User's Manual

t
HIGHER
ADDRESSES
LOWER
ADDRESSES
~
PARM 3
PARM 2
PARM 1
PROTECTION
SS:SP
FROM
OLDSS:SP_ .... ____ ...
NEW SS + SP
OLD STACK
(AT "OUTER"
PRIVILEGE
LEVEL)
TSS
OLO SS
OLDSP
PARM3
PARM 2
PARM 1
OLDCS
OLDIP
NEW STACK
(AT "INNER"
PRIVILEGE
LEVEL)
Figure 7-12. Stack Contents after an Inter-Level Call
DIRECTION
OF STACK
GROWTH
G30108
The old SS:SP value is then adjusted by the number of bytes indicated in the RET instruction and
loaded into
SS:SP. The new SP value is not checked for validity. If SP is invalid it is not recognized
until the first stack operation. The
SS:SP value of the returning program is not saved. (Note: this value
normally
is the same as that saved in the TSS.)
The last step in the return is checking the contents of the DS and ES descriptor register. If.DS or ES
refer to segments whose DPL is greater than the new CPL (excluding conforming code segments), the
segment registers are loaded with the null selector. Any subsequent memory reference that attempts
to use the segment register containing the null selector
will cause a general protection fault. This prevents
less privileged code from accessing more privileged data previously accessed by the more privileged
program.
7-21