sub

Subtract

SUB r/m, r

Subtracts src from dest.

Details

Subtracts the source operand from the destination operand and stores the result in the destination; the difference is computed modulo 2^(operand size). Sets OF, SF, ZF, AF, CF, and PF based on the result. Available in 16-bit, 32-bit, and 64-bit forms; operand size is determined by the prefix and REX.W in 64-bit mode.

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
29
+0
 
Format Legacy
Opcode 29
Extension Base

Operands

  • dest
    Register or memory operand
  • src
    General-purpose register

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID 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. SUB—Subtract Vol. 2B 4-684 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