Intel 80287, 80286 manual JMP-Jump

THE 80286 INSTRUCTION SET

JMP-Jump

Opcode		Instruction		Clocks'	Description
EB	cb	JMP cb		7	Jump short
EA	cd	JMP	cd	180	Jump to.task gate
E9	cw	JMP cw		7	Jump near
EA	cd	JMP	cd	11,pm=23	Jump far (4-byte immediate address)
EA	cd	JMP	cd	38	Jump to call gate, same privilege
EA	cd	JMP cd		175	Jump via Task State Segment
FF	/4	JMP	ew	7,mem=11	Jump near to EA word (absolute offset)
FF	/5	JMP	ed	15,pm=26	Jump far (4-byte effective address in memory
FF					doubleword)
	/5	JMP	ed	41	Jump to call gate, same privilege
FF	/5	JMP	ed	178	Jump via Task State Segment
FF	/5	JMP	ed	183	Jump to task gate

'Add one clock for every byte of the next instruction executed.

FLAGS MODIFIED

All if a task switch takes place; none if no task switch occurs.

FLAGS UNDEFINED

None

OPERATION

The JMP instruction transfers program control to a different instruction stream without recording any return information.

For inter-segment jumps, the destination can be a code segment, a call gate, a task gate, or a Task State Segment. The latter two destinations cause a complete task switch to take place.

Control transfers within a segment use the JMP cw or JMP cb forms. The operand is a relative offset added modulo 65536 to the offset of the instruction that follows the JMP. The result is the new value of IP; the value of CS is unchanged. The byte operand is sign-extended before it is added; it can therefore be used to address labels within 128 bytes in either direction from the next instruction.

Indirect jumps within a segment use the JMP ew form. The contents of the register or memory operand is an absolute offset, which becomes the new value of IP. Again, CS is unchanged.

Inter-segment jumps in real address mode simpiy set IP to tile offset part of inl: iOIl!;; poilii,;;r ..iid ,,;;.. CS to the selector part of the pointer.

In protected mode, inter-segment jumps cause the 80286 to consult the descriptor addressed by the selector part of the long pointer. The AR byte of the descriptor determines the type of the destination. (See table B-3 for possible values of the AR byte.) Following are the possible destinations:

1.Code segment-The addressability and visibility of the destination are verified, and CS and IP are loaded with the destination pointer values.

B-56