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Takes some steps towards five-and-three decoding.

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Now I 'just' need to figure out how bits are distributed within the decoded sector. The XORing and data checksum seem the same (?)
This commit is contained in:
Thomas Harte 2020-03-25 00:15:31 -04:00
parent e5cbdfc67c
commit 2320b5c1fe
1 changed files with 59 additions and 28 deletions
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43
Storage/Disk/Encodings/AppleGCR/SegmentParser.cpp
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@ -36,6 +36,25 @@ uint8_t unmap_six_and_two(uint8_t source) {
return six_and_two_unmapping[source - 0x96];
}
const uint8_t five_and_three_unmapping[] = {
/* 0xab */ 0x00, 0xff, 0x01, 0x02, 0x03,
/* 0xb0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x04, 0x05, 0x06,
/* 0xb8 */ 0xff, 0xff, 0x07, 0x08, 0xff, 0x09, 0x0a, 0x0b,
/* 0xc0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xc8 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xd0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0c, 0x0d,
/* 0xd8 */ 0xff, 0xff, 0x0e, 0x0f, 0xff, 0x10, 0x11, 0x12,
/* 0xe0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
/* 0xe8 */ 0xff, 0xff, 0x13, 0x14, 0xff, 0x15, 0x16, 0x17,
/* 0xf0 */ 0xff, 0xff, 0xff, 0xff, 0xff, 0x18, 0x19, 0x1a,
/* 0xf8 */ 0xff, 0xff, 0x1b, 0x1c, 0xff, 0x1d, 0x1e, 0x1f,
};
uint8_t unmap_five_and_three(uint8_t source) {
if(source < 0xab) return 0xff;
return five_and_three_unmapping[source - 0xab];
}
}
using namespace Storage::Encodings::AppleGCR;
@ -56,6 +75,7 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
// sector is still being parsed.
size_t bit = 0;
int header_delay = 0;
bool is_five_and_three = false;
while(bit < segment.data.size() || pointer != scanning_sentinel || header_delay) {
shift_register = static_cast<uint_fast8_t>((shift_register << 1) | (segment.data[bit % segment.data.size()] ? 1 : 0));
++bit;
@ -77,11 +97,16 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
scanner[0] == header_prologue[0] &&
scanner[1] == header_prologue[1] &&
(
scanner[2] == five_and_three_header_prologue[2] ||
scanner[2] == header_prologue[2] ||
scanner[2] == data_prologue[2]
)) {
pointer = 0;
if(scanner[2] != data_prologue[2]) {
is_five_and_three = scanner[2] == five_and_three_header_prologue[2];
}
// If this is the start of a data section, and at least
// one header has been witnessed, start a sector.
if(scanner[2] == data_prologue[2]) {
@ -97,7 +122,8 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
if(new_sector) {
// Check whether the value just read is a legal GCR byte, in six-and-two
// encoding (which is a strict superset of five-and-three).
if(unmap_six_and_two(value) == 0xff) {
const bool is_invalid = is_five_and_three ? (unmap_five_and_three(value) == 0xff) : (unmap_six_and_two(value) == 0xff);
if(is_invalid) {
// The second byte of the standard epilogue is illegal, so this still may
// be a valid sector. If the final byte was the first byte of an epilogue,
// chop it off and see whether the sector is otherwise intelligible.
@ -123,8 +149,7 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
default: // This is not a decodeable sector.
break;
// TODO: check for five-and-three / 13-sector form sectors.
case 411: // Potentially this is an Apple II five-and-three sector.
case 343: { // Potentially this is an Apple II six-and-two sector.
// Check for apparent four and four encoding.
uint_fast8_t header_mask = 0xff;
@ -137,13 +162,15 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
sector->address.sector = ((header[4] << 1) | 1) & header[5];
// Check the header checksum.
uint_fast8_t checksum = ((header[6] << 1) | 1) & header[7];
// The 0x11 is reverse engineered from the game 'Alien Rain' and is present even on the boot sector,
// so probably isn't copy protection?
uint_fast8_t checksum = (((header[6] << 1) | 1) & header[7]) ^ (is_five_and_three ? 0x11 : 0x00);
if(checksum != (sector->address.volume^sector->address.track^sector->address.sector)) continue;
// Unmap the sector contents as 6 and 2 data.
// Unmap the sector contents.
bool out_of_bounds = false;
for(auto &c : sector->data) {
c = unmap_six_and_two(c);
c = is_five_and_three ? unmap_five_and_three(c) : unmap_six_and_two(c);
if(c == 0xff) {
out_of_bounds = true;
break;
@ -160,6 +187,9 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
// Having checked the checksum, remove it.
sector->data.resize(sector->data.size() - 1);
if(is_five_and_three) {
// TODO
} else {
// Undo the 6 and 2 mapping.
const uint8_t bit_reverse[] = {0, 2, 1, 3};
#define unmap(byte, nibble, shift) \
@ -184,6 +214,7 @@ std::map<std::size_t, Sector> Storage::Encodings::AppleGCR::sectors_from_segment
// Add this sector to the map.
sector->encoding = Sector::Encoding::SixAndTwo;
}
result.insert(std::make_pair(sector_location, std::move(*sector)));
} break;