x86 Instruction Set Reference

# LEA

# Load Effective Address

Opcode Mnemonic Description
8D /r LEA r16,m Store effective address for m in register r16.
8D /r LEA r32,m Store effective address for m in register r32.
Description Operand Size Address Size Action Performed

Computes the effective address of the second operand (the source operand) and stores it in the first operand (destination operand). The source operand is a memory address (offset part) specified with one of the processors addressing modes; the destination operand is a general-purpose register. The address-size and operand-size attributes affect the action performed by this instruction, as shown in the following table. The operand-size attribute of the instruction is determined by the chosen register; the address-size attribute is determined by the attribute of the code segment.

Address and Operand Size Attributes Operand SizeAddress SizeAction Performed 161616-bit effective address is calculated and stored in requested 16-bit register destination. 163232-bit effective address is calculated. The lower 16 bits of the address are stored in the requested 16-bit register destination. 321616-bit effective address is calculated. The 16-bit address is zeroextended and stored in the requested 32-bit register destination. 323232-bit effective address is calculated and stored in the requested 32-bit register destination.

Different assemblers may use different algorithms based on the size attribute and symbolic reference of the source operand. |16|16|16-bit effective address is calculated and stored in requested 16-bit register destination.|16|32|32-bit effective address is calculated. The lower 16 bits of the address are stored in the requested 16-bit register destination.|32|16|16-bit effective address is calculated. The 16-bit address is zeroextended and stored in the requested 32-bit register destination.|32|32|32-bit effective address is calculated and stored in the requested 32-bit register destination.| | |16|16|16-bit effective address is calculated and stored in requested 16-bit register destination.| |16|32|32-bit effective address is calculated. The lower 16 bits of the address are stored in the requested 16-bit register destination.| |32|16|16-bit effective address is calculated. The 16-bit address is zeroextended and stored in the requested 32-bit register destination.| |32|32|32-bit effective address is calculated and stored in the requested 32-bit register destination.|

# Operation

if(OperandSize == 16 && AddressSize == 16) Destination = EffectiveAddress(Source); //16-bit address
else if(OperandSize == 16 && AddressSize == 32) {
	Temporary = EffectiveAddress(Source); //32-bit address
	Destination = Temporary[0..15]; //16-bit address
}
else if(OperandSize == 32 && AddressSize == 16) {
	Temporary = EffectiveAddress(Source); //16-bit address;
	Destination = ZeroExtend(Temporary); //32-bit address
}
else if(OperandSize == 32 && AddressSize == 32) Destination = EffectiveAddress(Source) //32-bit address
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# Flags affected

None.

# Protected Mode Exceptions
#UD If source operand is not a memory location.

# Real-Address Mode Exceptions
#UD If source operand is not a memory location.

# Virtual-8086 Mode Exceptions
#UD If source operand is not a memory location.

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