Lea instruction in 8086

Computes the effective address of the second operand the source operand and stores it in the first operand destination operand.

Команда LEA

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.

Different assemblers may use different algorithms based on the size attribute and symbolic reference of the source operand. Address calculation is governed by address size attribute, the default address size is bits.

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In bit mode, address size of 16 bits is not encodable. See Table The lower 16 bits of the address are stored in the requested bit register destination. The bit address is zero-extended and stored in the requested bit register destination. The lower 16 bits of the address are stored in the requested bit register destination using 66H prefix.Prepares two BCD values for division.

Corrects the result of multiplication of two BCD values. Add with Carry. Logical AND between all bits of two operands. Result is stored in operand1. Transfers control to procedure, return address is IP is pushed to stack.

Convert byte into word. Clear Carry flag. Clear Direction flag. Clear Interrupt enable flag. This disables hardware interrupts. Complement Carry flag. Inverts value of CF. Convert Word to Double word. Decimal adjust After Addition. Corrects the result of addition of two packed BCD values. Decimal adjust After Subtraction. Corrects the result of subtraction of two packed BCD values. Unsigned divide. Signed divide. Signed multiply. AL, im. Input from port into AL or AX. Second operand is a port number.

If required to access port number over - DX register should be used. Example: IN AX, 4 ; get status of traffic lights. IN AL, 7 ; get status of stepper-motor. Interrupt numbered by immediate byte Interrupt 4 if Overflow flag is 1.

LEA -- Load Effective Address

Interrupt Return. Short Jump if Carry flag is set to 1. Short Jump if CX register is 0. Unconditional Jump. Transfers control to another part of the program.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. Different assemblers may use different algorithms based on the size attribute and symbolic reference of the source operand.

Description Computes the effective address of the second operand the source operand and stores it in the first operand destination operand. Address and Operand Size Attributes Operand Size Address Size Action Performed 16 16 bit effective address is calculated and stored in requested bit register destination. The lower 16 bits of the address are stored in the requested bit register destination. The bit address is zeroextended and stored in the requested bit register destination.

UD If source operand is not a memory location.General purpose registers are used to store temporary data within the microprocessor. There are 8 general purpose registers in microprocessor. Figure — General purpose registers. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.

See your article appearing on the GeeksforGeeks main page and help other Geeks. Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below. Writing code in comment? Please use ide. Figure — General purpose registers AX — This is the accumulator.

It is of 16 bits and is divided into two 8-bit registers AH and AL to also perform 8-bit instructions. It is generally used for arithmetical and logical instructions but in microprocessor it is not mandatory to have accumulator as the destination operand.

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lea instruction in 8086

As others have pointed out, LEA load effective address is often used as a "trick" to do certain computations, but that's not its primary purpose. The x86 instruction set was designed to support high-level languages like Pascal and C, where arrays—especially arrays of ints or small structs—are common. Consider, for example, a struct representing x, y coordinates:. Assuming the base of the array is already in EBXand variable i is in EAXand xcoord and ycoord are each 32 bits so ycoord is at offset 4 bytes in the structthis statement can be compiled to:.

The scale factor of 8 is because each Point is 8 bytes in size. In this case, you don't want the value of ycoordbut its address. That's where LEA load effective address comes in. Instead of a MOVthe compiler can generate. LEAthe only instruction that performs memory addressing calculations but doesn't actually address memory.

LEA accepts a standard memory addressing operand, but does nothing more than store the calculated memory offset in the specified register, which may be any general purpose register. This feature decreases the level of dependency among instructions and thus makes room for further optimization by the compiler or hardware scheduler. The reason for that was already explained in the top rated answers i. Maybe just another thing about LEA instruction. You can also use LEA for fast multiplying registers by 3, 5 or 9.

It loads the address of the location reference by the source operand to the destination operand. For instance, you could use it to:. Basically, you benefit from complex addressing modes supported by x86 architecture to manipulate pointers efficiently.

The biggest reason that you use LEA over a MOV is if you need to perform arithmetic on the registers that you are using to calculate the address.

Effectively, you can perform what amounts to pointer arithmetic on several of the registers in combination effectively for "free. What's really confusing about it is that you typically write an LEA just like a MOV but you aren't actually dereferencing the memory. In other words:. The has a large family of instructions which accept a register operand and an effective address, perform some computations to compute the offset part of that effective address, and perform some operation involving the register and the memory referred to by the computed address.

It was fairly simple to have one of the instructions in that family behave as above except for skipping that actual memory operation. This, the instructions:. The difference is a skipped step. Both instructions work something like:. As for why Intel thought this instruction was worth including, I'm not exactly sure, but the fact that it was cheap to implement would have been a big factor. Another factor would have been the fact that Intel's assembler allowed symbols to be defined relative to the BP register.

If fnord was defined as a BP-relative symbol e. If one wanted to use something like stosw to store data to a BP-relative address, being able to say. As the existing answers mentioned, LEA has the advantages of performing memory addressing arithmetic without accessing memory, saving the arithmetic result to a different register instead of the simple form of add instruction.

The real underlying performance benefit is that modern processor has a separate LEA ALU unit and port for effective address generation including LEA and other memory reference addressthis means the arithmetic operation in LEA and other normal arithmetic operation in ALU could be done in parallel in one core. If an address is used repeatedly it is more effective to store it in a register instead of calculating the effective address every time it is used.

The LEA Load Effective Address instruction is a way of obtaining the address which arises from any of the Intel processor's memory addressing modes. But instead of the contents of the memory location, we get the location itself into the destination. LEA is not a specific arithmetic instruction; it is a way of intercepting the effective address arising from any one of the processor's memory addressing modes.First, looking in array above, we see that several jumps have same flag conditions and would get same opcode.

If you count enumerated jumps, you get 16 different cases. The number 16 is 2 4 or one hexadecimal digit or also a half of a byte. Intel have then allocated 4 bits a nibble in the opcode to define jump type. This value, which identifies the jump type, is part of opcode.

Programmer uses defined labels instruction blocks, line number, named label, etc. The linker's job is to compute and insert these addresses after compilation step to make machine code runnable.

Conditionnal jumps use relative addresses, jumps are done from current instruction and distances defined in bytes. Value can be stored on one byte or one word 2 ou 4 bytes. Machine code is composed by an opcode and optionally some extra bytes.

lea instruction in 8086

A conditional jump is composed by this way:. Machine code is 7 4 10 :.

lea instruction in 8086

Contrary to Motorola, Intel reverses the byte order for its instruction coding. This way of ordering is named little endian. Machine code is 0F8 5 :. To konw from how many bytes, the linker has to use a long jump instead a short onewe must know what is the maximal move of a short jump. If we add the instruction's size, the real range is from to Let's talk about most common and interesting jumps, the short and long relative jump: These jumps work like conditionnal jumps, only the opcode header changes.

For short jump, the header is EBfor long jumps opcode is E9. Changing a short conditional jump to unconditional is easy: just replace first byte 7 x with EB. In this case, we overwrite with a NOP instruction the first byte opcode of conditional long jump and replace the second opcode byte of instruction with the long JMP opcode.The has a large family of instructions which accept a register operand and an effective address, perform some computations to compute the offset part of that effective address, and perform some operation involving the register and the memory referred to by the computed address.

It was fairly simple to have one of the instructions in that family behave as above except for skipping that actual memory operation. This, the instructions:. The difference is a skipped step. Both instructions work something like:.

As for why Intel thought this instruction was worth including, I'm not exactly sure, but the fact that it was cheap to implement would have been a big factor. Another factor would have been the fact that Intel's assembler allowed symbols to be defined relative to the BP register. If fnord was defined as a BP-relative symbol e. If one wanted to use something like stosw to store data to a BP-relative address, being able to say. Take the unassembled tent out of the package.

The tent will likely consist of poles and a large, tarp-like sheet of cloth Some furniture assembly workers specialize in upholstered pieces. As with all assembly language An assembly line is a manufacturing process in which interchangable parts are added to a product in a sequential manner to Lea assembly instruction. What's the purpose of the LEA instruction?

October 3, Source: stackoverflow. You might also like. Latest Stories. Search for:. It's Interesting Like all assembly languages This product is based on the latest High-resolution graphics printed by cutting-edge printing The surface is made with long-lasting durable finish.

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