Avaya Partner 18 Connection Manual page 296

Appendix B: PC Serial Ports
represents a small region of the microprocessor's input/output memory space
which is used to pass data back and forth to the serial port. This memory
region acts something like a mailbox: outgoing mail (data from the
microprocessor to be transmitted to the peripheral device) is placed in the
mailbox by the owner (the microprocessor) to be picked up by the mail carrier
(the serial port hardware) for subsequent delivery to the destination party (the
peripheral device). In turn, the mail carrier (the serial port hardware) places
incoming mail (data from the peripheral device) into the box to be picked up by
the owner (the microprocessor). This analogy illustrates an additional
important point about I/O port addresses: just as individual mailboxes help the
residents in a neighborhood keep their mail from getting mixed up, each device
using an I/O port address to communicate with the microprocessor should
have a unique address that doesn't conflict with that of any other device.
The mailbox analogy is also helpful in understanding the IRQ mechanism.
Normally, we place our outgoing mail in our mailbox at any convenient time
before the mail carrier arrives to pick it up. The outgoing mail sits in our
mailbox until the mail carrier arrives, at which time it is picked up and possibly
some incoming mail is placed in the mailbox. Then, sometime later, we check
our mailbox and retrieve our new incoming mail. The problem with this
scheme is that it's not very efficient both the outgoing and incoming mail
spends some time just sitting in the mailbox. A better approach would be if the
mail carrier provided some sort of signal (for example, ringing the doorbell) to
announce his or her arrival, in which case we could hand over the outgoing
mail and pick up the incoming mail immediately. ln the PC architecture, IRQs
acts like the doorbell in our analogy they provide a method by which
hardware devices in the computer can get the microprocessor's attention to
efficiently deal with some process. The serial port hardware makes use of an
IRQ to announce that it's ready to receive more outgoing data and/or that new
data has arrived from the peripheral device which needs to be processed.
Like I/O port addresses, IRQs must generally be unique among the active
hardware devices in a computer system. In the mailbox analogy, the doorbell
is probably not a good signal since virtually anyone could ring the doorbell for
any number of reasons, not just to indicate the arrival of mail. Similarly, if a
particular IRQ signal is used (PCs generally support 16 unique IRQ signals,
B-3