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325 lines
9.0 KiB
C++
325 lines
9.0 KiB
C++
//
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// AppleGCR.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 21/04/2018.
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// Copyright 2018 Thomas Harte. All rights reserved.
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//
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#include "Encoder.hpp"
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namespace {
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const uint8_t five_and_three_mapping[] = {
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0xab, 0xad, 0xae, 0xaf, 0xb5, 0xb6, 0xb7, 0xba,
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0xbb, 0xbd, 0xbe, 0xbf, 0xd6, 0xd7, 0xda, 0xdb,
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0xdd, 0xde, 0xdf, 0xea, 0xeb, 0xed, 0xee, 0xef,
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0xf5, 0xf6, 0xf7, 0xfa, 0xfb, 0xfd, 0xfe, 0xff
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};
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const uint8_t six_and_two_mapping[] = {
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0x96, 0x97, 0x9a, 0x9b, 0x9d, 0x9e, 0x9f, 0xa6,
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0xa7, 0xab, 0xac, 0xad, 0xae, 0xaf, 0xb2, 0xb3,
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0xb4, 0xb5, 0xb6, 0xb7, 0xb9, 0xba, 0xbb, 0xbc,
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0xbd, 0xbe, 0xbf, 0xcb, 0xcd, 0xce, 0xcf, 0xd3,
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0xd6, 0xd7, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde,
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0xdf, 0xe5, 0xe6, 0xe7, 0xe9, 0xea, 0xeb, 0xec,
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0xed, 0xee, 0xef, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6,
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0xf7, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff
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};
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/*!
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Produces a PCM segment containing @c length sync bytes, each aligned to the beginning of
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a @c bit_size -sized window.
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*/
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Storage::Disk::PCMSegment sync(int length, int bit_size) {
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Storage::Disk::PCMSegment segment;
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// Reserve sufficient storage.
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segment.data.reserve(size_t(length * bit_size));
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// Write patters of 0xff padded with 0s to the selected bit size.
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while(length--) {
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int c = 8;
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while(c--)
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segment.data.push_back(true);
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c = bit_size - 8;
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while(c--)
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segment.data.push_back(false);
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}
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return segment;
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}
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}
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using namespace Storage::Encodings;
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Storage::Disk::PCMSegment AppleGCR::six_and_two_sync(int length) {
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return sync(length, 10);
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}
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Storage::Disk::PCMSegment AppleGCR::five_and_three_sync(int length) {
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return sync(length, 9);
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}
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Storage::Disk::PCMSegment AppleGCR::AppleII::header(uint8_t volume, uint8_t track, uint8_t sector) {
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const uint8_t checksum = volume ^ track ^ sector;
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// Apple headers are encoded using an FM-esque scheme rather than 6 and 2, or 5 and 3.
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std::vector<uint8_t> data(14);
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data[0] = header_prologue[0];
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data[1] = header_prologue[1];
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data[2] = header_prologue[2];
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#define WriteFM(index, value) \
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data[index+0] = uint8_t(((value) >> 1) | 0xaa); \
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data[index+1] = uint8_t((value) | 0xaa); \
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WriteFM(3, volume);
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WriteFM(5, track);
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WriteFM(7, sector);
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WriteFM(9, checksum);
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#undef WriteFM
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data[11] = epilogue[0];
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data[12] = epilogue[1];
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data[13] = epilogue[2];
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return Storage::Disk::PCMSegment(data);
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}
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Storage::Disk::PCMSegment AppleGCR::five_and_three_data(const uint8_t *source) {
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std::vector<uint8_t> data(410 + 7);
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data[0] = data_prologue[0];
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data[1] = data_prologue[1];
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data[2] = data_prologue[2];
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data[414] = epilogue[0];
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data[411] = epilogue[1];
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data[416] = epilogue[2];
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// TODO: encode.
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(void)source;
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// std::size_t source_pointer = 0;
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// std::size_t destination_pointer = 3;
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// while(source_pointer < 255) {
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// encode_five_and_three_block(&segment.data[destination_pointer], &source[source_pointer]);
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//
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// source_pointer += 5;
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// destination_pointer += 8;
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// }
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// Map five-bit values up to full bytes.
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for(std::size_t c = 0; c < 410; ++c) {
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data[3 + c] = five_and_three_mapping[data[3 + c]];
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}
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return Storage::Disk::PCMSegment(data);
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}
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// MARK: - Apple II-specific encoding.
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Storage::Disk::PCMSegment AppleGCR::AppleII::six_and_two_data(const uint8_t *source) {
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std::vector<uint8_t> data(349);
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// Add the prologue and epilogue.
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data[0] = data_prologue[0];
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data[1] = data_prologue[1];
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data[2] = data_prologue[2];
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data[346] = epilogue[0];
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data[347] = epilogue[1];
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data[348] = epilogue[2];
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// Fill in byte values: the first 86 bytes contain shuffled
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// and combined copies of the bottom two bits of the sector
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// contents; the 256 bytes afterwards are the remaining
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// six bits.
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const uint8_t bit_reverse[] = {0, 2, 1, 3};
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for(std::size_t c = 0; c < 84; ++c) {
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data[3 + c] =
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uint8_t(
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bit_reverse[source[c]&3] |
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(bit_reverse[source[c + 86]&3] << 2) |
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(bit_reverse[source[c + 172]&3] << 4)
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);
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}
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data[87] =
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uint8_t(
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(bit_reverse[source[84]&3] << 0) |
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(bit_reverse[source[170]&3] << 2)
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);
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data[88] =
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uint8_t(
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(bit_reverse[source[85]&3] << 0) |
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(bit_reverse[source[171]&3] << 2)
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);
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for(std::size_t c = 0; c < 256; ++c) {
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data[3 + 86 + c] = source[c] >> 2;
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}
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// Exclusive OR each byte with the one before it.
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data[345] = data[344];
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std::size_t location = 344;
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while(location > 3) {
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data[location] ^= data[location-1];
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--location;
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}
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// Map six-bit values up to full bytes.
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for(std::size_t c = 0; c < 343; ++c) {
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data[3 + c] = six_and_two_mapping[data[3 + c]];
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}
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return Storage::Disk::PCMSegment(data);
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}
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// MARK: - Macintosh-specific encoding.
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AppleGCR::Macintosh::SectorSpan AppleGCR::Macintosh::sectors_in_track(int track) {
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// A Macintosh disk has 80 tracks, divided into 5 16-track zones. The outermost
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// zone has 12 sectors/track, the next one in has only 11 sectors/track, and
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// that arithmetic progression continues.
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//
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// (... and therefore the elementary sum of an arithmetic progression formula
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// is deployed below)
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const int zone = track >> 4;
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const int prior_sectors = 16 * zone * (12 + (12 - (zone - 1))) / 2;
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AppleGCR::Macintosh::SectorSpan result;
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result.length = 12 - zone;
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result.start = prior_sectors + (track & 15) * result.length;
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return result;
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}
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Storage::Disk::PCMSegment AppleGCR::Macintosh::header(uint8_t type, uint8_t track, uint8_t sector, bool side_two) {
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std::vector<uint8_t> data(11);
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// The standard prologue.
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data[0] = header_prologue[0];
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data[1] = header_prologue[1];
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data[2] = header_prologue[2];
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// There then follows:
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//
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// 1) the low six bits of the track number;
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// 2) the sector number;
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// 3) the high five bits of the track number plus a side flag;
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// 4) the type; and
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// 5) the XOR of all those fields.
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//
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// (all two-and-six encoded).
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data[3] = track&0x3f;
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data[4] = sector;
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data[5] = (side_two ? 0x20 : 0x00) | ((track >> 6) & 0x1f);
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data[6] = type;
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data[7] = data[3] ^ data[4] ^ data[5] ^ data[6];
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for(size_t c = 3; c < 8; ++c) {
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data[c] = six_and_two_mapping[data[c]];
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}
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// Then the standard epilogue.
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data[8] = epilogue[0];
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data[9] = epilogue[1];
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data[10] = epilogue[2];
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return Storage::Disk::PCMSegment(data);
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}
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Storage::Disk::PCMSegment AppleGCR::Macintosh::data(uint8_t sector, const uint8_t *source) {
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std::vector<uint8_t> output(710);
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int checksum[3] = {0, 0, 0};
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// Write prologue.
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output[0] = data_prologue[0];
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output[1] = data_prologue[1];
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output[2] = data_prologue[2];
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// Add the sector number.
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output[3] = six_and_two_mapping[sector & 0x3f];
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// The Macintosh has a similar checksum-as-it-goes approach to encoding
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// to the Apple II, but works entirely differently. Each three bytes of
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// input are individually encoded to four GCR bytes, their output values
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// being a (mutating) function of the current checksum.
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//
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// Address references below, such as 'Cf. 18FA4' are to addresses in the
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// Macintosh Plus ROM.
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for(size_t c = 0; c < 175; ++c) {
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uint8_t values[3];
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// The low byte of the checksum is rotated left one position; Cf. 18FA4.
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checksum[0] = (checksum[0] << 1) | (checksum[0] >> 7);
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// See 18FBA and 18FBC: an ADDX (with the carry left over from the roll)
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// and an EOR act to update the checksum and generate the next output.
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values[0] = uint8_t(*source ^ checksum[0]);
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checksum[2] += *source + (checksum[0] >> 8);
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++source;
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// As above, but now 18FD0 and 18FD2.
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values[1] = uint8_t(*source ^ checksum[2]);
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checksum[1] += *source + (checksum[2] >> 8);
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++source;
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// Avoid a potential read overrun, but otherwise continue as before.
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if(c == 174) {
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values[2] = 0;
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} else {
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values[2] = uint8_t(*source ^ checksum[1]);
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checksum[0] += *source + (checksum[1] >> 8);
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++source;
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}
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// Throw away the top bits of checksum[1] and checksum[2]; the original
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// routine is byte centric, the longer ints here are just to retain the
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// carry after each add transientliy.
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checksum[0] &= 0xff;
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checksum[1] &= 0xff;
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checksum[2] &= 0xff;
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// Having mutated those three bytes according to the current checksum,
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// and the checksum according to those bytes, run them through the
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// GCR conversion table.
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output[4 + c*4 + 1] = six_and_two_mapping[values[0] & 0x3f];
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output[4 + c*4 + 2] = six_and_two_mapping[values[1] & 0x3f];
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output[4 + c*4 + 3] = six_and_two_mapping[values[2] & 0x3f];
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output[4 + c*4 + 0] = six_and_two_mapping[
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((values[0] >> 2) & 0x30) |
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((values[1] >> 4) & 0x0c) |
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((values[2] >> 6) & 0x03)
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];
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}
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// Also write the checksum.
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//
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// Caveat: the first byte written here will overwrite the final byte that
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// was deposited in the loop above. That's deliberate. The final byte from
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// the loop above doesn't contain any useful content, and isn't actually
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// included on disk.
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output[704] = six_and_two_mapping[checksum[2] & 0x3f];
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output[705] = six_and_two_mapping[checksum[1] & 0x3f];
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output[706] = six_and_two_mapping[checksum[0] & 0x3f];
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output[703] = six_and_two_mapping[
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((checksum[2] >> 2) & 0x30) |
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((checksum[1] >> 4) & 0x0c) |
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((checksum[0] >> 6) & 0x03)
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];
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// Write epilogue.
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output[707] = epilogue[0];
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output[708] = epilogue[1];
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output[709] = epilogue[2];
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return Storage::Disk::PCMSegment(output);
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}
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