From 2b24c1e78c9c5b51b19774e63959f422ed166f76 Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Fri, 28 Aug 2026 13:13:16 +0200
Subject: [PATCH] test(cartridge): add MBC3 RTC tests

---
 tests/GameBoy.Core.Tests/Mbc3RtcTests.cs | 137 +++++++++++++++++++++++
 1 file changed, 137 insertions(+)
 create mode 100644 tests/GameBoy.Core.Tests/Mbc3RtcTests.cs

diff --git a/tests/GameBoy.Core.Tests/Mbc3RtcTests.cs b/tests/GameBoy.Core.Tests/Mbc3RtcTests.cs
new file mode 100644
index 0000000..3c5393e
--- /dev/null
+++ b/tests/GameBoy.Core.Tests/Mbc3RtcTests.cs
@@ -0,0 +1,137 @@
+using Xunit;
+
+namespace GameBoy.Tests
+{
+    public sealed class Mbc3RtcTests
+    {
+        private const byte Mbc3TimerRamBattery = 0x10;
+
+        private static Emulator NewTimerCartridge()
+        {
+            return new Emulator(TestRoms.BuildCartridge(Mbc3TimerRamBattery, 0x03));
+        }
+
+        private static void EnableRtc(Emulator emulator)
+        {
+            emulator.WriteMemory(0x0000, 0x0A);
+        }
+
+        private static void SelectRtcRegister(Emulator emulator, byte register)
+        {
+            emulator.WriteMemory(0x4000, register);
+        }
+
+        private static void Latch(Emulator emulator)
+        {
+            emulator.WriteMemory(0x6000, 0x00);
+            emulator.WriteMemory(0x6000, 0x01);
+        }
+
+        private static byte ReadLatched(Emulator emulator, byte register)
+        {
+            SelectRtcRegister(emulator, register);
+            return emulator.ReadMemory(0xA000);
+        }
+
+        private static void WriteRtc(Emulator emulator, byte register, byte value)
+        {
+            SelectRtcRegister(emulator, register);
+            emulator.WriteMemory(0xA000, value);
+        }
+
+        [Fact]
+        public void CartridgeReportsRtcAndBattery()
+        {
+            var emulator = NewTimerCartridge();
+            Assert.True(emulator.HasRtc);
+            Assert.True(emulator.HasBattery);
+        }
+
+        [Fact]
+        public void SecondsRollIntoMinutes()
+        {
+            var emulator = NewTimerCartridge();
+            EnableRtc(emulator);
+
+            WriteRtc(emulator, 0x08, 58);
+            WriteRtc(emulator, 0x09, 0);
+
+            emulator.AdvanceRtcSeconds(5);
+            Latch(emulator);
+
+            Assert.Equal(3, ReadLatched(emulator, 0x08));
+            Assert.Equal(1, ReadLatched(emulator, 0x09));
+        }
+
+        [Fact]
+        public void LatchFreezesTheReadableTime()
+        {
+            var emulator = NewTimerCartridge();
+            EnableRtc(emulator);
+
+            WriteRtc(emulator, 0x08, 10);
+            Latch(emulator);
+            Assert.Equal(10, ReadLatched(emulator, 0x08));
+
+            emulator.AdvanceRtcSeconds(20);
+            Assert.Equal(10, ReadLatched(emulator, 0x08));
+
+            Latch(emulator);
+            Assert.Equal(30, ReadLatched(emulator, 0x08));
+        }
+
+        [Fact]
+        public void HaltStopsTheClock()
+        {
+            var emulator = NewTimerCartridge();
+            EnableRtc(emulator);
+
+            WriteRtc(emulator, 0x08, 0);
+            WriteRtc(emulator, 0x0C, 0x40);
+
+            emulator.AdvanceRtcSeconds(30);
+            Latch(emulator);
+            Assert.Equal(0, ReadLatched(emulator, 0x08));
+
+            WriteRtc(emulator, 0x0C, 0x00);
+            emulator.AdvanceRtcSeconds(30);
+            Latch(emulator);
+            Assert.Equal(30, ReadLatched(emulator, 0x08));
+        }
+
+        [Fact]
+        public void DayCounterOverflowSetsCarry()
+        {
+            var emulator = NewTimerCartridge();
+            EnableRtc(emulator);
+
+            WriteRtc(emulator, 0x08, 0);
+            WriteRtc(emulator, 0x09, 0);
+            WriteRtc(emulator, 0x0A, 0);
+            WriteRtc(emulator, 0x0B, 0xFF);
+            WriteRtc(emulator, 0x0C, 0x01);
+
+            emulator.AdvanceRtcSeconds(86400);
+            Latch(emulator);
+
+            Assert.Equal(0x00, ReadLatched(emulator, 0x0B));
+            byte dayHigh = ReadLatched(emulator, 0x0C);
+            Assert.Equal(0x00, dayHigh & 0x01);
+            Assert.Equal(0x80, dayHigh & 0x80);
+        }
+
+        [Fact]
+        public void ClockAdvancesFromEmulatedCycles()
+        {
+            var emulator = NewTimerCartridge();
+            EnableRtc(emulator);
+            WriteRtc(emulator, 0x08, 0);
+
+            for (long i = 0; i < 4194304 / 4 * 3; i++)
+                emulator.Tick4();
+
+            Latch(emulator);
+            Assert.Equal(3, ReadLatched(emulator, 0x08));
+        }
+    }
+}