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A2 Emulator - Disk Images - Technical Information Library
Universal 2img
+ 0 char magic[4]; // "2IMG"
+ 4 char creator[4]; // Creator signature
+ 8 word16 header_len; // Length of header in bytes
+10 word16 version; // Image version
+12 word32 image_format;// Image data format (see below)
+16 word32 flags; // Format flags (see below)
+20 word32 num_blocks; // Number of 512-byte blocks in this image
+24 word32 data_offset; // File offset to start of data
+28 word32 data_len; // Length of data in bytes
+32 word32 cmnt_offset; // File offset to start of comment
+36 word32 cmnt_len; // Length of comment in bytes
+40 word32 creator_offset; // File offset to start of creator data
+44 word32 creator_len; // Length of creator data in bytes
+48 word32 spare[4]; // Spare words (pads header to 64 bytes)
Below is a brief description:
+ 0 char magic[4];
Contains the string "2IMG"
+ 4 char creator[4];
A signature of the software that created this image. Important to
properly handle the creator-specific chunk.
+ 8 word16 header_len;
The length of this header which equals 52 bytes as of this writing.
+10 word16 version;
The version number of this header. We're at version 1 at this moment.
+12 word32 image_format;
Describes the type of disk image stored in the DATA chunk. Possible
values are;
0 for DOS 3.3 -order floppy images. This is the same kind of data as in
.do images. The disk data is already decoded, stored as logical disk
blocks, sorted by logical block number.
1 for ProDOS-order disk images.
This format is for hard disk images and "generic SmartPort devices".
Each block has 512 bytes. Blocks are sorted by their logical block
number.
2 for raw dumps of the disk nibbles. This is only valid for
floppy disk images.
+16 word32 flags;
flags has the following layout:
31 27 23 19 15 11 7 3 0
Lxxx xxxx xxxx xxxx xxxx xxxV vvvv vvvv
|\_________________________/| \_______/
| | | |
Disk Lock Reserved Vol #? DOS 3.3 volume #
x: reserved (0)
L: if true (1), disk image is locked
V: if true (1), the least significant 8 bits provide the volume number.
This is only meaningful for DOS 3.3 images. If V is zero, then the
volume number is not specified and 254 is assumed.
(Again, only in case of a DOS
3.3 image.)
v: volume number (byte).
+20 word32 num_blocks;
Number of 512-byte blocks in image. Only makes sense when format flag is
set to 1 (ProDOS image.)
+24 word32 data_offset;
+28 word32 data_len;
Indicates where the actual disk bytes begin. The offset is calculated
from the very beginning of the file, i.e. it includes the size of this
header. The following 4 bytes set the size in bytes of the data chunk.
Note: the DATA chunk must preceed the COMMENT and CREATOR chunks.
+32 word32 cmnt_offset;
+36 word32 cmnt_len;
Points at a text chunk for a user-defined "disk label". The text should
be ASCII encoded. Since the cmnt_len field determines the length of the
comment chunk, there's no C-style terminator or Pascal-style length
byte/word in the text chunk required.
Note: the COMMENT chunk must come after the DATA chunk and before the
CREATOR chunk. If it's not present, set both the offset and length
words to zero.
+40 word32 creator_offset;
+44 word32 creator_len;
You may add your own chunk where you can store data specific to your
emulator. Please keep in mind that this chunk is linked to the creator
code at the beginning of the header. If you really need to change the
creator, you should take appropriate steps to remove the CREATOR chunk
or convert it to your own format. Just don't change the creator
signature and keep the old creator chunk.
Note: the CREATOR chunk must be the last one of the three chunks. If
it's not present, set both the offset and length words to zero.
+48 word32 spare[4];
Reserved for future use.
.PO disks contain logical disk bytes, i.e. blocks of 256 bytes each
sorted by their logical block number. Since they are only used for
dumping floppies, .PO images always have 560 blocks originating from 35
tracks of 16 sectors each. You can use the following translation table
to find out about a sector's physical block number:
block: 0 1 2 3 4 5 6 7 8 9 A B C D E F
position: 0 8 1 9 2 A 3 B 4 C 5 D 6 E 7 F
This table mirrors ProDOS' hard-coded interleaving. It is different from
the one found on DOS 3.3 disks.
.DO disks contain logical disk bytes, i.e. blocks of 256 bytes each
sorted by their logical block number. Since they are only used for
dumping floppies, .DO images always have 560 blocks originating from 35
tracks of 16 sectors each. You can use the following translation table
to find out about a sector's physical block number:
block: 0 1 2 3 4 5 6 7 8 9 A B C D E F
position: 0 7 E 6 D 5 C 4 B 3 A 2 9 1 8 F
This table mirrors DOS 3.3' hard-coded interleaving. It is different
from the one found on ProDOS disks.
.NIB images are copies of a disk's nibblized disk bytes. Since they are
pure copies without having gone through any sort of decoding, they work
equally well for both DOS and ProDOS disks.
.NIB images are made up of 35 tracks. Each track has $1A00 (6656) bytes.
There's no header structure. The first disk byte of track 0 starts at
file offset +0, the first byte of track 1 at file offset +$1A00, and so
on.