HP Vectra Technical Reference Manual page 331

All ROM modules contain a header and checksum byte. The header format is shown below:
Byte 0-55H
Byte 1-0AAH
Byte 2-Length of ROM module in 512 byte blocks.
Byte 3-lnitialization entry point.
Bytes 0 and 1 are signature bytes. All ROM modules must contain this signature at the start of
the header in order to be identified by the SYSGEN process.
Byte 2 of the header contains the number of 512 byte blocks in the ROM module, except the
ROM module located on the processor extension card (memory address OEOOOOH). Byte 2 in that
ROM module header is reserved.
During the boot process, the address range from OC8000H to ODFFFFH is scanned in 2 KB blocks
looking for valid option ROM headers. In addition, memory location OEOOOOH is also examined
for a valid header. Since the scan does not proceed past OEOOOOH, only one ROM module can
reside in the address range OEOOOOH to OEFFFFH. The processor extension card will accept two
different size ROMs; 32 KB or 64 KB. If a 32 KB part is installed, the ROM will appear in the
system address space starting at location OE8000H instead of OEOOOOH. Therefore, the 32 KB
ROM will not be integrated into the system by SYSGEN.
If a valid ROM header is found, a checksum is computed for the ROM module. This is done by
summing each byte in the ROM module. The sum of all the bytes in the ROM, including the
checksum byte, must equal O. For ROM modules located from OCOOOOH to ODFFFFH, the
checksum is computed for the number of bytes indicated in the length field of the header. For a
ROM module located from OEOOOOH to OEFFFFH this checksum is calculated on the entire 64 KB
of address space.
If the checksum is valid, a FAR call to byte 3 of the module is is performed. The ROM module
should perform any initialization required and then execute a RETF instruction.
This integration process allows option ROMs to install vectors in either the HP_VECTOR_TABLE
or the low memory interrupt vectors. This re-vectoring process is the typical method used to
integrate ROM modules into the system.
322 System Processes