sub

Subtract

SUB r/m, r

Subtracts src from dest.

Pseudocode Operation

result ← dest - src;
OF ← overflow_occurred(dest, src, result);
SF ← (result < 0);
ZF ← (result == 0);
AF ← borrow_from_bit(3);
PF ← parity(result);
CF ← borrow_out;
dest ← result;

Example

SUB EAX, 10   ; Subtract 10 from EAX
SUB ECX, EDX  ; Subtract EDX from ECX

Encoding

Binary Layout

Format Legacy

Opcode 29

Extension Base

Operands

dest
Register or memory operand
src
General-purpose register

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
2C ib	SUB AL, imm8	I	Valid Valid	Subtract imm8 from AL.
2D iw	SUB AX, imm16	I	Valid Valid	Subtract imm16 from AX.
2D id	SUB EAX, imm32	I	Valid Valid	Subtract imm32 from EAX.
REX.W + 2D id	SUB RAX, imm32	I	Valid N.E.	Subtract imm32 sign-extended to 64-bits from RAX.
80 /5 ib	SUB r/m8, imm8	MI	Valid Valid	Subtract imm8 from r/m8.
81 /5 iw	SUB r/m16, imm16	MI	Valid Valid	Subtract imm16 from r/m16.
81 /5 id	SUB r/m32, imm32	MI	Valid Valid	Subtract imm32 from r/m32.
REX.W + 81 /5 id	SUB r/m64, imm32	MI	Valid N.E.	Subtract imm32 sign-extended to 64-bits from r/m64.
83 /5 ib	SUB r/m16, imm8	MI	Valid Valid	Subtract sign-extended imm8 from r/m16.
83 /5 ib	SUB r/m32, imm8	MI	Valid Valid	Subtract sign-extended imm8 from r/m32.
REX.W + 83 /5 ib	SUB r/m64, imm8	MI	Valid N.E.	Subtract sign-extended imm8 from r/m64.
28 /r	SUB r/m8, r8	MR	Valid Valid	Subtract r8 from r/m8.
29 /r	SUB r/m16, r16	MR	Valid Valid	Subtract r16 from r/m16.
29 /r	SUB r/m32, r32	MR	Valid Valid	Subtract r32 from r/m32.
REX.W + 29 /r	SUB r/m64, r64	MR	Valid N.E.	Subtract r64 from r/m64.
2A /r	SUB r8, r/m8	RM	Valid Valid	Subtract r/m8 from r8.
2B /r	SUB r16, r/m16	RM	Valid Valid	Subtract r/m16 from r16.
2B /r	SUB r32, r/m32	RM	Valid Valid	Subtract r/m32 from r32.
REX.W + 2B /r	SUB r64, r/m64	RM	Valid N.E.	Subtract r/m64 from r64.

Description

Subtracts the second operand (source operand) from the first operand (destination operand) and stores the result in the destination operand. The destination operand can be a register or a memory location; the source operand can be an immediate, register, or memory location. (However, two memory operands cannot be used in one instruction.) When an immediate value is used as an operand, it is sign-extended to the length of the destination operand format. The SUB instruction performs integer subtraction. It evaluates the result for both signed and unsigned integer operands and sets the OF and CF flags to indicate an overflow in the signed or unsigned result, respectively. The SF flag indicates the sign of the signed result.

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. This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.

Operation

DEST := (DEST - SRC);

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.

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. SUB-Subtract Vol. 2B 4-685

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