INT 21 u - European MSDOS 4.0 - "PBLOCK" - BLOCK A PROCESS
        AH = 99h
        DS:BX -> memory location to block on
        CX = timeout in milliseconds
        DH = nonzero if interruptible
Return: CF clear if awakened by event
            AX = 0000h
        CF set if unusual wakeup
            ZF set if timeout, clear if interrupted by signal
            AX = nonzero
Desc:   suspend calling process until another process sends a "restart" signal
          or a timeout occurs
SeeAlso: INT 2F/AX=0802h