x86doc › XBEGIN - Transactional Begin

XBEGIN—Transactional Begin

Opcode/Instruction	Op/En	64/32bit Mode Support	CPUID Feature Flag	Description
C7 F8 XBEGIN rel16	A	V/V	RTM	Specifies the start of an RTM region. Provides a 16-bit relative offset to compute the address of the fallback instruction address at which execution resumes following an RTM abort.
C7 F8 XBEGIN rel32	A	V/V	RTM	Specifies the start of an RTM region. Provides a 32-bit relative offset to compute the address of the fallback instruction address at which execution resumes following an RTM abort.

Instruction Operand Encoding

Op/En	Operand 1	Operand2	Operand3	Operand4
A	Offset	N/A	N/A	N/A

Description

The XBEGIN instruction specifies the start of an RTM code region. If the logical processor was not already in trans-actional execution, then the XBEGIN instruction causes the logical processor to transition into transactional execu-tion. The XBEGIN instruction that transitions the logical processor into transactional execution is referred to as the outermost XBEGIN instruction. The instruction also specifies a relative offset to compute the address of the fallback code path following a transactional abort. (Use of the 16-bit operand size does not cause this address to be trun-cated to 16 bits, unlike a near jump to a relative offset.)

On an RTM abort, the logical processor discards all architectural register and memory updates performed during the RTM execution and restores architectural state to that corresponding to the outermost XBEGIN instruction. The fallback address following an abort is computed from the outermost XBEGIN instruction.

Execution of XBEGIN while in a suspend read address tracking region causes a transactional abort.

Operation

XBEGIN

IF RTM_NEST_COUNT < MAX_RTM_NEST_COUNT AND SUSLDTRK_ACTIVE = 0
    THEN
         RTM_NEST_COUNT++
         IF RTM_NEST_COUNT = 1 THEN
              IF 64-bit Mode
                    THEN
                         IF OperandSize = 16
                              THEN fallbackRIP := RIP + SignExtend64(rel16);
                              ELSE fallbackRIP := RIP + SignExtend64(rel32);
                         FI;
                         IF fallbackRIP is not canonical
                              THEN #GP(0);
                         FI;
                    ELSE
                         IF OperandSize = 16
                              THEN fallbackEIP := EIP + SignExtend32(rel16);
                              ELSE fallbackEIP := EIP + rel32;
                         FI;
                         IF fallbackEIP outside code segment limit
                              THEN #GP(0);
                         FI;
              FI;
              RTM_ACTIVE := 1
              Enter RTM Execution (* record register state, start tracking memory state*)
         FI; (* RTM_NEST_COUNT = 1 *)
    ELSE (* RTM_NEST_COUNT = MAX_RTM_NEST_COUNT OR SUSLDTRK_ACTIVE = 1 *)
         GOTO RTM_ABORT_PROCESSING
FI;
(* For any RTM abort condition encountered during RTM execution *)
RTM_ABORT_PROCESSING:
    Restore architectural register state
    Discard memory updates performed in transaction
    Update EAX with status
    RTM_NEST_COUNT := 0
    RTM_ACTIVE := 0
    SUSLDTRK_ACTIVE := 0
    IF 64-bit mode
         THEN
              RIP := fallbackRIP
         ELSE
              EIP := fallbackEIP
    FI;
END

Flags Affected

None.

Intel C/C++ Compiler Intrinsic Equivalent

XBEGIN unsigned int _xbegin( void );

SIMD Floating-Point Exceptions

None.

Protected Mode Exceptions

#UD	CPUID.(EAX=7, ECX=0):EBX.RTM[bit 11]=0. If LOCK prefix is used.
#GP(0)	If the fallback address is outside the CS segment.

Real-Address Mode Exceptions

#GP(0)	If the fallback address is outside the address space 0000H and FFFFH.
#UD	CPUID.(EAX=7, ECX=0):EBX.RTM[bit 11]=0. If LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0)	If the fallback address is outside the address space 0000H and FFFFH.
#UD	CPUID.(EAX=7, ECX=0):EBX.RTM[bit 11]=0. If LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-bit Mode Exceptions

#UD	CPUID.(EAX=7, ECX=0):EBX.RTM[bit 11] = 0. If LOCK prefix is used.
#GP(0)	If the fallback address is non-canonical.