sbb

Subtract with Borrow

SBB r/m, r

Subtracts operands and the Carry Flag (CF).

Details

Subtracts the source operand and the Carry Flag (CF) from the destination operand, storing the result in the destination. Supports 8/16/32/64-bit sizes (64-bit in long mode only). Sets CF on unsigned borrow, SF/ZF/AF/OF/PF based on result; CF enables multi-precision subtraction chains.

Pseudocode Operation

result ← dest - src - CF
dest ← result
CF ← unsigned_borrow(dest_old, src, CF)
OF ← signed_overflow(dest_old, src, result)
ZF ← (result == 0)
SF ← (result_sign_bit == 1)
AF ← ((dest_old & 0xF) - (src & 0xF) - CF) has unsigned borrow
PF ← popcount(result & 0xFF) is even

Example

SBB rbx, rax

Encoding

Binary Layout

Format Legacy

Opcode 19

Extension Base

Operands

dest
Reg/Mem
src
Reg

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
1C ib	SBB AL, imm8	I	Valid Valid	Subtract with borrow imm8 from AL.
1D iw	SBB AX, imm16	I	Valid Valid	Subtract with borrow imm16 from AX.
1D id	SBB EAX, imm32	I	Valid Valid	Subtract with borrow imm32 from EAX.
REX.W + 1D id	SBB RAX, imm32	I	Valid N.E.	Subtract with borrow sign-extended imm.32 to 64-bits from RAX.
80 /3 ib	SBB r/m8, imm8	MI	Valid Valid	Subtract with borrow imm8 from r/m8.
81 /3 iw	SBB r/m16, imm16	MI	Valid Valid	Subtract with borrow imm16 from r/m16.
81 /3 id	SBB r/m32, imm32	MI	Valid Valid	Subtract with borrow imm32 from r/m32.
REX.W + 81 /3 id	SBB r/m64, imm32	MI	Valid N.E.	Subtract with borrow sign-extended imm32 to 64-bits from r/m64.
83 /3 ib	SBB r/m16, imm8	MI	Valid Valid	Subtract with borrow sign-extended imm8 from r/m16.
83 /3 ib	SBB r/m32, imm8	MI	Valid Valid	Subtract with borrow sign-extended imm8 from r/m32.
REX.W + 83 /3 ib	SBB r/m64, imm8	MI	Valid N.E.	Subtract with borrow sign-extended imm8 from r/m64.
18 /r	SBB r/m8, r8	MR	Valid Valid	Subtract with borrow r8 from r/m8.
19 /r	SBB r/m16, r16	MR	Valid Valid	Subtract with borrow r16 from r/m16.
19 /r	SBB r/m32, r32	MR	Valid Valid	Subtract with borrow r32 from r/m32.
REX.W + 19 /r	SBB r/m64, r64	MR	Valid N.E.	Subtract with borrow r64 from r/m64.
1A /r	SBB r8, r/m8	RM	Valid Valid	Subtract with borrow r/m8 from r8.
1B /r	SBB r16, r/m16	RM	Valid Valid	Subtract with borrow r/m16 from r16.
1B /r	SBB r32, r/m32	RM	Valid Valid	Subtract with borrow r/m32 from r32.
REX.W + 1B /r	SBB r64, r/m64	RM	Valid N.E.	Subtract with borrow r/m64 from r64.

Description

Adds the source operand (second operand) and the carry (CF) flag, and subtracts the result from the destination operand (first operand). The result of the subtraction is stored in the destination operand. The destination operand can be a register or a memory location; the source operand can be an immediate, a register, or a memory location. (However, two memory operands cannot be used in one instruction.) The state of the CF flag represents a borrow from a previous subtraction. When an immediate value is used as an operand, it is sign-extended to the length of the destination operand format. SBB—Integer Subtraction With Borrow Vol. 2B 4-612 The SBB instruction does not distinguish between signed or unsigned operands. Instead, the processor evaluates the result for both data types and sets the OF and CF flags to indicate a borrow in the signed or unsigned result, respectively. The SF flag indicates the sign of the signed result. The SBB instruction is usually executed as part of a multibyte or multiword subtraction in which a SUB instruction is followed by a SBB instruction. This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically. In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). Using a REX prefix in the form of REX.W promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits.

Operation

DEST := (DEST – (SRC + CF));

Intel C/C++ Compiler Intrinsic Equivalent

SBB extern unsigned char _subborrow_u8(unsigned char c_in, unsigned char src1, unsigned char src2, unsigned char *diff_out);
SBB extern unsigned char _subborrow_u16(unsigned char c_in, unsigned short src1, unsigned short src2, unsigned short *diff_out);
SBB extern unsigned char _subborrow_u32(unsigned char c_in, unsigned int src1, unsigned char int, unsigned int *diff_out);
SBB extern unsigned char _subborrow_u64(unsigned char c_in, unsigned __int64 src1, unsigned __int64 src2, unsigned __int64
*diff_out);

Flags Affected

The OF, SF, ZF, AF, PF, and CF flags are set according to the result.

Exceptions

Protected Mode Exceptions

#GP(0) If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used but the destination is not a memory operand.

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit. #UD If the LOCK prefix is used but the destination is not a memory operand.

Virtual-8086 Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made. #UD If the LOCK prefix is used but the destination is not a memory operand.

Compatibility Mode Exceptions

Same exceptions as in protected mode. SBB—Integer Subtraction With Borrow Vol. 2B 4-613

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #GP(0) If the memory address is in a non-canonical form. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used but the destination is not a memory operand. SBB—Integer Subtraction With Borrow Vol. 2B 4-614