A description of the hardware-level track format of the IBM media used on the BESM-6 I've found is as follows (translated by Google with minimal corrections):
Track header structure:
----------------------------
At the beginning of each track, its header is recorded, which
is a summary of track information used for hardware and software control and allowing
to organize work with a package that has defective tracks.
The header contains:
- Address marker (AM) - 12 bits,
- Identifier (I) - 2 bits,
- Track address (A) 16 bits,
- Address checksum (ACS) - 12 bits.
This information is duplicated if the track is good. When
the track is defective, the replacement track address is indicated in the 2nd part of the header.
Position of the header from the beginning of the track sweep:
| 270 | | 42 | | 258 |
| bit | 42 bits | bit | 42 bits | bit |
|zeros | | zeros | | units |
| | | | | |
0000000|AM|I|A|ACS|-------|AM|I|A|ACS|111111111
- Address marker code: 111111000001;
- ID values:
00 - working track,
01 - the path is defective, the one adjacent to it is good
10 - the track is defective, the one adjacent to it is defective
10 - replacement track {Likely a typo, should be 11? - Leo B.}
- Address structure:
16÷9 РР - cylinder number,
8÷5 PP - head number,
4÷1 РР = 0.
- Address checksum (includes ID)
is formed taking into account the features of the hardware circuit
control.
As reverse-engineered, ACS was a 12-bit "sum with carry to the right" of the lower 12 bits of A with the remaining bits.
The lower 5 bits of A are constant 0, presumably, because there was only one sector per track.
The questions are:
- Does that resemble the track/sector header structure actually used by the IBM disk controller?
- What was the actual structure of the whole sector? How was the beginning of the data part indicated, according to the IBM standard?
