Add notes to self on exceptions.

InstructionSets/x86/Descriptors.hpp

@@ -30,40 +30,38 @@ struct Descriptor {
 	}
 
 	void set(const uint16_t descriptor[4]) {
-		// TODO: something.
-
 		base_ = uint32_t(descriptor[1] | ((descriptor[2] & 0xff) << 16));
 		limit_ = descriptor[0];
 
 		printf("%04x %04x %04x %04x\n", descriptor[0], descriptor[1], descriptor[2], descriptor[3]);
 
+		present_ = descriptor[2] & 0x8000;
+		privilege_level_ = (descriptor[2] >> 13) & 3;
+
+		// TODO: need to know more about the below.
 		if(descriptor[2] & 0x1000) {
-			// Is segment descriptor.
-			present_ = descriptor[2] & 0x8000;
-			privilege_level_ = (descriptor[2] >> 13) & 3;
-			accessed_ = descriptor[2] & 0x100;
-			readable_ = descriptor[2] & 0x200;
-			conforming_ = descriptor[2] & 0x400;
 			executable_ = descriptor[2] & 0x800;
+
+			if(executable_) {
+				conforming_ = descriptor[2] & 0x400;
+				readable_ = descriptor[2] & 0x200;
+			} else {
+				// expand down = descriptor[2] & 0x400;
+				// writeable_ = descriptor[2] & 0x200;
+			}
 		} else {
 			assert(false);
 		}
 	}
 
-	// 286:
-	// 47...32 = 16-bit limit;
-	// 31 = P
-	// 30...29 = DPL
-	// 28 = S
-	// 27...24 = type;
-	// 23...00 = 4-bit base.
-
 	uint32_t to_linear(const uint32_t address) const {
 		return base_ + address;
 	}
+	uint32_t base() const {		return base_;	}
+	uint32_t limit() const {	return limit_;	}
 
-	uint32_t base() const {
+	int privilege_level() const {
-		return base_;
+		return privilege_level_;
 	}
 
 private:
@@ -71,8 +69,7 @@ private:
 	uint32_t limit_;
 	// TODO: permissions, type, etc.
 
-	bool present_;
+	int privilege_level_;
-	uint8_t privilege_level_;
 	enum class Type {
 		AvailableTaskStateSegment = 1,
 		LDTDescriptor = 2,
@@ -85,7 +82,8 @@ private:
 		Reserved9 = 9,	ReservedA = 10,	ReservedB = 11,	ReservedC = 12,
 		ReservedD = 13,	ReservedE = 14,	ReservedF = 15,
 	} type_;
-	bool accessed_;
+
+	bool present_;
 	bool readable_;
 	bool conforming_;
 	bool executable_;

InstructionSets/x86/Interrupts.hpp

@@ -33,4 +33,17 @@ enum Interrupt {
 
 };
 
+struct ProtectedException {
+	Interrupt cause;
+	uint16_t code;
+
+	// Code:
+	//	b3–b15: IDT/GDT/LDT entry
+	//	b2: 1 => in LDT; 0 => in GDT;
+	//	b1: 1 => in IDT, ignore b2; 0 => use b2;
+	//	b0:
+	//		1 => trigger was external to program code;
+	//		0 => trigger was caused by the instruction described by the CS:IP that is on the stack.
+};
+
 }

Machines/PCCompatible/SegmentedMemory.hpp

@@ -19,6 +19,7 @@
 
 namespace PCCompatible {
 
+// TODO: the following need to apply linear memory semantics, including potential A20 wrapping.
 template <InstructionSet::x86::Model model> struct ProgramFetcher {
 	std::pair<const uint8_t *, size_t> next_code(
 		const Registers<model> &registers,
@@ -26,11 +27,11 @@ template <InstructionSet::x86::Model model> struct ProgramFetcher {
 		LinearMemory<model> &linear_memory
 	) const {
 		const uint16_t ip = registers.ip();
-		const uint32_t start =
+		const auto &descriptor = segments.descriptors[InstructionSet::x86::Source::CS];
-			segments.descriptors[InstructionSet::x86::Source::CS].to_linear(ip) & (linear_memory.MaxAddress - 1);
+		const uint32_t start = descriptor.to_linear(ip) & (linear_memory.MaxAddress - 1);
 		return std::make_pair(
 			linear_memory.at(start),
-			std::min<size_t>(linear_memory.MaxAddress - start, 0x10000 - ip)
+			std::min<size_t>(linear_memory.MaxAddress - start, 1 + descriptor.limit() - ip)
 		);
 	}
 
@@ -38,10 +39,11 @@ template <InstructionSet::x86::Model model> struct ProgramFetcher {
 		const Segments<model> &segments,
 		LinearMemory<model> &linear_memory
 	) const {
-		const auto base = segments.descriptors[InstructionSet::x86::Source::CS].base();
+		const auto &descriptor = segments.descriptors[InstructionSet::x86::Source::CS];
+		const auto base = descriptor.base() & (linear_memory.MaxAddress - 1);
 		return std::make_pair(
 			linear_memory.at(base),
-			std::min<size_t>(0x1'0000, linear_memory.MaxAddress - base)
+			std::min<size_t>(0x1'0000, 1 + descriptor.limit() - base)
 		);
 	}
 

Machines/PCCompatible/Segments.hpp

@@ -43,12 +43,23 @@ public:
 				break;
 
 				case Mode::Protected286: {
-					// TODO: when to use the local table? Is this right?
+					// Check privilege level.
-					assert(!(value & 0x8000));
+					const auto requested_privilege_level = value & 3;
-					const auto &table = registers_.template get<DescriptorTable::Global>();
+					if(requested_privilege_level < descriptors[Source::CS].privilege_level()) {
-					const uint32_t table_address = table.base + value;
+						printf("TODO: privilege exception.");
+					}
 
-					// TODO: authorisation required, possibly.
+					// Check segment range.
+					const auto &table =
+						(value & 4) ?
+							registers_.template get<DescriptorTable::Local>() :
+							registers_.template get<DescriptorTable::Global>();
+					const uint32_t table_address = table.base + (value & ~7);
+					if(table_address > table.base + table.limit - 8) {
+						printf("TODO: descriptor table overrun exception.\n");
+					}
+
+					// Get descriptor contents.
 					using AccessType = InstructionSet::x86::AccessType;
 					const uint32_t table_end = table.base + table.limit;
 					const uint16_t entry[] = {
@@ -57,7 +68,20 @@ public:
 						memory_.template access<uint16_t, AccessType::Read>(table_address + 4, table_end),
 						memory_.template access<uint16_t, AccessType::Read>(table_address + 6, table_end)
 					};
+
+					// TODO: is this descriptor privilege within reach?
+					// TODO: is this an empty descriptor*? If so: exception!
+					// (* other than the 0 descriptor, which can be loaded to DS or ES)
+					// TODO:
+					//	bool is_compatible(const Source reg) const {
+					//		switch(reg)
+					//			DS/ES: either readable code, or any sort of data
+					//			SS: writeable data
+					//			CS: any sort of code
+					//	}
+
 					descriptors[segment].set(entry);
+					// TODO: set descriptor accessed bit in memory if it's a segment.
 				} break;
 			}
 		}