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When altering I/O-port bit values, the programmer should change
only those bits that are necessary to the current task. Upon
completion, the programmer should restore the original
environment. Failure to adhere to this practice may be
incompatible with present and future applications.
Adapter Cards with System-Accessible ROM Modules
The ROM BIOS provides a facility to integrate adapter cards with
on-board ROM code into the system. During the POST, interrupt
vectors are established for the BIOS calls. After the default
vectors are in place, a scan for additional ROM modules takes
place. At this point, a ROM routine on the adapter card may gain
control.. The routine may establish or intercept interrupt vectors
to hook themselves into the system.
The absolute addresses hex C8000 through hex F4000 are
scanned in 2K blocks in search of a valid adapter card ROM. A
valid ROM is defined as follows:
Byte 0:
Byte 1:
Byte 2:
Hex 55
HexAA
A length indicator representing the number of 512-byte
blocks in the ROM (length/512). A checksum is also
done to test the integrity of the ROM module. Each
byte in the defined ROM is summed modulo hex 100.
This sum must be 0 for the module to be deemed valid.
When the POST identifies a valid ROM, it does a far call to byte
3 of the ROM (which should be executable code). The adapter
card may now perform its power-on initialization tasks. The
feature ROM should return control to the BIOS routines by
executing a far return.
5-10
System BIOS
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