Date   : Wed, 10 Nov 1982 23:52:00-CST
From   : mknox@Utexas-11
Subject: CP/M disk logins in BIOS

My thanks to Ben Goldfarb for pointing out the secret.  An undocumented feature
in BDOS provides that the LSB of the E-register is a 0 if this is the first
time (i.e. after a cold or warm boot) that SELDSK has been called to select
the disk specified in the C-register.  Calls for a disk which has been 
previously logged in all have that LSB set.