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88 lines
2.4 KiB
C++
88 lines
2.4 KiB
C++
//
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// Shifter.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 24/09/2017.
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// Copyright 2017 Thomas Harte. All rights reserved.
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//
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#ifndef Shifter_hpp
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#define Shifter_hpp
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#include <cstdint>
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#include <memory>
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#include "../../../../Numeric/CRC.hpp"
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namespace Storage {
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namespace Encodings {
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namespace MFM {
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/*!
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The MFM shifter parses a stream of bits as input in order to produce
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a stream of MFM tokens as output. So e.g. it is suitable for use in parsing
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the output of a PLL windowing of disk events.
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It supports both FM and MFM parsing; see @c set_is_double_density.
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It will ordinarily honour sync patterns; that should be turned off when within
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a sector because false syncs can occur. See @c set_should_obey_syncs.
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It aims to implement the same behaviour as WD177x-series controllers when
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detecting a false sync — the received byte value will be either a 0xc1 or 0x14,
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depending on phase.
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Bits should be fed in with @c add_input_bit.
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The current output token can be read with @c get_token. It will usually be None but
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may indicate that an index, ID, data or deleted data mark was found, that an
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MFM sync mark was found, or that an ordinary byte has been decoded.
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It will properly reset and/or seed a CRC generator based on the data and ID marks,
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and feed it with incoming bytes. You can access that CRC generator to query its
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value via @c get_crc_generator(). An easy way to check whether the disk contained
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a proper CRC is to read bytes until you've just read whatever CRC was on the disk,
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then check that the generator has a value of zero.
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A specific instance of the CRC generator can be supplied at construction if preferred.
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*/
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class Shifter {
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public:
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Shifter();
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Shifter(CRC::CCITT *crc_generator);
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void set_is_double_density(bool is_double_density);
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void set_should_obey_syncs(bool should_obey_syncs);
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void add_input_bit(int bit);
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enum Token {
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Index, ID, Data, DeletedData, Sync, Byte, None
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};
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uint8_t get_byte() const;
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Token get_token() const {
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return token_;
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}
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CRC::CCITT &get_crc_generator() {
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return *crc_generator_;
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}
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private:
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// Bit stream input state.
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int bits_since_token_ = 0;
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unsigned int shift_register_ = 0;
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bool is_awaiting_marker_value_ = false;
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bool should_obey_syncs_ = true;
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Token token_ = None;
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// Input configuration.
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bool is_double_density_ = false;
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std::unique_ptr<CRC::CCITT> owned_crc_generator_;
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CRC::CCITT *crc_generator_;
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};
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}
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}
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}
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#endif /* Shifter_hpp */
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