User's Manual

ADVANCED TOPICS
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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