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Micropolis: Add Vector ECC support

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This commit is contained in:
Eric Anderson
2023-02-25 11:23:12 -08:00
parent c623d95a80
commit 96d4df296d
6 changed files with 141 additions and 18 deletions
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81
arch/micropolis/decoder.cc
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@@ -59,6 +59,70 @@ uint8_t mzosChecksum(const Bytes& bytes)
return checksum;
}
static uint8_t b(uint32_t field, uint8_t pos)
{
return (field >> pos) & 1;
}
static uint8_t eccNextBit(uint32_t ecc, uint8_t data_bit)
{
// This is 0x81932080 which is 0x0104C981 with reversed bits
return b(ecc, 7) ^ b(ecc, 13) ^ b(ecc, 16) ^ b(ecc, 17) ^ b(ecc, 20)
^ b(ecc, 23) ^ b(ecc, 24) ^ b(ecc, 31) ^ data_bit;
}
uint32_t vectorGraphicEcc(const Bytes& bytes)
{
uint32_t e = 0;
Bytes payloadBytes = bytes.slice(0, bytes.size()-4);
ByteReader payload(payloadBytes);
while (!payload.eof()) {
uint8_t byte = payload.read_8();
for (int i = 0; i < 8; i++) {
e = (e << 1) | eccNextBit(e, byte >> 7);
byte <<= 1;
}
}
Bytes trailerBytes = bytes.slice(bytes.size()-4);
ByteReader trailer(trailerBytes);
uint32_t res = e;
while (!trailer.eof()) {
uint8_t byte = trailer.read_8();
for (int i = 0; i < 8; i++) {
res = (res << 1) | eccNextBit(e, byte >> 7);
e <<= 1;
byte <<= 1;
}
}
return res;
}
/* Fixes bytes when possible, returning true if changed. */
static bool vectorGraphicEccFix(Bytes& bytes, uint32_t syndrome)
{
uint32_t ecc = syndrome;
int pos = (MICROPOLIS_ENCODED_SECTOR_SIZE-5)*8+7;
bool aligned = false;
while ((ecc & 0xff000000) == 0) {
pos += 8;
ecc <<= 8;
}
for (; pos >= 0; pos--) {
bool bit = ecc & 1;
ecc >>= 1;
if (bit)
ecc ^= 0x808264c0;
if ((ecc & 0xff07ffff) == 0)
aligned = true;
if (aligned && pos % 8 == 0)
break;
}
if (pos < 0)
return false;
bytes[pos/8] ^= ecc >> 16;
return true;
}
class MicropolisDecoder : public Decoder
{
public:
@@ -132,6 +196,17 @@ public:
auto rawbits = readRawBits(MICROPOLIS_ENCODED_SECTOR_SIZE * 16);
auto bytes =
decodeFmMfm(rawbits).slice(0, MICROPOLIS_ENCODED_SECTOR_SIZE);
bool eccPresent = bytes[274] == 0xaa;
uint32_t ecc = 0;
if (_config.ecc_type() == MicropolisDecoderProto::VECTOR && eccPresent) {
ecc = vectorGraphicEcc(bytes.slice(0, 274));
if (ecc != 0) {
vectorGraphicEccFix(bytes, ecc);
ecc = vectorGraphicEcc(bytes.slice(0, 274));
}
}
ByteReader br(bytes);
int syncByte = br.read_8(); /* sync */
@@ -193,8 +268,10 @@ public:
_sector->data = bytes;
else
error("Sector output size may only be 256 or 275");
_sector->status =
(wantChecksum == gotChecksum) ? Sector::OK : Sector::BAD_CHECKSUM;
if (wantChecksum == gotChecksum && (!eccPresent || ecc == 0))
_sector->status = Sector::OK;
else
_sector->status = Sector::BAD_CHECKSUM;
}
private: