I first noticed this when I first looked at the PC AT techref. "Why the heck did they code (fill in a routine name) in two pieces like that?" I asked myself. After I grabbled the PC 5150 techref it was obvious. Later, when I got the Phoenix BIOS books, routine locations were called out specifically in the literature, so I wasn't dreaming. I can well imagine that it's useful to some code to know the locations of entry points and ISRs. There was initially no standard convention for "hooking" interrupt vectors, so knowing where the default one was might be useful. I can think of at least one instance where that might be useful--reverse-engineering protection. Calling the ROM BIOS directly precludes a snoopy programmer from just hooking the INT 13H vector and logging a program's disk activity. When I was unraveling copy protection from some of the original PC games, I found having my own custom-hacked BIOS ROMs for such stuff very handy indeed. I do know that the convention continued into the PS/2 line--and, a spot check on the system I've got in front of me (an HP P4-equipped model) shows that the INT 10h vector still points to F000:F065; INT 16h points to F000:E82E--the same addresses used on the original 5150. Cheers, Chuck