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Chapter 6. System compatibility
4. Waits one I/O delay
5. Enables the interrupt through the Set Interrupt Enable Flag command
Hardware interrupt IRQ9 is defined as the replacement interrupt level for the cascade level IRQ2.
Program interrupt sharing is implemented on IRQ2, interrupt hex 0A. The following processing occurs to
maintain compatibility with the IRQ2 used by IBM Personal Computer products:
1. A device drives the interrupt request active on IRQ2 of the channel.
2. This interrupt request is mapped in hardware to IRQ9 input on the second interrupt controller.
3. When the interrupt occurs, the system microprocessor passes control to the IRQ9 (interrupt hex 71)
interrupt handler.
4. This interrupt handler performs an EOI command to the second interrupt controller and passes control
to the IRQ2 (interrupt hex 0A) interrupt handler.
5. This IRQ2 interrupt handler, when handling the interrupt, causes the device to reset the interrupt
request before performing an EOI command to the master interrupt controller that finishes servicing
the IRQ2 request.
Hard disk drives and controller
Reading from and writing to the hard disk is initiated in the same way as in IBM Personal Computer
products; however, new functions are supported.
Software compatibility
To maintain software compatibility, the interrupt polling mechanism that is used by IBM Personal Computer
products is retained. Software that interfaces with the reset port for the IBM Personal Computer
positive-edge interrupt sharing (hex address 02F
x
or 06F
x
, where
x
is the interrupt level) does not create
interference.
Software interrupts
With the advent of software interrupt sharing, software interrupt routines must daisy chain interrupts. Each
routine must check the function value, and if it is not in the range of function calls for that routine, it must
transfer control to the next routine in the chain. Because software interrupts are initially pointed to
address 0:0 before daisy chaining, check for this case. If the next routine is pointed to address 0:0 and
the function call is out of range, the appropriate action is to set the carry flag and do a RET 2 to indicate
an error condition.
Machine-sensitive programs
Programs can select machine-specific features, but they must first identify the machine and model type.
IBM has defined methods for uniquely determining the specific machine type. The machine model byte
can be found through Interrupt 15H, Return System Configuration Parameters function (AH)=C0H).
24 Technical Information Manual