User's Manual
Real
Address
Mode
In
real
mode
the
386SX
microprocessor
operates
as
a
very
fast
8086,
but
with
a
32-bit
extension
if
desired.
Real
mode
is
required
primarily
to
set
up
the
processor
for
protected
mode
operation.
The
segmentation
unit
shifts
the
selector
left
four
bits
and
adds
the
result
to
the
effective
address
to
form
the
linear
address.
This
linear
address
is
limited
to
1
megabyte.
in
addition,
real
mode
has
no
paging
capability.
System
Timers
The
system
has
three
programmable
timer/counters
controlled
by
the
Intel
8254-2
chip.
These
are
channels
0
through
2
defined
as
fol-
lows:
Table
6-1: Channel 0
Table
6-2:
Channel
1
Note:
Channel
1
is
programmed
to
generate
a
15
microsecond
sig-
nal.
Table 6-3:
Channel
2
18
Chapter
6:
Appendix