I gave it some thoughts and here's what I've come up with.
The Idea is to make it possible to handle it on a real SAM using
CD-ROM and CF in the future.
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SDI or Sam DiskImage is a new diskimage format to replace the
.DSK .MGT and .SAD diskimages. Key features are crc protection,
a flexible format and smaller diskimages. Diskimage sizes are
reduced by not including unused or empty sectors on a disk.
Information to keep track of the sectors that are used is
achieved with a minimum of overhead.
A SDI diskimage can be seen as just an array of sectors where the
first sector (header) tells where the other (used data) sectors belong.
The diskimage has following structure:
+- header sector -----------+
| 16 byte diskimage info |
| sectormap |
| discription |
| [zero filed gap] |
+---------------------------+
+---------------------------+
| First used sector |
+---------------------------+
...
+---------------------------+
| last used sector |
+---------------------------+
16 byte diskimage info
-----------------------
bytes Discription
[3] 'SDI' magic number in capitals.
[4] 32-bit crc. calculated from next byte onward to the last
byte of the header. This crc should be used to validate
the header.
[4] 32-bit crc. calculated over all the data sectors in the
image excluding the header.
32-bit crc are calculated in the same way as ZMODEM,ZLIB
and PKZIP
[2] disk geometry:
bit 7 = sides: 0 for single sided 1 for double sided
bits 6-0 = tracks 1 to 127 tracks
bit 15 = start sector: 0 start with 0, 1 start with 1
bits 14,13 = sector size: 00 128 byte sectors
01 256 byte sectors
10 512 byte sectors
11 1024 byte sectors
bits 12-8 = sectors per track 1..31 sectors
DEFAULT: value is D0CAh
[1] empty sector value. A sector completely filled with this
value is considered empty and are not included in the sector
data.
DEFAULT = 00h
[1] image flags:
bit 7 = Write Protect: When set no changes are allowed to
data sectors in the image. Header info can be edited
regardless of the state of this bit. To protect the
header. The read only file attribute of the image
file should be set.
DEFAULT = 0
bit 6 = Dynamic Image: When set, disk geometry may be
dynamically resized to minimum geometry or expanded
depending of the changes made. For example when this
bit is set and the image is formatted the a geometry
is changed to a single sided disk with just one track
contain one sector.
when writing a non empty sector outside the images
geometry then the geometry will be extended if the
sector allocation map can also be extended.
When this bit is clear then the geometry will not change
and any sector access outside the geometry will produce
an error.
DEFAULT = 0
bit 5 = emulator boot preference. When this bit is set then
it is prefered to always do a cold boot when F9 is
pressed or BOOT is executed.
DEFAULT = 0
bit 4 = reserved and should be 0
bits 3-0 = dos emulator. On the real hardware this flag can
be used to decide which diskimage emulator is
loaded to emulate the diskimage.
0 = no preference
1 = SAMDOS
2 = MasterDOS
3 = MASTERDOS+MasterBASIC
4 = Pro-DOS
6 = B-DOS
DEFAULT = 0 no preference
[1] Reserved and should be 00h
[xx] sector adress map or SAM. Like in a directory slot the SAM
represents which sectors are used and the position in the
map determines which track and sector status it reflects.
A '0' bit means that the sectors data is not included in
the diskimage a '1'bit means the sectors data is included.
the SAM has the same makeup as in a directory slot. starting
with bit 0 for the first track and sector up on side one.
Side two follows after the last sector on the last track starting
with bit 0. In case of odd track or sector numbering each side
may have some gap bits which should always be 0.
So incase of a standard disk layout of 2 sides 80 tracks 10
sectors starting with sector 1. Track 0, sector 1 starts at
bit 0,byte 0 and track 128, sector 1 starts at bit 0,byte 100
The sector allocation maps size can be calculated by the
following formula:
size = sides *int((tracks * sectors-per-track + 7)/8) any
unused bits should be set to 0.
DEFAULT SIZE = 200 (double sided 80 track 10 sector disk)
[xx] diskimage discription. The diskimage discription is optional
and can contain one or more zero terminated strings.
the first zero terminated string should have the following
make-up:
Title[0]
Copyright/Release[0]
Author(s)/Programmers/Artists[0]
Discription[0]
example:
SAMDOS V1.0[0]
(C) 1989 MGT[0]
BRUCE GORDON[CR]
ANDREW J.A. WRIGHT[0]
The DOS that wouldn't boot[0]
Note that empty strings consist of just a terminating zero.
This zero is used to count to the strings. and determine
the gap size.
DEFAULT SIZE = 4 bytes ( zero terminators)
[xx] gap filled with zeros to get the header length of 512 bytes.
This Gap may have a length of 0 in some cases. The size of
this gap should be calculated by substracting the position
after the discriptions terminating zero from 512.
DEFAULT SIZE =292 bytes
---------------------------------------------------------------
Edwin
