone to 1.
+								if((new_ths^previous_ths)&new_ths) {
+									update_video();
+									vdp_->latch_horizontal_counter();
+								}
+							} break;
 							case 0x40: case 0x41:
 								update_audio();
 								sn76489_.set_register(*cycle.value);
@@ -233,7 +251,7 @@ class ConcreteMachine:
 								time_until_interrupt_ = vdp_->get_time_until_interrupt();
 							break;
 							case 0xc0:
-//								printf("TODO: [output] I/O port A/N [%02x]\n", *cycle.value);
+								printf("TODO: [output] I/O port A/N [%02x]\n", *cycle.value);
 							break;
 							case 0xc1:
 								printf("TODO: [output] I/O port B/misc\n");
@@ -274,6 +292,17 @@ class ConcreteMachine:
 		}
 
 	private:
+		inline uint8_t get_th_values() {
+			// Quick not on TH inputs here: if either is setup as an output, then the
+			// currently output level is returned. Otherwise they're fixed at 1.
+			return
+				static_cast<uint8_t>(
+					((io_port_control_ & 0x02) << 5) | ((io_port_control_&0x20) << 1) |
+					((io_port_control_ & 0x08) << 4) | (io_port_control_&0x80)
+				);
+
+		}
+
 		inline void update_audio() {
 			speaker_.run_for(audio_queue_, time_since_sn76489_update_.divide_cycles(Cycles(sn76489_divider)));
 		}
@@ -299,6 +328,8 @@ class ConcreteMachine:
 		uint8_t bios_[8*1024];
 		std::vector<uint8_t> cartridge_;
 
+		uint8_t io_port_control_ = 0x0f;
+
 		// The memory map has a 1kb granularity; this is determined by the SG1000's 1kb of RAM.
 		const uint8_t *read_pointers_[64];
 		uint8_t *write_pointers_[64];