Impulse 3440 - Manual (Page 18)

Technical Description
Sealevel Systems ULTRA COMM+422 Page 15
Interrupts
A good description of an interrupt and it’s importance to the IBM PC can be
found in the book ‘Peter Norton’s Inside the PC, Premier Edition’:
“ One of the key things that makes a computer different from any other kind of
man-made machine is that computers have the capability to respond to the
unpredictable variety of work that comes to them. The key to this capability is a
feature known as interrupts. The interrupt feature enables the computer to
suspend whatever it is doing and switch to something else in response to an
interruption, such as the press of a key on the keyboard.”
A good analogy of a PC interrupt would be the phone ringing. The phone ‘bell’
is a request for us to stop what we are currently doing and take up another task
(speak to the person on the other end of the line). This is the same process the
PC uses to alert the CPU that a task must be preformed. The CPU upon receiving
an interrupt makes a record of what the processor was doing at the time and
stores this information on the ‘stack’; this allows the processor to resume its
predefined duties after the interrupt is handled, exactly where it left off. Every
main sub-system in the PC has it’s own interrupt, frequently called an IRQ (short
for Interrupt ReQuest)..
In these early days of PC’s Sealevel Systems decided that the ability to share
IRQs was an important feature for any add-in I/O card. Consider that in the IBM
XT the available IRQs were IRQ0 through IRQ7. Of these interrupts only IRQ2-
5 and IRQ7 were actually available for use. This made the IRQ a very valuable
system resource. To make the maximum use of these system resources Sealevel
Systems devised an IRQ sharing circuit that allowed more than one port to use a
selected IRQ. This worked fine as a hardware solution but presented the software
designer with a challenge to identify the source of the interrupt. The software
designer frequently used a technique referred to as ‘round robin polling’. This
method required the interrupt service routine to ‘poll’ or interrogate each UART
as to it’s interrupt pending status. This method of polling was sufficient for use
with slower speed communications, but as modems increased their through put
abilities this method of servicing shared IRQs became inefficient.