Zenith Z-100 Reference Manual page 156

Booting an Operating System
An operating system may be booted from a specified drive. Register DL should
contain the drive number, and if the drive is a Winchester, AL should contain
a partition number between 0 and 3 expressed in ASCII (30H to 33H) with 0
representing the default partition.
The routine will attempt to read from track 0, sector I of the spedfied device,
and execute the code it finds. If the boot fails, either because the drive did not
exist or because of a hardware failure, an error message is displayed and control
is passed to the monitor ROM (see Chapter 2).
Interrupt vector 19H executes the boot function.
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Disk Drives