This is the specification of the "tagged image" format as defined by Jamie Honan and Gero Kuhlmann in June of 1997.
Note: In order to provide more functionality to the boot rom code, Jamie's draft has been changed a little bit. All changes by Gero Kuhlmann are marked with (gk), and all changes be Rob Savoye are marked by (rs).
Whilst researching what other boot proms do (at least those implementing TCP/IP protocols) it is clear that each 'does their own thing' in terms of what they expect in a boot image.
If we could all agree on working toward an open standard, O/S suppliers and boot rom suppliers can build their products to this norm, and be confident that they will work with each other.
This is a description of how I will implement the boot rom for Linux. I believe it to be flexible enough for any OS that will be loaded when a PC boots from a network in the TCP/IP environment.
It would be good if this could be turned into some form of standard.
This is very much a first draft. I am inviting comment.
The ideas presented here should be independant of any implementation. In the end, where there is a conflict between the final of this draft, and an implementation, this description should prevail.
The terms I use are defined at the end.
(gk)IMPORTANT NOTE: The scope of this document starts at the point where the net boot process gains control from the BIOS, to where the booted image reaches a state from which there is no return to the net boot program possible.(gk)
The target is to have a PC retrieve a boot image from a network in the TCP/IP environment.
(gk)The boot may take place from a network adaptor rom, from a boot floppy.(gk)
(gk)The net boot process is started as a result of the PC boot process. The net boot program can reside on a rom, e.g. on an adaptor card, or in ram as a result of reading off disk.(gk)
The boot process may execute in any mode (e.g. 8086, 80386) it desires. When it jumps to the start location in the boot image, it must be in 8086 mode and be capable of going into any mode supported by the underlying processor.
The image cannot be loaded into address spaces below 10000h, or between A0000h through FFFFFh, or between 98000h through 9FFFFh. (gk)Only when the image is not going to return to the boot process, all the memory is available to it once it has been started, so it can relocate parts of itself to these areas.(gk)
The boot process must be capable of loading the image into all other memory locations. Specifically, where the machine supports this, this means memory over 100000h.
The net boot process must execute the bootp protocol, followed by the tftp protocol, as defined in the relevant rfc's.
The file name used in the tftp protocol must be that given by the bootp record.
If less than 512 bytes are loaded, the net boot process attempts to display on the screen any ascii data at the start of the image. The net boot process then exits in the normal manner. For a boot prom, this will allow normal disk booting. (gk)Reference to DOS deleted.(gk)
When the first 512 bytes have been loaded, the boot process checks for an initial magic number, which is defined later. If this number is present, the net process continues loading under the control of the image format. The image, which is described later, tells the net boot process where to put this record and all subsequent data.
If no initial magic number is present the net boot process checks for a second magic number at offset 510. If the magic number 510 = 55h, 511 = AAh, then the net process continues. If this second magic number is not present, then the net boot process terminates the tftp protocol, displays an error message and exits in the normal manner.
If no initial magic number is present and the second one is, the net boot process relocates the 512 bytes to location 7c00h. The net boot process continues to load any further image data to 10000h up. This data can overwrite the first 512 boot bytes. If the image reaches 98000h, then any further data is continued to be loaded above 100000h. When all the data has been loaded, the net boot process jumps to location 0:7c00.
(gk)When the net boot program calls the image, it places 2 far pointers onto the stack, in standard intel order (e.g. segment:offset representation). The first far pointer which immediately follows the return address on the stack, points to the loaded boot image header. The second far pointer which is placed above the first one, shows to the memory area where the net boot process saved the bootp reply.
If the boot image is flagged as being returnable to the boot process, the boot program has to provide the boot image with interrupt vector 78h. It's an interface to services provided by the net boot program (see below for further description).
If the boot image is not flagged as being returnable to the boot process, before the boot image is called, the boot program has to set the system into a state in which it was before the net boot process has started.(gk)
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 processis 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.
(gk)As mentioned above the net boot process has to provide interrupt 78h as an entry point in case, the returnable flag (bit 9 of the flags field in the image header) of the boot image has been set. When calling this interface interrupt, the caller has to load the AH register with a value indicating the type of operation requested:
00h - Installation check
Input: none
Output: AX - returns the value 474Bh
BX - flags indicating what further services are
provided by the net boot program:
Bit 0 - packet driver interface (see below)
Bits 1 to 15 are unused and have to be zero
01h - Cleanup and terminate the boot process services. This will
also remove the services provided by interrupt 87h.
Input: none
Output: none
|
Further functions are not yet defined. These functions are only available to boot images which have the first magic number at the beginning of the image header, and have the returnable flag set in the flags field.
In order to provide compatibility with net boot programs written to match an earlier version of this document, the loaded image should check for the existence of interrupt 78h by looking at it's vector. If that's 0:0, or if it does not return a proper magic ID after calling the installation check function, the boot image has to assume that the net boot program does not support this services interrupt.
If the bit 0 of register BX of function 00h is set, the boot program has to provide a packet driver interface at interrupt 79h as described in the packet driver interface standard, version 1.09, published by FTP Software, Inc., which is not repeated here. It serves as an interface to the system's network card. It is important to note that the net boot process has to provide a clean packet driver interface without any handles being defined when the boot image gets started. It is expected that the boot image sets up it's own TCP/IP or other network's stack on top of this packet driver interface. When the boot image returns to the net boot process, it has to return a clean packet driver interface as well, without any handles being defined.(gk)
Here is an example of how the boot image would look for Linux:
Example 8-1. Example of a boot image
0x1B031336, /* magic number */
0x4, /* length of header is 16 bytes, no vendor info */
0x90000000, /* location in ds:bx format */
0x90000200, /* execute address in cs:ip format */
/* 2048 setup.S bytes */
0x4, /* flags, not end, absolute address, 16 bytes this
record, no vendor info */
0x90200, /* load address - note format */
0x800, /* 4 8 512 byte blocks for linux */
0x800,
/* kernel image */
0x4, /* flags, not end, absolute address, 16 bytes this
record, no vendor info */
0x10000, /* load address - note format */
0x80000, /* 512K (this could be shorter */
0x80000,
/* ramdisk for root file system */
0x04000004, /* flags = last, absolute address, 16 bytes this
record, no vendor info *//
0x100000, /* load address - in extended memory */
0x80000, /* 512K for instance */
0x80000,
/* Then follows linux specific information */
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When I say 'the net boot process', I mean the act of loading the image into memory, setting up any tables, up until the jump to the required location in the image.
The net booting program executes the net boot process. The net boot program may be a rom, but not neccassarily. It is a set of instructions and data residing on the booting machine.
The image, or boot image, consists of the data loaded by the net boot process.
When I say 'the PC boot process', I mean the general PC rom bios boot process, the setting up of hardware, the scanning for adaptor roms, the execution of adaptor roms, the loading in of the initial boot track. The PC boot process will include the net boot process, if one is present.
When I say client, I mean the PC booting up.
When I say 'image host', I mean the host where the boot image is comming from. This may not have the same architecture as the client.
The bootp protocol is defined in RFC951 and RFC1084. The tftp protocol is defined in RFC783. These are available on many sites. See Comer 1991 for details on how to obtain them.
A bootp server is the machine that answers the bootp request. It is not neccessarily the image host.
"Can" and "may" means doesn't have to, but is allowed to and might. "Must" means just that. "Cannot" means must not.
Comer, D.E. 1991, Internetworking with TCP/IP Vol I: Principles, Protocols, and Architecture Second Edition, Prentice Hall, Englewood Cliffs, N.J., 1991
Stevens, W.R 1990, Unix Network Programming, Prentice Hall, Englewood Cliffs, N.J., 1990