IBM 5170 Technical Reference page 386

the coprocessor. If the interrupt was not generated by the
coprocessor, control should be passed to the original NMI
interrupt handler.
• Back to back I/O commands to the same I/O ports will not
permit enough recovery time for I/O chips. To insure enough
time, a JMP SHORT $+2 must be inserted between IN/OUT
~
instructions to the same I/O chip.
Note: MOV AL,AH type instruction does not allow
enough recovery time. An example of the correct
procedure follows:
OUT IO_ADD,AL
JMP SHORT $+2
MOV AL,AH
OUT IO_ADD,AL
• In the IBM Personal Computer AT IRQ
9
is redirected to INT
hex OA (hardware IRQ
2).
This insures that hardware
r"'.
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
9-8 Compatibility