(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)