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. It seems that lots of answers already complete, I'd like to add one more example code for showing how the lea and move instruction work differently when they have the same expression format. To make a long story short, lea instruction and mov instructions both can be used with the parentheses enclosing the src operand of the instructions. When they are enclosed with the , the expression in the is calculated in the same way; however, two instructions will interpret the calculated value in the src operand in a different way.
However, when it is used with the mov instruction, it tries to access the value pointed to by the address generated by the above expression and store it to the destination. In contrast of it, when the lea instruction is executed with the above expression, it loads the generated value as it is to the destination. The below code executes the lea instruction and mov instruction with the same parameter.
However, to catch the difference, I added a user-level signal handler to catch the segmentation fault caused by accessing a wrong address as a result of mov instruction. With -O optimize as compiler option, gcc will find the lea instruction for the indicated code line.
All normal "calculating" instructions like adding multiplication, exclusive or set the status flags like zero, sign. Now you may want to use the address multiple times. Loading such an addres into a register is never intended to set status flags and luckily it doesn't. The phrase "load effective address" makes the programmer aware of that. That is where the weird expression comes from. It is clear that once the processor is capable of using the complicated address to process its content, it is capable of calculating it for other purposes.
This is a general rule in assembly programming: Use the instructions however it rocks your boat. The only thing that counts is whether the particular transformation embodied by the instruction is useful for you.
This is ciasdis notation. When you use MOV , it calculates the address and accesses the memory. LEA just calculates the address, it doesn't actually access memory. This is the difference. In and later, LEA just sets a sum of up to two source registers and an immediate value to a destination register. You cannot achieve this calculation to save the result to a register with MOV. The processor introduced a series of scaling modes, in which the index register value can be multiplied by a valid scaling factor to obtain the displacement.
The valid scale factors are 1, 2, 4, and 8. So, these instructions load "far" pointer - a pointer consisting of a bit segment selector and a bit or a bit, depending on the mode offset, so the total far pointer size was bit in bit mode and bit in bit mode. Under bit mode, there is no need in these instructions since OSes set all segment bases to zero flat memory model , so there is no need to load segment registers. We just use bit pointers, not Under bit modes, these instructions are not implemented.
Their opcodes give access violation interrupt exception. That's why some of the bits are inverted in VEX prefixes". Forgive me if someone already mentioned, but in the days of x86 when memory segmentation was still relevant, you may not get the same results from these two instructions:.
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Ask Question. Asked 12 years ago. Active 2 months ago. Viewed k times. For me, it just seems like a funky MOV. What's its purpose and when should I use it?
Improve this question. Michael Petch It was probably added to because the hardware is already there to decode and calculate addressing modes, not because it's "intended" only for use with addresses. Remember that pointers are just integers in assembly. Add a comment. Active Oldest Votes. Improve this answer. Kennedy I. Kennedy Wouldn't it have been cleaner to extend the mov instruction and leave off the brackets?
Without brackets, you always deal with the pointer itself. It doesn't actually read from the computed address, just computes it. Erik, tour comment is not accurate. Show 13 more comments. From the "Zen of Assembly" by Abrash: LEA , the only instruction that performs memory addressing calculations but doesn't actually address memory. What does that give us? Two things that ADD doesn't provide: the ability to perform addition with either two or three operands, and the ability to store the result in any register; not just one of the source operands.
And LEA does not alter the flags. Community Bot 1 1 1 silver badge. Frank Krueger Frank Krueger It does not access memory i. Moreover, on Skylake there are two fast path functional units ports 1 and 5 and there's only one slow path functional unit port 1.
I felt a lack of exact numbers for this example, so here they are. Show 3 more comments. Evan Carroll Angus Lee Angus Lee 1, 1 1 gold badge 8 8 silver badges 6 6 bronze badges. Yes, lea is sometimes useful for the compiler or human coder to do math without clobbering a flag result. But lea isn't faster than add. Most x86 instructions write flags. High-performance x86 implementations have to rename EFLAGS or otherwise avoid the write-after-write hazard for normal code to run fast, so instructions that avoid flag writes aren't better because of that.
PeterCordes : Hate to bring this up here but - am I alone in thinking this new [xlea] tag is redundant and unnecessary? MichaelPetch: Yeah, I think it's too specific.
But having a tag for it implies that there's room for an open-ended number of future questions, when in fact there are about 2 or 3 total that aren't just duplicates. How to use it for multiplying integers? What does dword ptr mean? Basically, it means "the size of the target operand is 32 bits", so this will bitwise-AND the bit value at the address computed by taking the contents of the ebp register and subtracting four with 0.
Share a link to this answer. What is offset address in assembly language? In assembly language In computer engineering and low-level programming such as assembly language , an offset usually denotes the number of address locations added to a base address in order to get to a specific absolute address.
In this context an offset is sometimes called a relative address. What is CMP in assembly language? The CMP instruction compares two operands. It is generally used in conditional execution. This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. It is used along with the conditional jump instruction for decision making. What is Rip register? What is effective address? An effective address is any operand to an instruction which references memory.
Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in square brackets. 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.
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