The different diskdrives that are
available for SVI-318/328 uses 5.25" diskettes. For the most common
diskdrive, SV-902, are softsectored single sided diskettes with double
density 40 tracks used. The latest expansion unit, SV-605B, uses double
sided diskettes. There is also a rare diskdrive, SV-912, that uses 80
tracks single sided diskettes.
This means that the formatted capacity of a diskette is:
18 x 128 + 39 x 17 x 256 = 172 032 bytes
But in reality, only this can be used for datastorage:
36 x 17 x 256 = 156 672 bytes
The three outer tracks (0, 1 and 2) is
used for Disk Basic and track 20 for the file directory structure. The
other 36 tracks (3-19 and 21-39) is used for storage of files. Each
track is used for data storage, wich means that 1 to 36 files can be
stored on a diskette.
Track 20 structure
The 17 sectors in track 20 is used in the following way:
File directory (sector 1-13)
The file directory uses 16 bytes/file. This means that each sector can hold information for 16 files. The 16 bytes that are reserved for each file are used in following way:
Normally, only sectors 1-2 and sometimes 3
are used. While sectores 4-13 are unused. The file directory is ended
when a filename has the value 255 (&HFF).
Byte 9 can in other words have the following values:
A write protected Basic file has the value
90 (hex) and a ASCII file that will be control read while written has
the value 41 (hex).
Disk Allocation Table (sector 14)
The Disk Allocation Table contains
information regarding the diskette.
File Allocation Table (sectors 15-17)
Sectors 15, 16 and 17 are copies of each
other, and contains a copy of the File Allocation Table. For FAT are the
bytes 0-39 used, that means that there are one byte for each track on
the diskette. The other bytes (40-255) are not used, they just contains
When a file is deleted with the command
"KILL drive:filename.type", is only the first byte of the filename
changed to 0 and used bytes in FAT to 255. Other file information is not
changed until that space is used again.
The first file, the Basic file FIRST.GD,
uses 3 sectors of track 19 (&H13). The other file, binary file DEMO,
uses the whole track 21 (&H15) and 8 sectors of track 22 (&H16).
Now it's easy to understand what the
program "format" does. First is the file directory track 13 intialized,
all the bytes are set to the value 255 (&HFF). After that is the Disk
Allocation Table filled with 0. Finally is first 40 bytes of the File
Allocation Table (all 3 tracks) set to 255, except for the bytes 0, 1, 2
and 20 wich are set to 254 (&HFE).
Logical and Physical sector
Transfer a file from Disk Basic to CP/M
Disk Name: Spectravideo SV-328 DS Type: 5 1/4 Total usable space: 326k bytes Sector information: - 256 bytes/sector; 17 per track; numbered 1 to 17 (Side 2 numbered 1 to 17) - Disk is filled all first side, then second side - System tracks are formatted differently than data tracks. Logical Layout: - Allocation unit size is 2048 bytes; 163 AU's per disk - Directory starts at logical track 3; has 64 entries and occupies 8 sectors or 1 AU's Standard disk parameter table entry (values in HEX): 22 00 04 0F 01 A2 00 3F 00 80 00 10 00 03 00 01 01 Same format as Rank Xerox 820-II
The format of track 0 is FM encoded and the other tracks are MFM encoded. The data architecture of the MFM tracks are IBM System 34:
The data architecture of the FM tracks are IBM 3740:
This arcticle was written by Hans
Magnusson and published in NSVK's (The Nordic SpectraVideo Club)