(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)