Function:

WBINVD tells the processor that all data in both the internal as well as the
external caches is invalid. Data held in external write-back caches is
written back to memory before the flush.

If on some 486's a cache line fill is in progress while the WBINVD
instruction is being executed, that line is NOT invalidated and the buffer
contents is moved into the cache. Valid cache lines are ALWAYS used to
satisfy read requests on all 486's, regardless whether the cache is enabled
or not.

Workaround is to disable the cache prior to flushing it like this:

        MOV EAX,CR0
        OR  EAX,60000000h  ; cache disable bits
        PUSHFD
        CLI
        MOV BL,CS:here
        OUT dummyport,dummydata
        MOV CR0,EAX