Page 82 - slides.dvi

•
•
•
•
–
–
–
–
–
–
Operating
Then
Polled
(Host
What’s
Host
Host
Host
Systems
Consider
Device
Device
Device
can
—
a
the
sets
sets
polled
I/O
Mode
sees
read
clears
e.g.
simple
mode
repeatedly
performs
Subsystem
and
I/O
problem
*
command
write
command
reads
write
device
write
command
here?
bit
operation
ready
in
with
ready
these
bit
ready
device
via
works
in
&
operation.
and
three
as
busy
command
bus)
sets
then
command
status
data (r/w)
status
until
registers:
follows:
register,
device
device
error (R/O)
clear.
register.
and
read (W/O)
status,
write (W/O)
busy.
busy.
puts
device-busy (R/O)
command-ready (W/O)
data
data
and
into
data
command.
register.
78