Added description for Apple // disk image format

DiskImage_2MG_Info.txt

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+GS WorldView
+Summer 2003
+http://apple2.org.za/gswv/a2zine/Docs/DiskImage_2MG_Info.txt
+
+
+2MG (or 2IMG) Disk Image Files
+
+These are disk images (e.g. .dsk or .nib) with a prefix which
+includes information about size, format, sector ordering,
+volume number, locked/unlocked, etc..
+
+2MG files may also have a Comment and/or extra file information
+added following the disk image data.
+
+2MG format can accommodate disk images ranging from 5.25"
+diskette up through hard disk.
+
+Typically, the image files have names ending with ".2mg".
+
+ProDOS File Type-  $E0
+ProDOS Aux Type-   $0130
+
+Emulators which can use 2MG images include Bernie ][ the Rescue,
+Catakig, Sweet 16, XGS.
+
+On an Apple II, ASIMOV2 (for IIgs) is the usual utility for
+creating 2MG files and for converting them to disk.
+
+
+
+2MG (or 2IMG) Disk Image Format
+
+Part of File              Length
+
+Prefix-                   64 bytes (usual size of Prefix)
+Disk Data-                varies (e.g. 143,360 for 5.25" disk)
+Comment (optional)-       varies (often not present)
+Creator added (optional)- varies (often not present)
+
+
+
+Prefix Format
+
+0000-0003: 32 49 4D 47   "2IMG"    ID for 2MG format (ASCII Text)
+0004-0007: 58 47 53 21   "XGS!"    Creator ID (ASCII Text) **
+0008-0009: 40 00                   Header size ($0040= 64 bytes)
+000A-000B: 01 00                   Version number
+000C-000F: 01 00 00 00             Image Format
+                                    00= DOS 3.3 sector order
+                                    01= ProDOS sector order
+                                    02= NIB data
+
+**Note: "Creator" refers to application creating the image.
+Here are ID's in use by various applications:
+
+ASIMOV2-              "!nfc"
+Bernie ][ the Rescue- "B2TR"
+Catakig-              "CTKG"
+Sheppy's ImageMaker-  "ShIm"
+Sweet 16-             "WOOF"
+XGS-                  "XGS!"
+
+
+
+0010-0013: 00 00 00 00  (Flags & DOS 3.3 Volume Number)
+
+The four-byte flags field contains bit flags and data relating to
+the disk image. Bits not defined should be zero.
+
+Bit   Description
+
+31    Locked? If Bit 31 is 1 (set), the disk image is locked. The
+      emulator should allow no changes of disk data-- i.e. the disk
+      should be viewed as write-protected.
+
+8     DOS 3.3 Volume Number? If Bit 8 is 1 (set), then Bits 0-7
+      specify the DOS 3.3 Volume Number. If Bit 8 is 0 and the
+      image is in DOS 3.3 order (Image Format = 0), then Volume
+      Number will be taken as 254.
+
+7-0   The DOS 3.3 Volume Number, usually 1 through 254,
+      if Bit 8 is 1 (set). Otherwise, these bits should be 0.
+
+
+0014-0017: 18 01 00 00  (ProDOS Blocks = 280 for 5.25")
+
+The number of 512-byte blocks in the disk image- this value
+should be zero unless the image format is 1 (ProDOS order).
+Note: ASIMOV2 sets to $118 whether or not format is ProDOS.
+
+
+0018-001B: 40 00 00 00  (Offset to disk data = 64 bytes)
+
+Offset to the first byte of disk data in the image file
+from the beginning of the file- disk data must come before
+any Comment and Creator-specific chunks.
+
+
+001C-001F: 00 30 02 00  (Bytes of disk data = 143,360 for 5.25")
+
+Length of the disk data in bytes. (For ProDOS should be
+512 x Number of blocks)
+
+
+0020-0023: 00 00 00 00  (Offset to optional Comment)
+
+Offset to the first byte of the image Comment- zero if there
+is no Comment. The Comment must come after the data chunk,
+but before the creator-specific chunk. The Comment, if it
+exists, should be raw text; no length byte or C-style null
+terminator byte is required (that's what the next field is for).
+
+
+0024-0027: 00 00 00 00  (Length of optional Comment)
+
+Length of the Comment chunk- zero if there's no Comment.
+
+
+0028-002B: 00 00 00 00  (Offset to optional Creator data)
+
+Offset to the first byte of the Creator-specific data chunk-
+zero if there is none.
+
+
+002C-002F: 00 00 00 00  (Length of optional Creator data)
+
+Length of the Creator-specific data chunk- zero if there is no
+creator-specific data.
+
+
+0030-003F: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+
+Reserved space- at present all must be zero.
+
+
+
+
+Credits
+
+Thanks to Eric Shepherd and Roger Johnstone for 2MG info.
+
+
+Rubywand
+