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247 lines
6.4 KiB
C++
247 lines
6.4 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) : _file(nullptr), _bitPhase(3), _filePhase(FilePhaseLeadIn), _phaseOffset(0), _copy_mask(0x80)
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{
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struct stat file_stats;
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stat(file_name, &file_stats);
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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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_file = fopen(file_name, "rb");
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if(!_file) throw ErrorBadFormat;
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_load_address = (uint16_t)fgetc(_file);
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_load_address |= (uint16_t)fgetc(_file) << 8;
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_length = (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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PRG::~PRG()
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{
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if(_file) fclose(_file);
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}
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Storage::Tape::Tape::Pulse PRG::virtual_get_next_pulse()
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{
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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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_bitPhase = (_bitPhase+1)&3;
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if(!_bitPhase) 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 = (_bitPhase&1) ? Tape::Pulse::High : Tape::Pulse::Low;
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switch(_outputToken)
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{
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case Leader: pulse.length.length = leader_zero_length; break;
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case Zero: pulse.length.length = (_bitPhase&2) ? one_length : zero_length; break;
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case One: pulse.length.length = (_bitPhase&2) ? zero_length : one_length; break;
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case WordMarker: pulse.length.length = (_bitPhase&2) ? one_length : marker_length; break;
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case EndOfBlock: pulse.length.length = (_bitPhase&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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{
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_bitPhase = 3;
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fseek(_file, 2, SEEK_SET);
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_filePhase = FilePhaseLeadIn;
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_phaseOffset = 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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{
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return _filePhase == FilePhaseAtEnd;
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}
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void PRG::get_next_output_token()
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{
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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(_filePhase == FilePhaseHeaderDataGap || _filePhase == FilePhaseAtEnd)
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{
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_outputToken = Silence;
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if(_filePhase != FilePhaseAtEnd) _filePhase = 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(_filePhase == FilePhaseLeadIn || _phaseOffset < block_leadin_length)
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{
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_outputToken = Leader;
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_phaseOffset++;
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if(_filePhase == FilePhaseLeadIn && _phaseOffset == leadin_length)
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{
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_phaseOffset = 0;
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_filePhase = (_filePhase == 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 = _phaseOffset - 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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_phaseOffset++;
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if(!bit_offset &&
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(
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(_filePhase == FilePhaseHeader && byte_offset == block_length + countdown_bytes + 1) ||
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feof(_file)
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)
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)
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{
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_outputToken = EndOfBlock;
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_phaseOffset = 0;
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switch(_filePhase)
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{
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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) _filePhase = FilePhaseHeaderDataGap;
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break;
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case FilePhaseData:
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_copy_mask ^= 0x80;
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fseek(_file, 2, SEEK_SET);
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if(_copy_mask) _filePhase = 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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{
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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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{
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_output_byte = (uint8_t)(countdown_bytes - byte_offset) | _copy_mask;
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}
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else
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{
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if(_filePhase == FilePhaseHeader)
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{
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if(byte_offset == countdown_bytes + block_length)
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{
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_output_byte = _check_digit;
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}
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else
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{
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if(byte_offset == countdown_bytes) _check_digit = 0;
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if(_filePhase == FilePhaseHeader)
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{
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switch(byte_offset - countdown_bytes)
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{
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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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}
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else
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{
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_output_byte = (uint8_t)fgetc(_file);
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if(feof(_file))
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{
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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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{
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case 0:
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_outputToken = WordMarker;
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break;
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default: // i.e. 1–8
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_outputToken = (_output_byte & (1 << (bit_offset - 1))) ? One : Zero;
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break;
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case 9:
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{
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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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_outputToken = (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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