From cb89eb8c82dcf3f6816e2a7d8eb28521a10aa2df Mon Sep 17 00:00:00 2001
From: Adrian Conlon <Adrian.conlon@gmail.com>
Date: Thu, 4 Jan 2018 21:47:50 +0000
Subject: [PATCH] Add undocumented 6502 instruction: ISB

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>
---
 M6502/inc/mos6502.h       | 12 +++++++++---
 M6502/src/Disassembly.cpp | 17 ++++++++++++++++-
 M6502/src/mos6502.cpp     | 21 ++++++++++++++++++---
 3 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/M6502/inc/mos6502.h b/M6502/inc/mos6502.h
index 05f60ed..4370132 100644
--- a/M6502/inc/mos6502.h
+++ b/M6502/inc/mos6502.h
@@ -572,9 +572,15 @@ namespace EightBit {
 		}
 
 		void DCP(int bbb) {
-			const auto result = AM_11_x(bbb) - 1;
-			setByte(result);
-			CMP(A(), result);
+			auto operand = AM_11_x(bbb);
+			setByte(--operand);
+			CMP(A(), operand);
+		}
+
+		void ISB(int bbb) {
+			auto operand = AM_01(bbb);
+			setByte(++operand);
+			A() = SBC(A(), operand);
 		}
 
 		void ROR(uint8_t& output);
diff --git a/M6502/src/Disassembly.cpp b/M6502/src/Disassembly.cpp
index 5400b20..eaa9f7a 100644
--- a/M6502/src/Disassembly.cpp
+++ b/M6502/src/Disassembly.cpp
@@ -399,7 +399,22 @@ std::string EightBit::Disassembly::disassemble(uint16_t current) const {
 			output << disassemble_AM_11_x(bbb, "*DCP");
 			break;
 		case 0b111:
-			output << disassemble_AM_11(bbb, "*SBC");
+			switch (bbb) {
+			case 0b000:	// *ISB
+			case 0b001:
+			case 0b011:
+			case 0b100:
+			case 0b101:
+			case 0b110:
+			case 0b111:
+				output << disassemble_AM_01(bbb, "*ISB");
+				break;
+			case 0b010:
+				output << disassemble_AM_11(bbb, "*SBC");
+				break;
+			default:
+				UNREACHABLE;
+			}
 			break;
 		default:
 			throw std::domain_error("Illegal instruction group");
diff --git a/M6502/src/mos6502.cpp b/M6502/src/mos6502.cpp
index 6fd18af..760c23d 100644
--- a/M6502/src/mos6502.cpp
+++ b/M6502/src/mos6502.cpp
@@ -19,8 +19,8 @@ EightBit::MOS6502::MOS6502(Bus& bus)
 		/* B */	2, 5, 0, 5, 4, 4, 4, 4, 2, 4, 2, 0, 4, 4, 4, 4,
 		/* C */	2, 6, 0, 8, 3, 3, 5, 5, 2, 2, 2, 0, 4, 4, 6, 6,
 		/* D */	2, 5, 0, 7, 4, 4, 6, 6, 2, 4, 2, 6, 4, 4, 7, 6,
-		/* E */	2, 6, 0, 0, 3, 3, 5, 0, 2, 2, 2, 2, 4, 4, 6, 0,
-		/* F */	2, 5, 0, 0, 4, 4, 6, 0, 2, 4, 2, 0, 4, 4, 7, 0,
+		/* E */	2, 6, 0, 8, 3, 3, 5, 5, 2, 2, 2, 2, 4, 4, 6, 6,
+		/* F */	2, 5, 0, 7, 4, 4, 6, 6, 2, 4, 2, 6, 4, 4, 7, 6,
 	};
 
 
@@ -421,7 +421,22 @@ int EightBit::MOS6502::execute(uint8_t cell) {
 			DCP(decoded.bbb);
 			break;
 		case 0b111:	// *SBC
-			A() = SBC(A(), AM_11(decoded.bbb));
+			switch (decoded.bbb) {
+			case 0b000:	// *ISB
+			case 0b001:
+			case 0b011:
+			case 0b100:
+			case 0b101:
+			case 0b110:
+			case 0b111:
+				ISB(decoded.bbb);
+				break;
+			case 0b010:
+				A() = SBC(A(), AM_11(decoded.bbb));
+				break;
+			default:
+				UNREACHABLE;
+			}
 			break;
 		default:
 			throw std::domain_error("Illegal instruction group");