Offset /Name/ /Meaning/
0 log2secsize Log2 of sector size (Sector size = 2^x)
1 secspertrack number of sectors per track
2 heads number of disc heads, if interleaved otherwise -1 (1 for old directories)
3 density disc density
4 idlen length of id field of a map fragment (in bits)
5 log2bpmp log2 of number of bytes per map bits
6 skew track to track sector skew for random access file allocation
7 bootoption boot option (as in *Opt 4,n)
8 lowsector lowest numbered sector + disc flags
9 nzones number of zones in the map
A-B zone_spare number of non-allocation bits between zones
C-F root disc address of root directory
10-13 disc_size disc size in bytes
14-15 disc_id disc cycle id
16-1F disc_name disc name (spaced to 10 chars, no terminator)
20-23 disctype filetype given to disc
'Small' discs (up to 512MB, pre-RISC OS 3.60) :
24-3B reserved, must be 0
'Large' discs (over 512MB, RISC OS 3.60 and later) :
24-27 disc_size2 high word of disc size
28 share_size b0-3: log2 share size granularity (in sectors)
b4-7: reserved, must be 0
29 big_flag b0: set if this is a 'large' disc
b1-7: reserved, must be 0
2A-3B reserved, must be 0
'Big map' discs (more efficient use of space, RISC OS 3.80 and later) :
2A nzones2 high byte of nzones
2B-3B reserved, must be 0
'Big dir' discs (long filenames, RISC OS 3.80 and later) :
2A nzones2 high byte of nzones
2B reserved
2C-2F discversion Currently, 1
30-33 root_size root directory size in bytes
34-3B reserved, must be 0
Disc description flags
bits /Meaning/
0-5 lowest numbered sector id on a track
6 if set, treat sides as a sequence (not interleaved)
7 if set, disc is 40 track
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