Loader uses CBIOS Disk I/O prior to CBOOT/WBOOT being run. As a 
result, DIOBUF is not properly initialized.  At present, it is 
initialized to $7C00 which will work unless the location of the 
physical disk buffer in HBIOS ever changes!
 
The clock drivers supplied with BPBIOS (LDDS, LDP2D, and LDNZT) 
load into ZSYS user memory.  If the user segment is not at the 
same location as the original BPBIOS ($E900), then the clock 
driver will just overlay other code.  At present, the N config 
is OK because it does not relocate the user segment, but the T 
config fails because it uses a user segment at $E700. Note that 
this does not affect ZSDOS2 variants because they do not require 
a loadable clock driver.

BPBIOS needs to assign disk units dynamically via discovery of disk type