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Switch strategy on 8086 instruction lengths.

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This commit is contained in:
Thomas Harte 2022-03-11 09:48:26 -05:00
parent dc8cff364f
commit 91d75d7704
1 changed files with 15 additions and 7 deletions
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22
InstructionSets/x86/Decoder.cpp
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@ -18,13 +18,16 @@ template <Model model>
std::pair<int, typename Decoder<model>::InstructionT> Decoder<model>::decode(const uint8_t *source, size_t length) {
// Instruction length limits:
//
// 8086/80186: none
// 8086/80186: none*
// 80286: 10 bytes
// 80386: 15 bytes
constexpr int max_instruction_length = model >= Model::i80386 ? 15 : (model == Model::i80286 ? 10 : 0);
const uint8_t *const buffer_end = source + length;
const uint8_t *const end = max_instruction_length ? std::min(buffer_end, source + max_instruction_length - consumed_) : buffer_end;
//
// * but, can treat internally as a limit of 65536 bytes — after that distance the IP will
// be back to wherever it started, so it's safe to spit out a NOP and reset parsing
// without any loss of context. This reduces the risk of the decoder tricking a caller into
// an infinite loop.
constexpr int max_instruction_length = model >= Model::i80386 ? 15 : (model == Model::i80286 ? 10 : 65536);
const uint8_t *const end = source + std::min(length, size_t(max_instruction_length - consumed_));
// MARK: - Prefixes (if present) and the opcode.
@ -932,8 +935,13 @@ std::pair<int, typename Decoder<model>::InstructionT> Decoder<model>::decode(con
}
// Check for a too-long instruction.
if(max_instruction_length && consumed_ == max_instruction_length) {
const auto result = std::make_pair(consumed_, InstructionT());
if(consumed_ == max_instruction_length) {
std::pair<int, InstructionT> result;
if(max_instruction_length == 65536) {
result = std::make_pair(consumed_, InstructionT(Operation::NOP, consumed_));
} else {
result = std::make_pair(consumed_, InstructionT());
}
reset_parsing();
return result;
}