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Intel Pentium CPU Instruction Set Reference

SUB instruction - Subtract

Code Mnemonic Description
2C ib SUB AL, imm8 Subtract imm8 from AL
2D iw SUB AX, imm16 Subtract imm16 from AX
2D id SUB EAX, imm32 Subtract imm32 from EAX
80 /5 ib SUB r/m8, imm8 Subtract imm8 from r/m8
81 /5 iw SUB r/m16, imm16 Subtract imm16 from r/m16
81 /5 id SUB r/m32, imm32 Subtract imm32 from r/m32
83 /5 ib SUB r/m16, imm8 Subtract sign-extended imm8 from r/m16
83 /5 ib SUB r/m32, imm8 Subtract sign-extended imm8 from r/m32
28 / r SUB r/m8, r8 Subtract r8 from r/m8
29 / r SUB r/m16, r16 Subtract r16 from r/m16
29 / r SUB r/m32, r32 Subtract r32 from r/m32
2A / r SUB r8, r/m8 Subtract r/m8 from r8
2B / r SUB r16, r/m16 Subtract r/m16 from r16
2B / r SUB r32, r/m32 Subtract r/m32 from r32


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 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.

Operands Bytes Clocks
reg, reg 2 1 UV
mem, reg 2 + d(0, 2) 3 UV
reg, mem 2 + d(0, 2) 2 UV
reg, imm 2 + i(1, 2) 1 UV
mem, imm 2 + d(0, 2) + i(1, 2) 3 UV ( not pairable if there is a displacement and immediate)
acc, imm 1 + i(1, 2) 1 UV


ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF sets according to the result
VM unaffected ZF sets according to the result
RF unaffected AF sets according to the result
NT unaffected PF sets according to the result
IOPL unaffected CF sets according to the result
OF sets according to the result