Intel 80286, 80287 manual Esc, Wait, IfTS=1

SYSTEM CONTROL AND INITIALIZATION

The EM flag indicates a processor extension function is to be emulated by software. If EM = 1 and MP=O, all ESCAPE instructions will be trapped via the processor extension not-present exception (#7).

MP flag tells whether a processor extension is present. If MP=l and TS=I, escape and wait instruc- tions will cause exception 7.

If ESC instructions are to be used, either the MP or the EM bit must be set, but not both.

The PE flag indicates that the 80236 is in the protected virtual address mode. Once the PE flag is set, it can be cleared only by a reset, which then puts the system in real address mode emulating the 8086.

Table 10-2 shows the recommended usage of the MSW. Other encodings of these bits are not recommended.

10.2.3 Other Instructions

Instructions that verify or adjust access rights, segment limits, or privilege levels can be used to avoid exceptions or faults that are correctable. Section 10.3 describes such instructions.

10.3 PRIVILEGED AND TRUSTED INSTRUCTIONS

Instructions that execute only at CPL=O are called "privileged." An attempt to execute the privileged instructions at any other privilege level causes a general protection exception (#13) with an error code of zero. The privileged instructions manipulate descriptor tables or system registers. Incorrect use of these instructions can produce unrecoverable conditions. Some of these instructions (LGDT, LLDT, and LTR) are discussed in section 10.2.

Table 10-2. Recommended MSW Encodings for Processor Extension Control

15				Instructions
	MP	EM	Recommended Use	Causing
				Exception 7
0	0	0	Initial encoding after RESET. 80286 operation is identical to 8086,	None
			8088. Use this encoding only if no ESC instructions are to be
			executed.
0	0	1	No processor extension is available. Software will emulate its	ESC
			function. Wait instructions do not cause exception 7.
1	0	1	No processor extension is available. Software will emulate its	ESC
			function. The current processor extension context may belong to
			another task.
0	1	0	A processor extension exists.	WAIT
				(ifTS=1)
1	1	0	A processor extension exists. The current processor extension	ESC or
			context may belong to another task. The exception 7 on WAIT	WAIT
			allows software to test for an error pending from a previous	(if TS=1)
			processor extension operation.

10-5