User's Manual

PROTECTION
P DPL 5 E C R A
P DPL 5 E
ED W A
1 0 1 0
I 1 01 1 0 0
7
o
7
o
Readable Code Segment
Writable Code Segment
G3010B
Figure 7-8. Access Byte Examples
Table 7-2. Allowed Segment Types in Segment Registers
(
Allowed Segnlent Types
Segment Register
Read
Only
Read-Write Execute Only
Execute'Read
Data
Segment Data Segment
Code Segment Code Segment
OS Yes Yes No Yes
ES
Yes Yes No Yes
SS No Yes No
No
CS No No
Yes Yes
NOTE
The Intel reserved bytes in the segment descriptor must be set to 0 for compatibility with
the 80386.
7.4.1 Data Accesses
Data may be accessed in data segments or readable code segments. When DS or ES is ioaded with a
new selector, e.g., by an
LDS, LES, or MOV to ES, SS, or DS instruction, the bits in the access byte
are checked to verify legitimate descriptor type and access (see table 7-2).
If any test fails, an error
code
is pushed onto the stack identifying the selector involved (see figure 7-5 for the error code format).
A privilege check
is made when the segment register is loaded. In general, a data segment's DPL must
be numerically greater than or equal to the
CPL. The DPL of a descriptor loaded into the SS must
eq'1!"J the C.PL Conforming code segments are an exception to privilege checking rules (see
section 11. 2).
Once the segment descriptor and selector are loaded, the offset of subsequent accesses within the
segment are checked against the limit given in the segment descriptor. Violating the segment size limit
causes a General Protection exception with an error code of
O.
A normal data segment is addressed with offset values ranging from 0 to the size of the segment. When
the ED bit of the access rights byte in the segment descriptor
is 0, the allowed range of offsets is
OOOOH to the limit. If limit is OFFFFH, the data segment contains 65,536 bytes.
7-12