  Maximum I/O size? 
 The Question is:
 
How can I find the upper limit on the number of bytes which can be read in a
 single I/O call?
 
 
 The Answer is:
 
  Most obviously, try it.  Code up a $qio call to the device.
 
  The exact limits of a particular I/O operation will depend on the
  device driver, on the process quotas, and potentially on the system
  parameter settings (eg: MAXBUF) and the available system resources.
 
  The OpenVMS I/O User's Reference Manual documents most of the common
  I/O devices.   For example, Section 4.3.1 specifies that the maximum
  READ I/O to a mailbox is limited by the mailbox maximum message size.
  (Note also, for instance, that the mailbox driver includes some small
  data structure overhead into the mailbox message itself, reducing the
  maximum size that is available to the user.)
 
