Adapters With System-Accessible Rom Modules - IBM 5170 Technical Reference

If an error is reported by the disk or diskette code, you should
reset the drive adapter and retry the operation. A specified
number of retries should be required for diskette reads to ensure
that the problem is not due to motor startup.
When altering I/O-port bit values, the programmer should change
only those bits necessary to the current task. Upon completion,
the programmer should restore the original environment. Failure
to adhere to this practice may cause incompatibility with present
and future applications.
Additional information for BIOS programming can be found in
Section 9 of this manual.
Adapters with System-Accessible ROM Modules
The ROM BIOS provides a way to integrate adapters with
on-board ROM code into the system. During POST, interrupt
vectors are established for the BIOS calls. After the default
vectors are in place, a scan for additional ROM modules occurs.
At this point, a ROM routine on an adapter may gain control and
~,
establish or intercept interrupt vectors to hook themselves into
the system.
The absolute addresses hex C8000 through EOOOO are scanned in
2K blocks in search of a valid adapter ROM. A valid ROM is
defined as follows:
Byte 0 Hex 55
Byte 1 HexAA
Byte 2 A length indicator representing the number of 512-byte
blocks in the ROM.
Byte 3 Entry via a CALL FAR
A checksum is also done to test the integrity of the ROM module.
Each byte in the defined ROM module is summed modulo hex
100. This sum must be 0 for the module to be valid.
System BIOS 5-11