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Chapter 6. System compatibility
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