IBM PC 300GL User Manual download pdf (Page 39)

32 PC 300 GL and 300 PL

3. Sends the EOI

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.

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

02Fx or 06Fx, 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 the function

value is not in the range of function calls, that routine 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

initiate 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).

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IBM PC 300GL User Manual Page 39