using the bootstrap loader

Generic Third Stage
OVERVIEW OF BOOTSTRAP LOADER OPERATIONS
The generic third stage is so named because it can load application systems from any
device that the first stage recognizes. This stage contains no device driver of its own.
Instead, it uses the same device driver used by the first and second stages. This means that
you won't need to write a separate device driver to work in protected mode, but it also
means that the generic third stage runs in real address mode. In real address mode,
addressability is restricted to the first (lowest) megabyte of memory. Therefore, the
generic third stage can load only those application systems that are smaller than 840K
bytes. The remaining space is used by the Bootstrap Loader, the monitor and the SCT. To
load larger applications, you must use a device-specific third stage.
When the generic third stage receives control, it uses the device driver supplied in the first
stage to load the application system. It then switches the processor into protected virtual
address mode and passes control to the application.
Device-Specific Third Stage
The device-specific third stage switches the processor to protected virtual address mode
before loading the application system. This enables this stage to load into memory
addresses higher than one megabyte. However, because this stage switches the processor
into protected mode, it cannot use the first stage's device drivers (which operate only in
real mode). Instead, it must contain its own device driver, operating in protected mode, to
control the device from which the application system is loaded.
The device-specific third stage supplied in your Bootstrap Loader package supports the
following devices:
• iSBC® 215G/iSBX N 218A winchester and diskette controller combination or the
iSBC 214 controller, or the iSBC 221 controller
• iSBC 264 bubble memory controller
• iSBC 186/224A multi-peripheral controller
• SCSI (Small Computer Systems Interface) and SASI (Shugart Associates Systems
Interface) peripheral bus controllers having the iSBC 286/100A CPU board as the
host.
If you want to boot from any other device, you must write a protected mode device driver
for the device and link the driver in when you configure the device specific third stage (see
Chapter 6).
When the device-specific third stage receives control, it performs the same operations as
the generic third stage. However, before invoking the device driver to load the application
system, it switches the processor into protected mode. This enables the third stage to load
applications that are located outside the first megabyte.
Bootstrap Loader 1-5