Intel 80286, 80287 manual Stack Contents after an Inter-Level Call

PROTECTION

t		SS:SP
HIGHER		FROM TSS	OLO SS
ADDRESSES
			OLDSP	DIRECTION
				OF STACK
			PARM3	GROWTH
			PARM 2
	PARM 3		PARM 1
	PARM 2		OLDCS
	PARM 1		OLDIP
LOWER	OLDSS:SP_....____...	NEW SS + SP
ADDRESSES
~	OLD STACK		NEW STACK
	(AT "OUTER"		(AT "INNER"
	PRIVILEGE		PRIVILEGE
	LEVEL)		LEVEL)

G30108

Figure 7-12. Stack Contents after an Inter-Level Call

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.

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