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─────Run-Time Library───────────────────────────────────────────────────────
 
     The _vlock function loads a virtual memory block into DOS memory,
     locks it, and returns a far pointer to it. The argument <handle>
     points to a virtual memory block previously allocated through a
     call to _vmalloc or _vrealloc.
 
     A locked virtual memory block will not be swapped out until it is
     unlocked. A virtual memory block can be locked up to 255 times.
     The pointer returned by _vlock remains valid until an equal
     number of unlock operations is performed.
 
     Since DOS memory may be scarce, try to keep the number of blocks
     locked at one time to a minimum and use _vunlock to unlock them as
     soon as possible.
 
     Return Value
 
     The _vlock function returns a far pointer to DOS memory if the
     virtual memory block is successfully loaded and locked. If
     insufficient DOS memory is available, _vload returns NULL.
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