Intel 80286, 80287 manual OP-A MOD OP-S Displacement Format OP-AMOD, Reg

APPENDIX A

MACHINE INSTRUCTION ENCODING AND DECODING

Machine instructions for the 80287 come in one of five different forms as shown in table A-I. In all cases, the instructions are at least two bytes long and begin with the bit pattern 11011B, which identi- fies the ESCAPE class of instructions. Instructions that reference memory operands are encoded much like similar CPU instructions, because all of the CPU memory-addressing modes may be used with ESCAPE instructions.

Note that several of the processor control instruction~ (see table 2-11 in Chapter Two) may be preceded by an assembler-generated CPU WAIT instruction (encoding: 10011011B) if they are programmed using the WAIT form of their mnemonics. The ASM286 assembler inserts a WAIT instruction only before these specific processor control instructions-all of the numeric instructions are automatically synchronized by the 80286 CPU and an explicit WAIT instruction, though allowed, is not necessary.

						Table A-1.			80287 Instruction Encoding
			Lower-Addressed Byte								Higher-Addressed Byte			0, 1, or 2 bytes
(1)	1	1	0	1	1	OP-A		1	MOD		1	OP-S	R/M	DISPLACEMENT
(2)	1	1	0	1	1	FORMAT		OP-AMOD				OP-S	R/M	DISPLACEMENT
(3)	1	1	0	1	1	R	P	OP-A 1		1		OP~S	REG
(4)	1	1	0	1	1	0	0	1	1	1	1		OP
(5)	1	1	0	1	1	0	1	1	1	1	1		OP
	7	6	5	4	3	2		0765432						o
NOTES:

(l)Memory transfers, including applicable processor control instructions; 0, 1, or 2 displacement bytes may follow.

(2)Memory arithmetic and comparison instructions; 0, 1, or 2 displacement bytes may follow.

(3)Stack arithmetic and comparison instructions.

(4)Constant, transcendental, some arithmetic instructions.

(5)Processor control instructions that do not reference memory.

OP, OP-A, OP-S: Instruction opcode, possibly split into two fields.

MOD: Same as 80286 CPU mode field.

R/M: Same as 80286 CPU register/memory field.

FORMAT: Defines memory operand 00 = short real

01 = short integer

10 = long real

11 = word integer

R:0 = return result to stack top

1 = return result to other register

A-1