From 90e932e12c0e093d9ff35989bf12778d7ff079eb Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Fri, 5 Jun 2020 20:29:31 -0700
Subject: [PATCH] SM83: Emulate HALT bug

---
 CHANGES                           |  1 +
 include/mgba/internal/sm83/sm83.h |  3 ++-
 src/gb/gb.c                       |  5 +++--
 src/sm83/sm83.c                   | 11 +++++++++++
 4 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/CHANGES b/CHANGES
index fef76ce8f..0953fc677 100644
--- a/CHANGES
+++ b/CHANGES
@@ -27,6 +27,7 @@ Emulation fixes:
  - GBA Video: Fix disabling OBJWIN in GL renderer (fixes mgba.io/i/1759)
  - GBA Video: Add missing parts of 256-color mode 0 mosaic (fixes mgba.io/1701)
  - GBA Video: Fix double-size OBJ wrapping in GL renderer (fixes mgba.io/1712)
+ - SM83: Emulate HALT bug
 Other fixes:
  - 3DS: Fix framelimiter on newer citro3d (fixes mgba.io/i/1771)
  - All: Improve export headers (fixes mgba.io/i/1738)
diff --git a/include/mgba/internal/sm83/sm83.h b/include/mgba/internal/sm83/sm83.h
index e2cf89540..3e36e4e26 100644
--- a/include/mgba/internal/sm83/sm83.h
+++ b/include/mgba/internal/sm83/sm83.h
@@ -47,7 +47,8 @@ enum SM83ExecutionState {
 	SM83_CORE_MEMORY_STORE = 11,
 	SM83_CORE_READ_PC = 15,
 	SM83_CORE_STALL = 19,
-	SM83_CORE_OP2 = 23
+	SM83_CORE_OP2 = 23,
+	SM83_CORE_HALT_BUG = 27,
 };
 struct SM83Memory {
 	uint8_t (*cpuLoad8)(struct SM83Core*, uint16_t address);
diff --git a/src/gb/gb.c b/src/gb/gb.c
index 0aaa6ae54..fda3f214a 100644
--- a/src/gb/gb.c
+++ b/src/gb/gb.c
@@ -749,8 +749,9 @@ void GBHalt(struct SM83Core* cpu) {
 	if (!(gb->memory.ie & gb->memory.io[REG_IF] & 0x1F)) {
 		cpu->cycles = cpu->nextEvent;
 		cpu->halted = true;
-	} else if (gb->model < GB_MODEL_CGB) {
-		mLOG(GB, STUB, "Unimplemented HALT bug");
+	} else {
+		mLOG(GB, GAME_ERROR, "HALT bug");
+		cpu->executionState = SM83_CORE_HALT_BUG;
 	}
 }
 
diff --git a/src/sm83/sm83.c b/src/sm83/sm83.c
index eca13bbb8..28e9e4bd3 100644
--- a/src/sm83/sm83.c
+++ b/src/sm83/sm83.c
@@ -131,6 +131,17 @@ static void _SM83Step(struct SM83Core* cpu) {
 	case SM83_CORE_STALL:
 		cpu->instruction = _sm83InstructionTable[0]; // NOP
 		break;
+	case SM83_CORE_HALT_BUG:
+		if (cpu->irqPending) {
+			cpu->index = cpu->sp;
+			cpu->irqPending = false;
+			cpu->instruction = _SM83InstructionIRQ;
+			cpu->irqh.setInterrupts(cpu, false);
+			break;
+		}
+		cpu->bus = cpu->memory.cpuLoad8(cpu, cpu->pc);
+		cpu->instruction = _sm83InstructionTable[cpu->bus];
+		break;
 	default:
 		break;
 	}