Technical information

NewBIOS Reference Serial Solutions
up when the machine is used, and minimises the (ugly) risk of
beginning to use a port and then altering it’s address.
The reader who is not well acquainted with the architecture
of the 8086 series processors may be a little confused by
references to I/O addresses. Under the 8086 architecture all I/O
devices are accessed by a bus (logically) separate to that which
accesses memory, although there is strictly nothing to prevent
I/O devices being placed on the memory bus (video memory can
in fact be considered in this category), or even memory being
placed on the I/O bus. The I/O bus uses a 16-bit address to
select which I/O device to access. An 16450 serial chip occupies
seven contiguous I/O addresses, and the first of these is what is
meant by ’the I/O address of the chip’. The value of the address
is defined by the wiring of the serial card which holds the chip,
and can usually be swapped between several different values via
switches on the card, or by software for a Micro Channel
Architecture card or Plug and Play cards.
The same I/O address can be given to more than one serial
port, as no checking is performed.
Chapter 5 Page 91