From 57ce10418f29a01a82c9f27423421d64abd26706 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 20 Dec 2019 23:33:14 -0500
Subject: [PATCH] Switches prescale logic, the better to deal with changes in
 prescaler.

According to my assumptions about the behaviour, anyway.
---
 Components/68901/MFP68901.cpp                 | 33 +++++++++++++++----
 Components/68901/MFP68901.hpp                 |  2 +-
 .../Clock Signal.xcodeproj/project.pbxproj    |  2 +-
 3 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/Components/68901/MFP68901.cpp b/Components/68901/MFP68901.cpp
index efb1de927..95c2a1992 100644
--- a/Components/68901/MFP68901.cpp
+++ b/Components/68901/MFP68901.cpp
@@ -8,6 +8,7 @@
 
 #include "MFP68901.hpp"
 
+#include <algorithm>
 #include <cstring>
 
 #define LOG_PREFIX "[MFP] "
@@ -179,12 +180,26 @@ void MFP68901::run_for(HalfCycles time) {
 	cycles_left_ += time;
 
 	const int cycles = int(cycles_left_.flush<Cycles>().as_integral());
+	if(!cycles) return;
+
 	for(int c = 0; c < 4; ++c) {
 		if(timers_[c].mode >= TimerMode::Delay) {
-			const int dividend = (cycles + timers_[c].prescale - timers_[c].divisor);
-			const int decrements = dividend / timers_[c].prescale;
-			timers_[c].divisor = timers_[c].prescale - (dividend % timers_[c].prescale);
-			if(decrements) decrement_timer(c, decrements);
+			// This code applies the timer prescaling only. prescale_count is used to count
+			// upwards rather than downwards for simplicity, but on the real hardware it's
+			// pretty safe to assume it actually counted downwards. So the clamp to 0 is
+			// because gymnastics may need to occur when the prescale value is altered, e.g.
+			// if a prescale of 256 is set and the prescale_count is currently 2 then the
+			// counter should roll over in 254 cycles. If the user at that point changes the
+			// prescale_count to 1 then the counter will need to be altered to -253 and
+			// allowed to keep counting up until it crosses both 0 and 1.
+			const int dividend = timers_[c].prescale_count + cycles;
+			const int decrements = std::max(dividend / timers_[c].prescale, 0);
+			if(decrements) {
+				decrement_timer(c, decrements);
+				timers_[c].prescale_count = dividend % timers_[c].prescale;
+			} else {
+				timers_[c].prescale_count += cycles;
+			}
 		}
 	}
 }
@@ -198,11 +213,17 @@ HalfCycles MFP68901::get_next_sequence_point() {
 void MFP68901::set_timer_mode(int timer, TimerMode mode, int prescale, bool reset_timer) {
 	LOG("Timer " << timer << " mode set: " << int(mode) << "; prescale: " << prescale);
 	timers_[timer].mode = mode;
-	timers_[timer].prescale = prescale;
 	if(reset_timer) {
-		timers_[timer].divisor = prescale;
+		timers_[timer].prescale_count = 0;
 		timers_[timer].value = timers_[timer].reload_value;
+	} else {
+		// This hoop is because the prescale_count here goes upward but I'm assuming it goes downward in
+		// real hardware. Therefore this deals with the "switched to a lower prescaling" case whereby the
+		// old cycle should be allowed naturally to expire.
+		timers_[timer].prescale_count = prescale - (timers_[timer].prescale - timers_[timer].prescale_count);
 	}
+
+	timers_[timer].prescale = prescale;
 }
 
 void MFP68901::set_timer_data(int timer, uint8_t value) {
diff --git a/Components/68901/MFP68901.hpp b/Components/68901/MFP68901.hpp
index d49fe55bb..974e195e7 100644
--- a/Components/68901/MFP68901.hpp
+++ b/Components/68901/MFP68901.hpp
@@ -93,7 +93,7 @@ class MFP68901: public ClockingHint::Source {
 			uint8_t value = 0;
 			uint8_t reload_value = 0;
 			int prescale = 1;
-			int divisor = 1;
+			int prescale_count = 1;
 			bool event_input = false;
 		} timers_[4];
 		uint8_t timer_ab_control_[2] = { 0, 0 };
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index e7513b49a..3c527c9e1 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -4916,7 +4916,7 @@
 					"$(USER_LIBRARY_DIR)/Frameworks",
 				);
 				GCC_C_LANGUAGE_STANDARD = gnu11;
-				GCC_OPTIMIZATION_LEVEL = 3;
+				GCC_OPTIMIZATION_LEVEL = fast;
 				MACOSX_DEPLOYMENT_TARGET = 10.12;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 			};