From 60cf8486bb6c4b4c8095a795c76731798f0ff967 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Tue, 15 Jun 2021 18:57:14 -0400
Subject: [PATCH] Makes a genuine attempt at real line counting.

No output yet though.
---
 Machines/Enterprise/Nick.cpp | 60 ++++++++++++++++++++++++++++++++++--
 Machines/Enterprise/Nick.hpp |  9 ++++++
 2 files changed, 67 insertions(+), 2 deletions(-)

diff --git a/Machines/Enterprise/Nick.cpp b/Machines/Enterprise/Nick.cpp
index 3cd39de86..0641b85da 100644
--- a/Machines/Enterprise/Nick.cpp
+++ b/Machines/Enterprise/Nick.cpp
@@ -36,8 +36,13 @@ void Nick::write(uint16_t address, uint8_t value) {
 			// Still a mystery to me: the exact meaning of the top two bits here. For now
 			// just treat a 0 -> 1 transition of the MSB as a forced frame restart.
 			if((value^line_parameter_control_) & value & 0x80) {
-				// TODO: restart frame.
 				printf("Should restart frame from %04x\n", line_parameter_base_);
+
+				// For now: just force this to be the final line of this mode block.
+				// I'm unclear whether I should also reset the horizontal counter
+				// (i.e. completely abandon current video phase).
+				lines_remaining_ = 0xff;
+				line_parameters_[1] |= 1;
 			}
 			line_parameter_control_ = value & 0xc0;
 		break;
@@ -48,7 +53,58 @@ uint8_t Nick::read([[maybe_unused]] uint16_t address) {
 	return 0xff;
 }
 
-void Nick::run_for(HalfCycles) {
+void Nick::run_for(HalfCycles duration) {
+	constexpr int line_length = 912;
+
+	int clocks_remaining = duration.as<int>();
+	while(clocks_remaining) {
+		// Determine how many cycles are left this line.
+		const int clocks_this_line = std::min(clocks_remaining, line_length - horizontal_counter_);
+
+		// Convert that into a [start/current] and end window.
+		int window = horizontal_counter_ >> 4;
+		const int end_window = (horizontal_counter_ + clocks_this_line) >> 4;
+
+		// Advance the line counters.
+		clocks_remaining -= clocks_this_line;
+		horizontal_counter_ = (horizontal_counter_ + clocks_this_line) % line_length;
+
+		// Do nothing if a window boundary isn't crossed.
+		if(window == end_window) continue;
+
+		// If this is within the first 8 cycles of the line, [possibly] fetch
+		// the relevant part of the line parameters.
+		if(should_reload_line_parameters_ && window < 8) {
+			int fetch_spot = window;
+			while(fetch_spot < end_window && fetch_spot < 8) {
+				line_parameters_[(fetch_spot << 1)] = ram_[line_parameter_pointer_];
+				line_parameters_[(fetch_spot << 1) + 1] = ram_[line_parameter_pointer_ + 1];
+				line_parameter_pointer_ += 2;
+				++fetch_spot;
+			}
+
+			// If all parameters have been loaded, set appropriate fields.
+			if(fetch_spot == 8) {
+				should_reload_line_parameters_ = false;
+
+				// Set length of mode line.
+				lines_remaining_ = line_parameters_[0];
+			}
+		}
+
+		// Check for end of line.
+		if(!horizontal_counter_) {
+			++lines_remaining_;
+			if(!lines_remaining_) {
+				should_reload_line_parameters_ = true;
+
+				// Check for end-of-frame.
+				if(line_parameters_[1] & 1) {
+					line_parameter_pointer_ = line_parameter_base_;
+				}
+			}
+		}
+	}
 }
 
 // MARK: - CRT passthroughs.
diff --git a/Machines/Enterprise/Nick.hpp b/Machines/Enterprise/Nick.hpp
index d38aa59a3..a5088cc6f 100644
--- a/Machines/Enterprise/Nick.hpp
+++ b/Machines/Enterprise/Nick.hpp
@@ -31,9 +31,18 @@ class Nick {
 		Outputs::CRT::CRT crt_;
 		const uint8_t *const ram_;
 
+		// CPU-provided state.
 		uint8_t line_parameter_control_ = 0xc0;
 		uint16_t line_parameter_base_ = 0x0000;
+
+		// Ephemerals, related to current video position.
 		int horizontal_counter_ = 0;
+		uint16_t line_parameter_pointer_ = 0x0000;
+		uint8_t line_parameters_[16];
+		bool should_reload_line_parameters_ = false;
+
+		// Current mode line parameters.
+		uint8_t lines_remaining_ = 0x00;
 };