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211 lines
6.1 KiB
C++
211 lines
6.1 KiB
C++
//
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// TapePRG.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 14/08/2016.
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// Copyright © 2016 Thomas Harte. All rights reserved.
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//
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#include "TapePRG.hpp"
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/*
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My interpretation of Commodore's tape format is such that a PRG is encoded as:
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[long block of lead-in tone]
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[short block of lead-in tone]
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[count down][header; 192 bytes fixed length]
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[short block of lead-in tone]
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[count down][copy of header; 192 bytes fixed length]
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[gap]
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[short block of lead-in tone]
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[count down][data; length as in file]
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[short block of lead-in tone]
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[count down][copy of data]
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... and repeat ...
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Individual bytes are composed of:
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word marker
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least significant bit
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...
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most significant bit
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parity bit
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Both the header and data blocks additionally end with an end-of-block marker.
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Encoding is via square-wave cycles of four lengths, in ascending order: lead-in, zero, one, marker.
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Lead-in tone is always just repetitions of the lead-in wave.
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A word marker is a marker wave followed by a one wave.
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An end-of-block marker is a marker wave followed by a zero wave.
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A zero bit is a zero wave followed by a one wave.
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A one bit is a one wave followed by a zero wave.
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Parity is 1 if there are an even number of bits in the byte; 0 otherwise.
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*/
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#include <sys/stat.h>
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using namespace Storage::Tape;
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PRG::PRG(const char *file_name) :
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file_(file_name)
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{
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// There's really no way to validate other than that if this file is larger than 64kb,
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// of if load address + length > 65536 then it's broken.
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if(file_.stats().st_size >= 65538 || file_.stats().st_size < 3)
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throw ErrorBadFormat;
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load_address_ = file_.get16le();
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length_ = static_cast<uint16_t>(file_.stats().st_size - 2);
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if (load_address_ + length_ >= 65536)
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throw ErrorBadFormat;
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}
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Storage::Tape::Tape::Pulse PRG::virtual_get_next_pulse() {
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// these are all microseconds per pole
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static const unsigned int leader_zero_length = 179;
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static const unsigned int zero_length = 169;
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static const unsigned int one_length = 247;
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static const unsigned int marker_length = 328;
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bit_phase_ = (bit_phase_+1)&3;
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if(!bit_phase_) get_next_output_token();
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Tape::Pulse pulse;
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pulse.length.clock_rate = 1000000;
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pulse.type = (bit_phase_&1) ? Tape::Pulse::High : Tape::Pulse::Low;
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switch(output_token_) {
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case Leader: pulse.length.length = leader_zero_length; break;
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case Zero: pulse.length.length = (bit_phase_&2) ? one_length : zero_length; break;
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case One: pulse.length.length = (bit_phase_&2) ? zero_length : one_length; break;
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case WordMarker: pulse.length.length = (bit_phase_&2) ? one_length : marker_length; break;
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case EndOfBlock: pulse.length.length = (bit_phase_&2) ? zero_length : marker_length; break;
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case Silence: pulse.type = Tape::Pulse::Zero; pulse.length.length = 5000; break;
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}
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return pulse;
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}
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void PRG::virtual_reset() {
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bit_phase_ = 3;
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file_.seek(2, SEEK_SET);
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file_phase_ = FilePhaseLeadIn;
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phase_offset_ = 0;
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copy_mask_ = 0x80;
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}
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bool PRG::is_at_end() {
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return file_phase_ == FilePhaseAtEnd;
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}
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void PRG::get_next_output_token() {
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static const int block_length = 192; // not counting the checksum
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static const int countdown_bytes = 9;
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static const int leadin_length = 20000;
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static const int block_leadin_length = 5000;
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if(file_phase_ == FilePhaseHeaderDataGap || file_phase_ == FilePhaseAtEnd) {
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output_token_ = Silence;
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if(file_phase_ != FilePhaseAtEnd) file_phase_ = FilePhaseData;
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return;
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}
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// the lead-in is 20,000 instances of the lead-in pair; every other phase begins with 5000
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// before doing whatever it should be doing
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if(file_phase_ == FilePhaseLeadIn || phase_offset_ < block_leadin_length) {
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output_token_ = Leader;
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phase_offset_++;
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if(file_phase_ == FilePhaseLeadIn && phase_offset_ == leadin_length) {
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phase_offset_ = 0;
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file_phase_ = (file_phase_ == FilePhaseLeadIn) ? FilePhaseHeader : FilePhaseData;
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}
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return;
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}
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// determine whether a new byte needs to be queued up
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int block_offset = phase_offset_ - block_leadin_length;
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int bit_offset = block_offset % 10;
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int byte_offset = block_offset / 10;
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phase_offset_++;
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if(!bit_offset &&
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(
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(file_phase_ == FilePhaseHeader && byte_offset == block_length + countdown_bytes + 1) ||
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file_.eof()
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)
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) {
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output_token_ = EndOfBlock;
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phase_offset_ = 0;
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switch(file_phase_) {
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default: break;
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case FilePhaseHeader:
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copy_mask_ ^= 0x80;
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if(copy_mask_) file_phase_ = FilePhaseHeaderDataGap;
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break;
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case FilePhaseData:
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copy_mask_ ^= 0x80;
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file_.seek(2, SEEK_SET);
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if(copy_mask_) file_phase_ = FilePhaseAtEnd;
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break;
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}
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return;
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}
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if(bit_offset == 0) {
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// the first nine bytes are countdown; the high bit is set if this is a header
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if(byte_offset < countdown_bytes) {
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output_byte_ = static_cast<uint8_t>(countdown_bytes - byte_offset) | copy_mask_;
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} else {
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if(file_phase_ == FilePhaseHeader) {
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if(byte_offset == countdown_bytes + block_length) {
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output_byte_ = check_digit_;
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} else {
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if(byte_offset == countdown_bytes) check_digit_ = 0;
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if(file_phase_ == FilePhaseHeader) {
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switch(byte_offset - countdown_bytes) {
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case 0: output_byte_ = 0x03; break;
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case 1: output_byte_ = load_address_ & 0xff; break;
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case 2: output_byte_ = (load_address_ >> 8)&0xff; break;
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case 3: output_byte_ = (load_address_ + length_) & 0xff; break;
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case 4: output_byte_ = ((load_address_ + length_) >> 8) & 0xff; break;
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case 5: output_byte_ = 0x50; break; // P
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case 6: output_byte_ = 0x52; break; // R
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case 7: output_byte_ = 0x47; break; // G
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default:
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output_byte_ = 0x20;
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break;
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}
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}
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}
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} else {
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output_byte_ = file_.get8();
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if(file_.eof()) {
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output_byte_ = check_digit_;
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}
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}
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check_digit_ ^= output_byte_;
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}
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}
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switch(bit_offset) {
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case 0:
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output_token_ = WordMarker;
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break;
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default: // i.e. 1–8
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output_token_ = (output_byte_ & (1 << (bit_offset - 1))) ? One : Zero;
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break;
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case 9: {
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uint8_t parity = output_byte_;
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parity ^= (parity >> 4);
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parity ^= (parity >> 2);
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parity ^= (parity >> 1);
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output_token_ = (parity&1) ? Zero : One;
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}
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break;
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}
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}
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