Chapter 6. System Compatibility
Diskette drive controls:
– Rotational speed: The time between two events in a diskette drive is a function of the controller.
– Access time: Diskette BIOS routines must set the track-to-track access time for the different types
of media that are used in the drives.
– ‘Diskette change’ signal: Copy protection might not be able to reset this signal.
Write-current control: Copy protection that uses write-current control does not work, because the
controller selects the proper write current for the media that is being used.
Hard Disk Drives and Controller
Reading from and writing to the hard disk is initiated in the same way as in other 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 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 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).
Chapter 6. System Compatibility
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