Intel 80286, 80287 manual Tag Word Format

OVERVIEW OF NUMERIC PROCESSING

TAG VALUES:

00 ~ VALID

01 ~ ZERO

10 ~ INVALID OR INFINITY

11 ~ EMPTY

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Figure 1-6. 80287 Tag Word Format

				MEMORY		MEMORY OFFSET
				OFFSET
15
					15
	CONTROL WORD			to	CONTROL WORD	+0
	STATUS WORD				'.	+2
					STATUS WORD
	TAG WORD			+4	TAG WORD	+4
	INSTRUCTION POINTER (15-0)			+6	IPOFFSET	+6
	INSTRUCTION ;)1	'I	INSTRUCTION	+8·
			OPCODE (10-0)		CSSELECTOR	+8
POINTER (19-16)		0
	DATA POINTER (15-01			+10	DATA OPERAND OFFSET	+10
DATA POINTER 1			0	+12		+12
	(19-16)				DATA OPERAND SELECTOR
15	1211
	REAL MODE				PROTECTED MODE

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Figure 1-7. 80287 Instruction and Data POinter Image in Memory

When stored in memory, the instruction and data pointers appear in one of two formats, depending on the operating mode of the 80287. Figure 1-7 shows these pointers as they are stored following an FSTENV instruction. In Real-Address mode, these values are the 20-bit physical address and ll-bit opcode formatted like the 8087. In Protected mode, these values are the 32-bit virtual addresses used by the program that executed the ESC instruction.

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