Remote Initial Program Load - RPL and DOS

RPL and DOS

For DOS remote boot to work, the RPL boot loader is loaded into the client's memory over the network before the operating system starts. Without special precautions the operating system would easily overwrite the RPL code during boot, since the RPL code resides in unallocated memory (typically at the top of the available conventional memory). The RPL code hides and thereby protects itself from being overwritten by hooking INT 12h and reducing the memory reported by this BIOS service by its own size. INT 12h is used by DOS to query the amount of available memory when initializing its own real-mode memory allocation scheme. This causes problems on more modern DOS systems, where free real-mode address ranges may be utilized by the operating system in order to relocate parts of itself and load drivers high, so that the amount of available conventional memory is maximized. Typically various operating system vendor and version specific "dirty tricks" had to be used by the RPL code in order to survive this very dynamic boot process and let DOS regain control over the memory occupied by RPL once the boot is complete in a seamless manner.

Since MS-DOS/PC DOS 5.0 and DR DOS 6.0, the operating system checks, if the RPL has hooked INT 2Fh by looking for a "RPL" signature at the code pointed to by INT 2Fh. If present, DOS calls INT 2Fh/AX=4A06h to retrieve the amount of memory from the RPL and integrate it into its own memory allocation, thereby protecting the RPL code from being overwritten by other programs. Still, it remained the RPL's difficult responsibility to cleanly remove itself from memory at the end of the boot phase, if possible.