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Inter-segment Returns that refer
to
conforming code segments use the RPL field of the code selector
of the return address to determine the
new
CPL. The RPL becomes the
new
CPL if the conforming
code segment
DPL::5RPL.
If
a conforming segment
is
readable, it can
be
read from any privilege level without restriction. This
is
the only exception to the protection rules. This allows constants
to
be stored with conforming code. For
example, a read-only look-up table can be embedded
in
a conforming code segment that can
be
used
to convert system-wide logical ID's into character strings that represent those logical entities.
11.2.2 Expand-Down Data Segments
\
If
bit 2 in the
a~cess
byte of a data segment
is
1,
the segment
is
an
expand-down segment: All the
offsets that reference such a segment must
be
strictly greater than the segment limit, as opposed to
normal data segments (bit
2=0)
where all offsets must
be
less than or equal
to
the segment limit.
Figure
11-1
shows an expand-down segment.
The size of the expand down segment can
be
changed
by
changing either the base or the limit. An
expand down segment with Limit=O
will
have a size of 2
16
-1
bytes. With a limit value of FFFFH,
the expand down segment
will
have a size of 0 bytes. In an expand
down
segment, the base + offset
value should always be greater than the base
+ limit value. Therefore, a full size segment
(2
i6
bytes)
can only
be
obtained
by
using an expand up segment.
The operating system should check the Expand-Down bit when a protection fault indicates that the
limit of a data segment has been reached.
If
the Expand-Down bit
is
not set, the operating system
should increase the segment limit; if it
is
set, the limit should be lowered. This supplies more
room
in
either case (assuming the segment
is
not write-protected, i.e., that bit 1
is
not 0). In some cases, if the
operating system can ascertain that there
is
not enough room
to
expand the data segment to meet the
need that caused the fault, it can
move
the data segment
to
a region of memory where there
is
enough
room.
See figure
11-2.
.
BASE + FFFEH
-~===d
BASE + OFFSET
> BASE + LIMIT
1
EXPAND DOWN
SEGMENT
BASE + LIMIT
-~f'~=:=t
J
Figure 11-1. Expand-Down Segment
11-2
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