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123 lines
3.0 KiB
C++
123 lines
3.0 KiB
C++
//
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// SpectrumTAP.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 19/03/2021.
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// Copyright © 2021 Thomas Harte. All rights reserved.
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//
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#include "ZXSpectrumTAP.hpp"
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using namespace Storage::Tape;
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/*
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The understanding of idiomatic Spectrum data encoding below
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is taken from the TZX specifications at
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https://worldofspectrum.net/features/TZXformat.html ;
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specifics of the TAP encoding were gained from
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https://sinclair.wiki.zxnet.co.uk/wiki/TAP_format
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*/
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ZXSpectrumTAP::ZXSpectrumTAP(const std::string &file_name) :
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file_(file_name)
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{
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// Check for a continuous series of blocks through to
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// exactly file end.
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//
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// To consider: could also check those blocks of type 0
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// and type ff for valid checksums?
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while(true) {
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const uint16_t block_length = file_.get16le();
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if(file_.eof()) throw ErrorNotZXSpectrumTAP;
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file_.seek(block_length, SEEK_CUR);
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if(file_.tell() == file_.stats().st_size) break;
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}
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virtual_reset();
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}
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bool ZXSpectrumTAP::is_at_end() {
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return file_.tell() == file_.stats().st_size && phase_ == Phase::Gap;
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}
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void ZXSpectrumTAP::virtual_reset() {
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file_.seek(0, SEEK_SET);
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read_next_block();
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}
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Tape::Pulse ZXSpectrumTAP::virtual_get_next_pulse() {
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// Adopt a general pattern of high then low.
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Pulse pulse;
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pulse.type = (distance_into_phase_ & 1) ? Pulse::Type::High : Pulse::Type::Low;
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switch(phase_) {
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default: break;
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case Phase::PilotTone: {
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// Output: pulses of length 2168;
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// 8063 pulses if block type is 0, otherwise 3223;
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// then a 667-length pulse followed by a 735-length pulse.
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pulse.length = Time(271, 437'500); // i.e. 2168 / 3'500'000
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++distance_into_phase_;
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// Check whether in the last two.
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if(distance_into_phase_ >= (block_type_ ? 8063 : 3223)) {
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pulse.length = (distance_into_phase_ & 1) ? Time(667, 3'500'000) : Time(735, 3'500'000);
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// Check whether this is the last one.
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if(distance_into_phase_ == (block_type_ ? 8064 : 3224)) {
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distance_into_phase_ = 0;
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phase_ = Phase::Data;
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}
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}
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} break;
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case Phase::Data: {
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// Output two pulses of length 855 for a 0; two of length 1710 for a 1,
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// from MSB to LSB.
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pulse.length = (data_byte_ & 0x80) ? Time(1710, 3'500'000) : Time(855, 3'500'000);
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++distance_into_phase_;
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if(!(distance_into_phase_ & 1)) {
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data_byte_ <<= 1;
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}
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if(!(distance_into_phase_ & 15)) {
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if((distance_into_phase_ >> 4) == block_length_) {
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if(block_type_) {
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distance_into_phase_ = 0;
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phase_ = Phase::Gap;
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} else {
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read_next_block();
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}
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} else {
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data_byte_ = file_.get8();
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}
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}
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} break;
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case Phase::Gap:
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Pulse gap;
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gap.type = Pulse::Type::Zero;
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gap.length = Time(1);
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read_next_block();
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return gap;
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}
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return pulse;
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}
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void ZXSpectrumTAP::read_next_block() {
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if(file_.tell() == file_.stats().st_size) {
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phase_ = Phase::Gap;
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} else {
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block_length_ = file_.get16le();
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data_byte_ = block_type_ = file_.get8();
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phase_ = Phase::PilotTone;
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}
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distance_into_phase_ = 0;
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}
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