Victor 9000 format

Overview

A very strange format used by the Sirius One / Victor 9000 computer. This machine ran MS-DOS and CP/M, but had a very unusual disc format:

• Variable speed -- 9-zone Z-CLV. The linear velocity of the disc is held fairly constant by rotating certain blocks of tracks slower than others, and storing more data sectors on the outer ("longer") tracks.
• GCR coding -- specifically, a 4-to-5 GCR code which looks like MFM on a timing histogram.

Other than that, they're pretty standard. A DiscFerret can accurately sample and (in theory) decode these discs.

Low level format

GCR code table

From the source code to CW 0.13 by Karsten Scheibler:

input (hex)	output (bin)	output (hex)
0	01010	0x0a
1	01011	0x0b
2	10010	0x12
3	10011	0x13
4	01110	0x0e
5	01111	0x0f
6	10110	0x16
7	10111	0x17
8	01001	0x09
9	11001	0x19
A	11010	0x1a
B	11011	0x1b
C	01101	0x0d
D	11101	0x1d
E	11110	0x1e
F	10101	0x15