From ce8bd011d702dbce6bbcf111cfe02d84ba317630 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 18 May 2023 16:50:46 -0400
Subject: [PATCH] Add commentary, and TODOs.

---
 Components/9918/Implementation/9918.cpp | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/Components/9918/Implementation/9918.cpp b/Components/9918/Implementation/9918.cpp
index 34af6a73b..7011e3681 100644
--- a/Components/9918/Implementation/9918.cpp
+++ b/Components/9918/Implementation/9918.cpp
@@ -35,12 +35,16 @@ Base<personality>::Base() :
 
 		// "For a line interrupt, /INT is pulled low 608 mclks into the appropriate scanline relative to pixel 0.
 		// This is 3 mclks before the rising edge of /HSYNC which starts the next scanline."
+		//
+		// i.e. it's 304 internal clocks after the end of the left border.
 		mode_timing_.line_interrupt_position = (LineLayout<personality>::EndOfLeftBorder + 304) % LineLayout<personality>::CyclesPerLine;
 
 		// For a frame interrupt, /INT is pulled low 607 mclks into scanline 192 (of scanlines 0 through 261) relative to pixel 0.
 		// This is 4 mclks before the rising edge of /HSYNC which starts the next scanline.
+		//
+		// i.e. it's 1/2 cycle before the line interrupt position, which I have rounded. Ugh.
 		mode_timing_.end_of_frame_interrupt_position.column = mode_timing_.line_interrupt_position - 1;
-		mode_timing_.end_of_frame_interrupt_position.row = 193;
+		mode_timing_.end_of_frame_interrupt_position.row = 192 + (LineLayout<personality>::EndOfLeftBorder + 304) / LineLayout<personality>::CyclesPerLine;
 	}
 
 	if constexpr (is_yamaha_vdp(personality)) {
@@ -81,6 +85,7 @@ TMS9918<personality>::TMS9918() {
 
 template <Personality personality>
 void TMS9918<personality>::set_tv_standard(TVStandard standard) {
+	// TODO: the Yamaha is programmable on this at runtime.
 	this->tv_standard_ = standard;
 	switch(standard) {
 		case TVStandard::PAL:
@@ -214,8 +219,10 @@ void TMS9918<personality>::run_for(const HalfCycles cycles) {
 			// ---------------------------------------
 			// Latch scrolling position, if necessary.
 			// ---------------------------------------
+			// TODO: shouldn't this happen one per frame?
 			if constexpr (is_sega_vdp(personality)) {
-				if(this->fetch_pointer_.column < 61 && end_column >= 61) {
+				constexpr auto latch_time = to_internal<personality, Clock::Grauw>(61);	// TODO: where did this magic constant come from? Is it the same for the Game Gear, etc?
+				if(this->fetch_pointer_.column < latch_time && end_column >= latch_time) {
 					if(!this->fetch_pointer_.row) {
 						Storage<personality>::latched_vertical_scroll_ = Storage<personality>::vertical_scroll_;