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apple2: encoder: Fix writing of "FF40"

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I was writing "FF40" as "FF48": 1111 1111 0000 instead of 1111 1111 00.
This sequence of four zeros will not allow the real hardware to synchronize
to the bitstream.

With this change, I was able to boot a DOS 3.3 disk I rearranged into
"physical" order; some 'there's a disk error' raspberries occur (probably
indicating a data error within a sector) during the boot process, so
something is still obviously marginal, but this is a huge step forward.
This commit is contained in:
Jeff Epler
2022-03-08 07:44:03 -06:00
parent 0aed615ee5
commit 61be3714a5
1 changed files with 19 additions and 21 deletions
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40
arch/apple2/encoder.cc
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@@ -57,6 +57,7 @@ public:
int logicalTrack = physicalTrack / 2;
double clockRateUs = 4.;
// apple2 drives spin at 300rpm. 300/minute in us = 200000.
int bitsPerRevolution = 200000.0 / clockRateUs;
std::vector<bool> bits(bitsPerRevolution);
@@ -110,26 +111,28 @@ private:
write_bits(encode_data_gcr(value), 8);
};
auto write_ff40 = [&]() {
write_bits(0xff0, 12);
};
auto write_ff36 = [&]() {
write_bits(0xff << 2, 10);
};
auto write_ff32 = [&]() {
write_bits(0xff, 8);
// The special "FF40" sequence is used to synchronize the receiving
// shift register. It's written as "1111 1111 00"; FF indicates the
// 8 consecutive 1-bits, while "40" indicates the total number of
// microseconds.
auto write_ff40 = [&](int n=1) {
for(;n--;) {
write_bits(0xff << 2, 10);
}
};
// There is data to encode to disk.
if ((sector.data.size() != APPLE2_SECTOR_LENGTH))
Error() << fmt::format("unsupported sector size {} --- you must pick 256", sector.data.size());
// Write address syncing leader : A sequence of "FF40"s followed by an "FF32", 5 to 40 of them
// "FF40" seems to indicate that the actual data written is "1111 1111 0000" i.e., 8 1s and a total of 40 microseconds
for(int i=0; i<4; i++) { write_ff40(); }
write_ff32();
// Write address syncing leader : A sequence of "FF40"s; 5 of them
// are said to suffice to synchronize the decoder.
// "FF40" indicates that the actual data written is "1111
// 1111 00" i.e., 8 1s and a total of 40 microseconds
//
// In standard formatting, the first logical sector apparently gets
// extra padding.
write_ff40(sector.logicalSector == 0 ? 32 : 8);
// Write address field: APPLE2_SECTOR_RECORD + sector identifier + DE AA EB
write_bits(APPLE2_SECTOR_RECORD, 24);
@@ -139,13 +142,8 @@ private:
write_gcr44(volume_id ^ sector.logicalTrack ^ sector.logicalSector);
write_bits(0xDEAAEB, 24);
// Write the "zip": a gap of 50 (we actually do 52, hopefully it's OK).
// In real HW this is actually turning OFF the write head for 50 cycles
write_bits(0, 13);
// Write data syncing leader: FF40 x4 + FF36 + APPLE2_DATA_RECORD + sector data + sum + DE AA EB (+ mystery bits cut off of the scan?)
for(int i=0; i<4; i++) write_ff40();
write_ff36();
// Write data syncing leader: FF40 + APPLE2_DATA_RECORD + sector data + sum + DE AA EB (+ mystery bits cut off of the scan?)
write_ff40(8);
write_bits(APPLE2_DATA_RECORD, 24);
// Convert the sector data to GCR, append the checksum, and write it out