JMP — Jump

Instruction Operand Encoding ¶

Description ¶

Transfers program control to a different point in the instruction stream without recording return information. The destination (target) operand specifies the address of the instruction being jumped to. This operand can be an immediate value, a general-purpose register, or a memory location.

This instruction can be used to execute four different types of jumps:

• Near jump—A jump to an instruction within the current code segment (the segment currently pointed to by the CS register), sometimes referred to as an intrasegment jump.
• Short jump—A near jump where the jump range is limited to –128 to +127 from the current EIP value.
• Far jump—A jump to an instruction located in a different segment than the current code segment but at the same privilege level, sometimes referred to as an intersegment jump.
• Task switch—A jump to an instruction located in a different task.

A task switch can only be executed in protected mode (see Chapter 7, in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A, for information on performing task switches with the JMP instruction).

Near and Short Jumps. When executing a near jump, the processor jumps to the address (within the current code segment) that is specified with the target operand. The target operand specifies either an absolute offset (that is an offset from the base of the code segment) or a relative offset (a signed displacement relative to the current

value of the instruction pointer in the EIP register). A near jump to a relative offset of 8-bits (rel8) is referred to as a short jump. The CS register is not changed on near and short jumps.

An absolute offset is specified indirectly in a general-purpose register or a memory location (r/m16 or r/m32). The operand-size attribute determines the size of the target operand (16 or 32 bits). Absolute offsets are loaded directly into the EIP register. If the operand-size attribute is 16, the upper two bytes of the EIP register are cleared, resulting in a maximum instruction pointer size of 16 bits.

A relative offset (rel8, rel16, or rel32) is generally specified as a label in assembly code, but at the machine code level, it is encoded as a signed 8-, 16-, or 32-bit immediate value. This value is added to the value in the EIP register. (Here, the EIP register contains the address of the instruction following the JMP instruction). When using relative offsets, the opcode (for short vs. near jumps) and the operand-size attribute (for near relative jumps) determines the size of the target operand (8, 16, or 32 bits).

Far Jumps in Real-Address or Virtual-8086 Mode. When executing a far jump in real-address or virtual-8086 mode, the processor jumps to the code segment and offset specified with the target operand. Here the target operand specifies an absolute far address either directly with a pointer (ptr16:16 or ptr16:32) or indirectly with a memory location (m16:16 or m16:32). With the pointer method, the segment and address of the called procedure is encoded in the instruction, using a 4-byte (16-bit operand size) or 6-byte (32-bit operand size) far address immediate. With the indirect method, the target operand specifies a memory location that contains a 4-byte (16-bit operand size) or 6-byte (32-bit operand size) far address. The far address is loaded directly into the CS and EIP registers. If the operand-size attribute is 16, the upper two bytes of the EIP register are cleared.

Far Jumps in Protected Mode. When the processor is operating in protected mode, the JMP instruction can be used to perform the following three types of far jumps:

• A far jump to a conforming or non-conforming code segment.
• A far jump through a call gate.
• A task switch.

(The JMP instruction cannot be used to perform inter-privilege-level far jumps.)

In protected mode, the processor always uses the segment selector part of the far address to access the corresponding descriptor in the GDT or LDT. The descriptor type (code segment, call gate, task gate, or TSS) and access rights determine the type of jump to be performed.

If the selected descriptor is for a code segment, a far jump to a code segment at the same privilege level is performed. (If the selected code segment is at a different privilege level and the code segment is non-conforming, a general-protection exception is generated.) A far jump to the same privilege level in protected mode is very similar to one carried out in real-address or virtual-8086 mode. The target operand specifies an absolute far address either directly with a pointer (ptr16:16 or ptr16:32) or indirectly with a memory location (m16:16 or m16:32). The operand-size attribute determines the size of the offset (16 or 32 bits) in the far address. The new code segment selector and its descriptor are loaded into CS register, and the offset from the instruction is loaded into the EIP register. Note that a call gate (described in the next paragraph) can also be used to perform far call to a code segment at the same privilege level. Using this mechanism provides an extra level of indirection and is the preferred method of making jumps between 16-bit and 32-bit code segments.

When executing a far jump through a call gate, the segment selector specified by the target operand identifies the call gate. (The offset part of the target operand is ignored.) The processor then jumps to the code segment specified in the call gate descriptor and begins executing the instruction at the offset specified in the call gate. No stack switch occurs. Here again, the target operand can specify the far address of the call gate either directly with a pointer (ptr16:16 or ptr16:32) or indirectly with a memory location (m16:16 or m16:32).

Executing a task switch with the JMP instruction is somewhat similar to executing a jump through a call gate. Here the target operand specifies the segment selector of the task gate for the task being switched to (and the offset part of the target operand is ignored). The task gate in turn points to the TSS for the task, which contains the segment selectors for the task’s code and stack segments. The TSS also contains the EIP value for the next instruction that was to be executed before the task was suspended. This instruction pointer value is loaded into the EIP register so that the task begins executing again at this next instruction.

The JMP instruction can also specify the segment selector of the TSS directly, which eliminates the indirection of the task gate. See Chapter 7 in Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A, for detailed information on the mechanics of a task switch.

Note that when you execute at task switch with a JMP instruction, the nested task flag (NT) is not set in the EFLAGS register and the new TSS’s previous task link field is not loaded with the old task’s TSS selector. A return to the previous task can thus not be carried out by executing the IRET instruction. Switching tasks with the JMP instruction differs in this regard from the CALL instruction which does set the NT flag and save the previous task link information, allowing a return to the calling task with an IRET instruction.

In 64-Bit Mode. The instruction’s operation size is fixed at 64 bits. If a selector points to a gate, then RIP equals the 64-bit displacement taken from gate; else RIP equals the zero-extended offset from the far pointer referenced in the instruction.

See the summary chart at the beginning of this section for encoding data and limits.

Instruction ordering. Instructions following a far jump may be fetched from memory before earlier instructions complete execution, but they will not execute (even speculatively) until all instructions prior to the far jump have completed execution (the later instructions may execute before data stored by the earlier instructions have become globally visible).

Certain situations may lead to the next sequential instruction after a near indirect JMP being speculatively executed. If software needs to prevent this (e.g., in order to prevent a speculative execution side channel), then an INT3 or LFENCE instruction opcode can be placed after the near indirect JMP in order to block speculative execution.

Operation ¶

IF near jump
    IF 64-bit Mode
            THEN
                    IF near relative jump
                        THEN
                            tempRIP ← RIP + DEST; (* RIP is instruction following JMP instruction*)
                        ELSE (* Near absolute jump *)
                            tempRIP ← DEST;
                    FI;
            ELSE
                    IF near relative jump
                        THEN
                            tempEIP ← EIP + DEST; (* EIP is instruction following JMP instruction*)
                        ELSE (* Near absolute jump *)
                            tempEIP ← DEST;
                    FI;
    FI;
    IF (IA32_EFER.LMA = 0 or target mode = Compatibility mode)
    and tempEIP outside code segment limit
            THEN #GP(0); FI
    IF 64-bit mode and tempRIP is not canonical
            THEN #GP(0);
    FI;
    IF OperandSize = 32
                THEN
                    EIP ← tempEIP;
                ELSE
                    IF OperandSize = 16
                            THEN (* OperandSize = 16 *)
                                    EIP ← tempEIP AND 0000FFFFH;
                                ELSE (* OperandSize = 64)
                                    RIP ← tempRIP;
                    FI;
        FI;
FI;
IF far jump and (PE = 0 or (PE = 1 AND VM = 1)) (* Real-address or virtual-8086 mode *)
        THEN
                tempEIP ← DEST(Offset); (* DEST is ptr16:32 or [m16:32] *)
                IF tempEIP is beyond code segment limit
                    THEN #GP(0); FI;
                CS ← DEST(segment selector); (* DEST is ptr16:32 or [m16:32] *)
                IF OperandSize = 32
                        THEN
                            EIP ← tempEIP; (* DEST is ptr16:32 or [m16:32] *)
                        ELSE (* OperandSize = 16 *)
                            EIP ← tempEIP AND 0000FFFFH; (* Clear upper 16 bits *)
                FI;
FI;
IF far jump and (PE = 1 and VM = 0)
(* IA-32e mode or protected mode, not virtual-8086 mode *)
        THEN
                IF effective address in the CS, DS, ES, FS, GS, or SS segment is illegal
            or segment selector in target operand NULL
                            THEN #GP(0); FI;
                IF segment selector index not within descriptor table limits
                    THEN #GP(new selector); FI;
            Read type and access rights of segment descriptor;
            IF (EFER.LMA = 0)
                    THEN
                            IF segment type is not a conforming or nonconforming code
                            segment, call gate, task gate, or TSS
                                    THEN #GP(segment selector); FI;
                    ELSE
                            IF segment type is not a conforming or nonconforming code segment
                            call gate
                                    THEN #GP(segment selector); FI;
            FI;
            Depending on type and access rights:
                    GO TO CONFORMING-CODE-SEGMENT;
                    GO TO NONCONFORMING-CODE-SEGMENT;
                    GO TO CALL-GATE;
                    GO TO TASK-GATE;
                    GO TO TASK-STATE-SEGMENT;
        ELSE
                #GP(segment selector);
FI;
CONFORMING-CODE-SEGMENT:
    IF L-Bit = 1 and D-BIT = 1 and IA32_EFER.LMA = 1
            THEN GP(new code segment selector); FI;
        IF DPL > CPL
            THEN #GP(segment selector); FI;
        IF segment not present
            THEN #NP(segment selector); FI;
    tempEIP ← DEST(Offset);
    IF OperandSize = 16
                THEN tempEIP ← tempEIP AND 0000FFFFH;
    FI;
    IF (IA32_EFER.LMA = 0 or target mode = Compatibility mode) and
    tempEIP outside code segment limit
            THEN #GP(0); FI
    IF tempEIP is non-canonical
            THEN #GP(0); FI;
    CS ← DEST[segment selector]; (* Segment descriptor information also loaded *)
    CS(RPL) ← CPL
    EIP ← tempEIP;
END;
NONCONFORMING-CODE-SEGMENT:
    IF L-Bit = 1 and D-BIT = 1 and IA32_EFER.LMA = 1
            THEN GP(new code segment selector); FI;
    IF (RPL > CPL) OR (DPL ≠ CPL)
            THEN #GP(code segment selector); FI;
    IF segment not present
            THEN #NP(segment selector); FI;
    tempEIP ← DEST(Offset);
    IF OperandSize = 16
                THEN tempEIP ← tempEIP AND 0000FFFFH; FI;
    IF (IA32_EFER.LMA = 0 OR target mode = Compatibility mode)
    and tempEIP outside code segment limit
            THEN #GP(0); FI
    IF tempEIP is non-canonical THEN #GP(0); FI;
    CS ← DEST[segment selector]; (* Segment descriptor information also loaded *)
    CS(RPL) ← CPL;
    EIP ← tempEIP;
END;
CALL-GATE:
    IF call gate DPL < CPL
    or call gate DPL < call gate segment-selector RPL
                    THEN #GP(call gate selector); FI;
    IF call gate not present
            THEN #NP(call gate selector); FI;
    IF call gate code-segment selector is NULL
            THEN #GP(0); FI;
    IF call gate code-segment selector index outside descriptor table limits
            THEN #GP(code segment selector); FI;
    Read code segment descriptor;
    IF code-segment segment descriptor does not indicate a code segment
    or code-segment segment descriptor is conforming and DPL > CPL
    or code-segment segment descriptor is non-conforming and DPL ≠ CPL
                    THEN #GP(code segment selector); FI;
    IF IA32_EFER.LMA = 1 and (code-segment descriptor is not a 64-bit code segment
    or code-segment segment descriptor has both L-Bit and D-bit set)
                    THEN #GP(code segment selector); FI;
    IF code segment is not present
            THEN #NP(code-segment selector); FI;
        tempEIP ← DEST(Offset);
        IF GateSize = 16
                THEN tempEIP ← tempEIP AND 0000FFFFH; FI;
    IF (IA32_EFER.LMA = 0 OR target mode = Compatibility mode) AND tempEIP
    outside code segment limit
            THEN #GP(0); FI
    CS ← DEST[SegmentSelector); (* Segment descriptor information also loaded *)
    CS(RPL) ← CPL;
    EIP ← tempEIP;
END;
TASK-GATE:
    IF task gate DPL < CPL
    or task gate DPL < task gate segment-selector RPL
            THEN #GP(task gate selector); FI;
    IF task gate not present
            THEN #NP(gate selector); FI;
    Read the TSS segment selector in the task-gate descriptor;
    IF TSS segment selector local/global bit is set to local
    or index not within GDT limits
    or descriptor is not a TSS segment
    or TSS descriptor specifies that the TSS is busy
            THEN #GP(TSS selector); FI;
        IF TSS not present
            THEN #NP(TSS selector); FI;
        SWITCH-TASKS to TSS;
        IF EIP not within code segment limit
            THEN #GP(0); FI;
END;
TASK-STATE-SEGMENT:
    IF TSS DPL < CPL
    or TSS DPL < TSS segment-selector RPL
    or TSS descriptor indicates TSS not available
            THEN #GP(TSS selector); FI;
    IF TSS is not present
            THEN #NP(TSS selector); FI;
    SWITCH-TASKS to TSS;
    IF EIP not within code segment limit
            THEN #GP(0); FI;
END;

Flags Affected ¶

All flags are affected if a task switch occurs; no flags are affected if a task switch does not occur.

Protected Mode Exceptions ¶

Real-Address Mode Exceptions ¶

Virtual-8086 Mode Exceptions ¶

Compatibility Mode Exceptions ¶

Same as 64-bit mode exceptions.

64-Bit Mode Exceptions ¶