From 86239469e729a7c2ddb2ff32d5866d0d232b1649 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 1 Mar 2018 18:51:05 -0500
Subject: [PATCH] Allows SN76489 consumers to apply an additional divider that
 reduces computation.

---
 Components/SN76489/SN76489.cpp         |  7 ++++++-
 Components/SN76489/SN76489.hpp         |  2 +-
 Machines/ColecoVision/ColecoVision.cpp | 14 ++++++++------
 3 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/Components/SN76489/SN76489.cpp b/Components/SN76489/SN76489.cpp
index 495a43607..a321b3898 100644
--- a/Components/SN76489/SN76489.cpp
+++ b/Components/SN76489/SN76489.cpp
@@ -8,11 +8,12 @@
 
 #include "SN76489.hpp"
 
+#include <cassert>
 #include <cmath>
 
 using namespace TI;
 
-SN76489::SN76489(Personality personality, Concurrency::DeferringAsyncTaskQueue &task_queue) : task_queue_(task_queue) {
+SN76489::SN76489(Personality personality, Concurrency::DeferringAsyncTaskQueue &task_queue, int additional_divider) : task_queue_(task_queue) {
 	// Build a volume table.
 	double multiplier = pow(10.0, -0.1);
 	double volume = 8191.0f;
@@ -37,6 +38,10 @@ SN76489::SN76489(Personality personality, Concurrency::DeferringAsyncTaskQueue &
 			shifter_is_16bit_ = true;
 		break;
 	}
+
+	assert((master_divider_period_ % additional_divider) == 0);
+	assert(additional_divider < master_divider_period_);
+	master_divider_period_ /= additional_divider;
 }
 
 void SN76489::set_register(uint8_t value) {
diff --git a/Components/SN76489/SN76489.hpp b/Components/SN76489/SN76489.hpp
index eb58af331..12f2b2d3f 100644
--- a/Components/SN76489/SN76489.hpp
+++ b/Components/SN76489/SN76489.hpp
@@ -23,7 +23,7 @@ class SN76489: public Outputs::Speaker::SampleSource {
 		};
 
 		/// Creates a new SN76489.
-		SN76489(Personality personality, Concurrency::DeferringAsyncTaskQueue &task_queue);
+		SN76489(Personality personality, Concurrency::DeferringAsyncTaskQueue &task_queue, int additional_divider = 1);
 
 		/// Writes a new value to the SN76489.
 		void set_register(uint8_t value);
diff --git a/Machines/ColecoVision/ColecoVision.cpp b/Machines/ColecoVision/ColecoVision.cpp
index 77ecded30..d742baa74 100644
--- a/Machines/ColecoVision/ColecoVision.cpp
+++ b/Machines/ColecoVision/ColecoVision.cpp
@@ -21,6 +21,10 @@
 
 #include "../../Outputs/Speaker/Implementation/LowpassSpeaker.hpp"
 
+namespace {
+const int sn76489_divider = 4;
+}
+
 namespace Coleco {
 namespace Vision {
 
@@ -107,10 +111,9 @@ class ConcreteMachine:
 	public:
 		ConcreteMachine() :
 			z80_(*this),
-			sn76489_(TI::SN76489::Personality::SN76489, audio_queue_),
+			sn76489_(TI::SN76489::Personality::SN76489, audio_queue_, sn76489_divider),
 			speaker_(sn76489_) {
-//			speaker_.set_input_rate(3579545.0f);
-			speaker_.set_input_rate(3579545.0f / 2.0f);
+			speaker_.set_input_rate(3579545.0f / static_cast<float>(sn76489_divider));
 			set_clock_rate(3579545);
 			joysticks_.emplace_back(new Joystick);
 			joysticks_.emplace_back(new Joystick);
@@ -176,7 +179,7 @@ class ConcreteMachine:
 				case CPU::Z80::PartialMachineCycle::ReadOpcode:
 				case CPU::Z80::PartialMachineCycle::Read:
 					if(address < 0x2000) {
-						*cycle.value = bios_[address];
+						*cycle.value = bios_[address & 0x1fff];
 					} else if(address >= 0x6000 && address < 0x8000) {
 						*cycle.value = ram_[address & 1023];
 					} else if(address >= 0x8000) {
@@ -264,8 +267,7 @@ class ConcreteMachine:
 
 	private:
 		void update_audio() {
-			speaker_.run_for(audio_queue_, time_since_sn76489_update_.divide_cycles(Cycles(2)));
-//			speaker_.run_for(audio_queue_, time_since_sn76489_update_.cycles());
+			speaker_.run_for(audio_queue_, time_since_sn76489_update_.divide_cycles(Cycles(sn76489_divider)));
 		}
 		void update_video() {
 			vdp_->run_for(time_since_vdp_update_.flush());