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jvc_dsk: Added notes (nw)

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Nigel Barnes 2018-03-06 17:41:45 +00:00
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src/lib/formats/jvc_dsk.cpp
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@ -6,10 +6,101 @@
Disk image format
Named after its creator, Jeff Vavasour
Used by Jeff Vavasour's CoCo Emulators
Documentation taken from Tim Linder's web site:
http://tlindner.macmess.org/?page_id=86
A. Header length
The header length is determined by the file length modulo 256:
headerSize = fileLength % 256;
This means that the header is variable length and the minimum size
is zero bytes, and the maximum size of 255 bytes.
B. Header
Here is a description of the header bytes:
Byte Offset Description Default
----------- ----------- -------
0 Sectors per track 18
1 Side count 1
2 Sector size code 1
3 First sector ID 1
4 Sector attribute flag 0
If the sector attribute flag is zero then the track count is determined
by the formula:
(fileLength - headerSize) / (sectorsPerTrack * (128 <<
sectorSizeCode)) / sideCount
If the sector attribute flag is non zero then the track count is
determined by the more complex formula:
(fileLength - headerSize) / (sectorsPerTrack * ((128 <<
sectorSizeCode) + 1) ) / sideCount
If the length of the header is to short to contain the geometry desired,
then the default values are assumed. If the header length is zero the all
of the geometry is assumed. When creating disk images it is desirable to
make the header length as short as possible. The header should only be
used to deviate from the default values.
The sector data begins immediately after the header. If the header length
is zero then the sector data is at the beginning file.
C. Sectors per track
This is the number of sectors per track (ones based). A value of 18
means there are 18 sectors per track
D. Side Count
This is the number of sides in the disk image. Values of 1 or 2 are
acceptable. If there are two sides then the tracks are interleaved.
The first track in the image file is track zero side 1, the second
track in the image file is track zero side 2.
E. Sector size
The is the same value that is stored in the wd179x ID field to
determine sector size:
0x00 128 bytes
0x01 256 bytes
0x02 512 bytes
0x03 1024 bytes
Other values are undefined. Every sector in the disk image must be the
same size.
F. First sector ID
This determines the first sector ID for each track. Each successive
sector adds one to the previous ID. If the first sector ID is 1, then
the second sector has an ID of 2, and the third has an ID of 3.
G. Sector Attribute Flag
If this byte is non zero, then each sector contains an additional
byte prepended to the sector data. If the attribute flag is zero then
there are no extra bytes in front of the sector data.
H. Sector attribute byte
This byte is put at the beginning of every sector if the header flag
is turned on. The information this byte contains is the same as the
status register (of the wd179x) would contain when a 'Read Sector'
command was issued. The bit fields are defined as:
Bit position:
---------------
7 6 5 4 3 2 1 0
| | | | | | | |
| | | | | | | +--- Not used. Set to zero.
| | | | | | +----- Not used. Set to zero.
| | | | | +------- Not used. Set to zero.
| | | | +--------- Set on CRC error.
| | | +----------- Set if sector not found.
| | +------------- Record type: 1 - Deleted Data Mark, 0 - Data Mark.
| +--------------- Not Used. Set to zero.
+----------------- Not Used. Set to zero.
TODO:
- Support writing unusal formats?
- Support writing unusual formats?
***************************************************************************/