The first 512 bytes of the image file contain the image header, and image loading information records. This contains all the information needed by the net boot process as to where data is to be loaded.
The magic number (in time-honoured tradition (well why not?)) is:
0 = 36h
1 = 13h
2 = 03h
3 = 1Bh
Apart from the two magic numbers, all words and double words are in PC native endian.
Including the initial magic number the header record is:
+---------------------+
| |
| Initial Magic No. | 4 bytes
+---------------------+
| |
| Flags and length | double word
+---------------------+
| |
| Location Address | double word in ds:bx format
+---------------------+
| |
| Execute Address | double word in cs:ip format
+---------------------+
The Location address is where to place the 512 bytes. The net boot process does this before loading the rest of the image. The location address cannot be one of the reserved locations mentioned above, but must be an address lower than 100000h.
The rest of the image must not overwrite these initial 512 bytes, placed at the required location. The writing of data by the net boot process into these 512 bytes is deprecated. These 512 bytes must be available for the image to interogate once it is loaded and running.
The execute address is the location in cs:ip of the initial instruction once the full image has been loaded. This must be lower than 100000h, since the initial instructions will be executed in 8086 mode. When the jump (actaully a far call) is made to the boot image, the stack contains a far return address, with a far pointer parameter above that, pointing to the location of this header.
The flags and length field is broken up in the following way:
Bits 0 to 3 (lowest 4 bits) define the length of the non vendor header in double words. Currently the value is 4.
Bits 4 to 7 define the length required by the vendor extra information in double words. A value of zero indicates no extra vendor information.
(gk)Bit 8 is set if the boot image can return to the net boot process after execution. If this bit is not set the boot image does never return to the net boot process, and the net boot program has to set the system into a clean state before calling the boot image.
Bits 9 to 31 are reserved for future use and must be set to zero.(gk)
After this header, and any vendor header, come the image loading information records. These specify where data is to be loaded, how long it is, and communicates to the loaded image what sort of data it is.
The format of each image loading information record is :
+---------------------+
| Flags, tags and | double word
| lengths |
+---------------------+
| |
| Load Address | double word
+---------------------+
| |
| Image Length | double word
+---------------------+
| |
| Memory Length | double word
+---------------------+
Each image loading information record follows the previous, or the header.
The memory length, image length and load address fields are unsigned 32 numbers. They do not have the segment:offset format used by the 8086.
The flags, tags and lengths field is broken up as follows:
Bits 0 to 3 (lowest 4 bits) are the length of the non vendor part of this header in double words. Currently this value is 4.
Bits 4 to 7 indicate the length of any vendor information, in double words.
Bits 8 to 15 are for vendor's tags. The vendor tag is a private number that the loaded image can use to determine what sort of image is at this particular location.
Bits 16 to 23 are for future expansion and should be set to zero.
Bits 24 to 31 are for flags, which are defined later.
Vendors may place further information after this information record, and before the next. Each information record may have a different vendor length.
There are two restrictions on vendor information.
One is that the header and all information records that the net boot process is to use fall within the first 512 bytes.
The second restriction is that the net boot process must ignore all vendor additions. The net boot process may not overwrite vendor supplied information, or other undefined data in the initial 512 bytes.
The flags are used to modify the load address field, and to indicate that this is the last information record that the net boot process should use.
Bit 24 works in conjunction with bit 25 to specify the meaning of the load address.
B24 B25
0 0 load address is an absolute 32 number
1 0 add the load address to the location one past the last byte
of the memory area required by the last image loaded.
If the first image, then add to 512 plus the location
where the 512 bytes were placed
0 1 subtract the load address from the one past the
last writeable location in memory. Thus 1 would
be the last location one could write in memory.
1 1 load address is subtracted from the start of
the last image loaded. If the first image, then
subtract from the start of where the 512 bytes were
placed
(For convenience bit 24 is byte 0 of the flag field)
Bit 26 is the end marker for the net boot process. It is set when this is the last information record the net boot process should look at. More records may be present, but the net boot process will not look at them. (Vendors can continue information records out past the 512 boundary for private use in this manner).
The image length tells the net boot process how many bytes are to be loaded. Zero is a valid value. This can be used to mark memory areas such as shared memory for interprocessor communication, flash eproms, data in eproms.
The image length can also be different from the memory length. This allows decompression programs to fluff up the kernel image. It also allows a file system to be larger then the loaded file system image.
Bits 27 through 31 are not defined as yet and must be set to zero until they are.