## Virtual Device Driver Reference for OS/2

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### VDHInstallIOHook Parameter - Flags

Flags (FLAGS) Indicates interface and trapping options.

Possible values:

VDHIIH_ASM_HOOK Use assembler language hook interface (register-based). Otherwise, use a C hook interface (stack-based).

VDHIIH_ALWAYS_TRAP Always trap this range of addresses. The virtual device driver cannot call VDHSetIOHookState on any ports in the range.

VDHIIH_NO_SIMULATE Does not change WORD or other I/O handlers to simulation routines; allows WORD handlers for 1-port ranges.

VDHIIH_IGNORE Sets a system handler that does the following:

On WRITES: The OUT succeeds, but no actual I/O is performed.
On READS: The IN succeeds, and always returns -1 (all bits set to 1).

This is also the default behavior of unhooked ports. This behavior is compatible with the behavior of a machine when there is no hardware on a particular I/O address.