Inside the Einstein: The Disc, a TatungBytes course: XtalDOS listing the first practice disc image, on a rainbow sunburst

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Appendix II: The Disc At A Glance

Everything on an Einstein disc, on one page. Sections in brackets.

Shape


Tracks	40, numbered 0-39 (S1)
Sides	1
Sectors per track	10, IDs 0-9 - in any order on the track (S10)
Bytes per sector	512
Whole disc	40 x 10 x 512 = 204,800 bytes

Plan (S12)

The disc's plan

Where	What	Size
Tracks 0-1	Kept for the system: what the Einstein starts from	10,240 bytes
Track 2, sectors 0-3	The directory: 64 slots of 32 bytes; slot n at sector n // 16, offset (n % 16) x 32	2K
Track 2 sector 4 to track 39 sector 9	Files	188K
Tracks 2-39 together	What `DIR` calls `Total`	190K

Blocks (S15)

2K each: four sectors.
Block n's i-th sector: logical sector 4n + i, at track (4n + i) // 10 + 2, sector (4n + i) % 10 - worked out for each sector, because a block can cross tracks.
Block 0 is the directory. Files use blocks 1-94.
Space is given a whole block at a time, lowest free block first.

A Directory Entry (S13)

A directory entry

Byte	Holds
0	`00` in use; `E5` free or deleted
1-8	Name, space-padded, capitals by convention
9-11	Extension, space-padded. Top bit of byte 9 = locked (`DIR` shows `*`)
12	Extent: 0, 1, 2 ... - one entry per 16K
13-14	Not used (0)
15	Records: 128-byte records in this extent, at most 128
16-31	Block numbers: 8 x 2 bytes, low byte first, as the Einstein writes them; 0 = none. A few discs use 16 x 1 byte - if any of bytes 17, 19 ... 31 is not 0, it is that kind

Files


Size on disc (`DIR`'s figure)	2K per block number in its entries (S14)
Size of the data	Records x 128, summed over its extents - not an exact length (S16)
End of a text file	The byte `1A`; the rest of its last record is `1A` too (S16)
Text line endings	`0D 0A` (S22)
A file over 16K	One entry per 16K, same name, extents 0, 1, 2 ...; all but the last have 128 records and 8 blocks (S17)
Names	Case is not part of a name: `story.txt` is `STORY.TXT` (S14). Names can hold characters a computer's file system cannot, such as `/` (S18)
Deleting	Byte 0 of each entry becomes `E5`; nothing else changes; the blocks become free (S24)

Space


Free space	188K minus 2K for every block named by a live entry (S20)
Free slots	Slots whose byte 0 is `E5` (S20)
Most files on a disc	64 (one slot each, more for files over 16K)

A Blank Disc (S19)

Every sector E5. That is an empty directory (every slot starts E5) and empty space at once.

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