From d6ac0dc6f526ac111c4e750e282050c034724b76 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 4 Mar 2019 18:28:27 -0800
Subject: [PATCH] LR35902: Support PC-relative opcode decoding

---
 CHANGES                                 |  1 +
 include/mgba/internal/lr35902/decoder.h |  3 ++-
 src/lr35902/decoder.c                   | 16 +++++++++++-----
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/CHANGES b/CHANGES
index 962e2e6a2..c5c787633 100644
--- a/CHANGES
+++ b/CHANGES
@@ -22,6 +22,7 @@ Misc:
  - Qt: Don't unload ROM immediately if it crashes
  - Debugger: Add breakpoint and watchpoint listing
  - Qt: Add missing HEVC NVENC option (fixes mgba.io/i/1323)
+ - LR35902: Support PC-relative opcode decoding
 
 0.7.1: (2019-02-24)
 Bugfixes:
diff --git a/include/mgba/internal/lr35902/decoder.h b/include/mgba/internal/lr35902/decoder.h
index 101a104c3..ba9ceec36 100644
--- a/include/mgba/internal/lr35902/decoder.h
+++ b/include/mgba/internal/lr35902/decoder.h
@@ -86,6 +86,7 @@ enum {
 	LR35902_OP_FLAG_MEMORY = 2,
 	LR35902_OP_FLAG_INCREMENT = 4,
 	LR35902_OP_FLAG_DECREMENT = 8,
+	LR35902_OP_FLAG_RELATIVE = 16,
 };
 
 struct LR35902Operand {
@@ -104,7 +105,7 @@ struct LR35902InstructionInfo {
 };
 
 size_t LR35902Decode(uint8_t opcode, struct LR35902InstructionInfo* info);
-int LR35902Disassemble(struct LR35902InstructionInfo* info, char* buffer, int blen);
+int LR35902Disassemble(struct LR35902InstructionInfo* info, uint16_t pc, char* buffer, int blen);
 
 CXX_GUARD_END
 
diff --git a/src/lr35902/decoder.c b/src/lr35902/decoder.c
index 76c6b9f02..089b52c69 100644
--- a/src/lr35902/decoder.c
+++ b/src/lr35902/decoder.c
@@ -199,6 +199,7 @@ DEFINE_DECODER_LR35902(ADDSP, info->mnemonic = LR35902_MN_ADD; \
 	DEFINE_DECODER_LR35902(JR ## CONDITION_NAME, \
 		info->mnemonic = LR35902_MN_JR; \
 		info->condition = CONDITION; \
+		info->op1.flags = LR35902_OP_FLAG_RELATIVE; \
 		return 1;)
 
 #define DEFINE_CALL_INSTRUCTION_LR35902(CONDITION_NAME, CONDITION) \
@@ -497,7 +498,7 @@ static const char* _lr35902MnemonicStrings[] = {
 };
 
 
-static int _decodeOperand(struct LR35902Operand op, char* buffer, int blen) {
+static int _decodeOperand(struct LR35902Operand op, uint16_t pc, char* buffer, int blen) {
 	int total = 0;
 	if (op.flags & LR35902_OP_FLAG_IMPLICIT) {
 		return 0;
@@ -511,7 +512,12 @@ static int _decodeOperand(struct LR35902Operand op, char* buffer, int blen) {
 		int written = snprintf(buffer, blen - 1, "%s", _lr35902Registers[op.reg]);
 		ADVANCE(written);
 	} else {
-		int written = snprintf(buffer, blen - 1, "$%02X", op.immediate);
+		int written;
+		if (op.flags & LR35902_OP_FLAG_RELATIVE) {
+			written = snprintf(buffer, blen - 1, "$%04X", pc + (int8_t) op.immediate);
+		} else {
+			written = snprintf(buffer, blen - 1, "$%02X", op.immediate);
+		}
 		ADVANCE(written);
 		if (op.reg) {
 			strncpy(buffer, "+", blen - 1);
@@ -533,7 +539,7 @@ static int _decodeOperand(struct LR35902Operand op, char* buffer, int blen) {
 	return total;
 }
 
-int LR35902Disassemble(struct LR35902InstructionInfo* info, char* buffer, int blen) {
+int LR35902Disassemble(struct LR35902InstructionInfo* info, uint16_t pc, char* buffer, int blen) {
 	const char* mnemonic = _lr35902MnemonicStrings[info->mnemonic];
 	int written;
 	int total = 0;
@@ -553,7 +559,7 @@ int LR35902Disassemble(struct LR35902InstructionInfo* info, char* buffer, int bl
 	}
 
 	if (info->op1.reg || info->op1.immediate || info->op2.reg || info->op2.immediate) {
-		written = _decodeOperand(info->op1, buffer, blen);
+		written = _decodeOperand(info->op1, pc, buffer, blen);
 		ADVANCE(written);
 	}
 
@@ -562,7 +568,7 @@ int LR35902Disassemble(struct LR35902InstructionInfo* info, char* buffer, int bl
 			strncpy(buffer, ", ", blen - 1);
 			ADVANCE(2);
 		}
-		written = _decodeOperand(info->op2, buffer, blen);
+		written = _decodeOperand(info->op2, pc, buffer, blen);
 		ADVANCE(written);
 	}