using the bootstrap loader

WRITING A CUSTOM THIRD-STAGE DRIVER
6.2 WHAT A THIRD-STAGE DEVICE DRIVER MUST CONTAIN
6-2
The third stage device driver, like the first stage, must contain a device initialization and a
device read procedure. For the most part, these procedures are similar to their first-stage
counterparts. However, two differences exist.
• Both procedures must be in the same code segment.
• You must also create a PUBLIC symbol that contains a pointer to the device driver
data segment. The third stage needs this information so it can create a descriptor for
the data segment, enabling the driver to access the segment in protected mode.
When developing code for your third stage driver, you must remember that the second
stage always loads the third stage, including the drivers you write. The only type of code
that the second stage can load is code that uses the 8086 object module format (OMF-86).
Therefore, you must use 8086 tools (ASM86, PL/M-86, LINK86, etc.) to develop the thirdstage
device initialization and read procedures. (
Even though you use 8086 tools to develop your driver code, the resulting initialization and
read procedures must be able to run in protected mode. One ramification of running in
protected mode is that all long pointers produced by PL/M-86 (or by any other means)
that were correct in real mode cause an ILLEGAL SELECTOR exception in protected
mode. Therefore, if you must use long pointers, your device initialization and read
procedure must determine if the processor is in protected mode. If protected mode is
active, the procedure must replace all the selector portions of long pointers with a new
selector that is valid in protected mode.
You can determine the processor mode by using the following assembly code:
DB OFH,OlH,OE3H ;Opcode for the ASM286 instruction
;SMSW BX. You must use
;DB OFH,OlH,OE3H because SMSW is an
;ASM286 instruction unrecognized by
;ASM86.
AND BX, OlH ;Examine lowest bit of MSW to see if
; CPU is running in PVAM.
JZ REAL ;No, not running in PVAM .
. code to override ;Yes, running in PVAM.
selectors of
. long pointers ;
Bootstrap Loader
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