IBM AT 5170 Technical Reference page 438

that hardware designed to use IRQ 2 will operate in the IBM
Personal Computer AT.
• The system can mask hardware sensitivity. New devices can
change the ROM BIOS to accept the same programming
interface on the new device.
• In cases where BIOS provides parameter tables, such as for
video or diskette, a program may substitute new parameter
values by building a new copy of the table and changing the
vector to point to that table. However, the program should
copy the current table, using the current vector, and then
modify those locations in the table that need to be changed.
In this way, the program will not inadvertently change any
values that should be left the same.
• Disk Base consists of 11 parameters required for diskette
operation. They are pointed at by the data variable,
Disk Pointer, at absolute address 0:78. It is strongly
recommended that the values supplied in ROM be used. If it
becomes necessary to modify any of the parameters, build
another parameter block and modify the address in
Disk_Pointer to point to the new block.
The parameters were established to operate both the High
Capacity Diskette Drive and the Double Sided Diskette
Drive. Three of the parameters in this table are under
control of BIOS in the following situations.
The Gap Length Parameter is no longer retrieved from
the parameter block.
The gap length used during diskette read, write, and
verify operations is derived from within diskette BIOS.
The gap length for format operations is still obtained
from the parameter block.
Special considerations are required for fomlatting operations.
See the prolog of Diskette BIOS for the required details. If a
parameter block contains a head settle time parameter value
of 0 milliseconds, and a write operation is being performed,
at least 15 milliseconds of head settle time will be enforced
r--...
9-12 Compatibility