What is the difference between MOV and LEA

Question

I would like to know what the difference between these instructions is   MOV AX   TABLE-ADDR    and   LEA AX   TABLE-ADDR

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As stated in the other answers    MOV will grab the data at the address inside the brackets and place that data into the destination operand  LEA will perform the calculation of the address inside the brackets and place that calculated address into the destination operand  This happens without actually going out to the memory and getting the data  The work done by LEA is in the calculating of the  effective address     Because memory can be addressed in several different ways  see examples below   LEA is sometimes used to add or multiply registers together without using an explicit ADD or MUL instruction  or equivalent    Since everyone is showing examples in Intel syntax  here are some in AT amp T syntax   MOVL 16  ebp    eax          put long  at  ebp 16  into eax    LEAL 16  ebp    eax          add 16 to ebp and store in eax     MOVQ   rdx  rcx 8    rax     put qword at  rcx 8   rdx  into rax    LEAQ   rdx  rcx 8    rax     put value of  rcx 8   rdx  into rax     MOVW 5  bp  si    ax         put word  at  si   bp   5  into ax    LEAW 5  bp  si    ax         put value of  si   bp   5  into ax     MOVQ 16  rip    rax          put qword at rip   16 into rax                    LEAQ 16  rip    rax          add 16 to instruction pointer and store in rax     MOVL label  1    eax         put long at label into eax               LEAL label  1    eax         put the address of the label into eax

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It depends on the used assembler  because  mov ax table addr   in MASM works as  mov ax word ptr table addr    So it loads the first bytes from table addr and NOT the offset to table addr  You should use instead  mov ax offset table addr   or  lea ax table addr   which works the same   lea version also works fine if table addr is a local variable e g   some procedure proc  local table addr 64  word  lea ax table addr

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Basically      Move into REG     after computing it     it seems to be nice for other purposes as well     if you just forget that the value is a pointer  you can use it for code optimizations minimization    what ever    MOV EBX   1 MOV ECX   2     with 1 instruction you got result of 2 registers in 3rd one     LEA EAX    EBX ECX 5    EAX   8   originaly it would be   MOV EAX  EBX ADD EAX  ECX ADD EAX  5

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None of the previous answers quite got to the bottom of my own confusion  so I d like to add my own   What I was missing is that lea operations treat the use of parentheses different than how mov does   Think of C  Let s say I have an array of long that I call array  Now the expression array i  performs a dereference  loading the value from memory at the address array   i   sizeof long   1    On the other hand  consider the expression  amp array i   This still contains the sub-expression array i   but no dereferencing is performed  The meaning of array i  has changed  It no longer means to perform a deference but instead acts as a kind of a specification  telling  amp  what memory address we re looking for   If you like  you could alternatively think of the  amp  as  cancelling out  the dereference   Because the two use-cases are similar in many ways  they share the syntax array i   but the existence or absence of a  amp  changes how that syntax is interpreted  Without  amp   it s a dereference and actually reads from the array  With  amp   it s not  The value array   i   sizeof long  is still calculated  but it is not dereferenced   The situation is very similar with mov and lea  With mov  a dereference occurs that does not happen with lea  This is despite the use of parentheses that occurs in both  For instance  movq   r8    r9 and leaq   r8    r9  With mov  these parentheses mean  dereference   with lea  they don t  This is similar to how array i  only means  dereference  when there is no  amp    An example is in order   Consider the code  movq   rdi   rsi  8    rbp   This loads the value at the memory location  rdi    rsi   8 into the register  rbp  That is  get the value in the register  rdi and the value in the register  rsi  Multiply the latter by 8  and then add it to the former  Find the value at this location and place it into the register  rbp   This code corresponds to the C line x   array i    where array becomes  rdi and i becomes  rsi and x becomes  rbp  The 8 is the length of the data type contained in the array   Now consider similar code that uses lea   leaq   rdi   rsi  8    rbp   Just as the use of movq corresponded to dereferencing  the use of leaq here corresponds to not dereferencing  This line of assembly corresponds to the C line x    amp array i    Recall that  amp  changes the meaning of array i  from dereferencing to simply specifying a location  Likewise  the use of leaq changes the meaning of   rdi   rsi  8  from dereferencing to specifying a location   The semantics of this line of code are as follows  get the value in the register  rdi and the value in the register  rsi  Multiply the latter by 8  and then add it to the former  Place this value into the register  rbp   No load from memory is involved  just arithmetic operations  2    Note that the only difference between my descriptions of leaq and movq is that movq does a dereference  and leaq doesn t  In fact  to write the leaq description  I basically copy pasted the description of movq  and then removed  Find the value at this location    To summarize  movq vs  leaq is tricky because they treat the use of parentheses  as in   rsi  and   rdi   rsi  8   differently  In movq  and all other instruction except lea   these parentheses denote a genuine dereference  whereas in leaq they do not and are purely convenient syntax      1  I ve said that when array is an array of long  the expression array i  loads the value from the address array   i   sizeof long   This is true  but there s a subtlety that should be addressed  If I write the C code  long x   array 5     this is not the same as typing  long x     array   5   sizeof long      It seems that it should be based on my previous statements  but it s not   What s going on is that C pointer addition has a trick to it  Say I have a pointer p pointing to values of type T  The expression p   i does not mean  the position at p plus i bytes   Instead  the expression p   i actually means  the position at p plus i   sizeof T  bytes    The convenience of this is that to get  the next value  we just have to write p   1 instead of p   1   sizeof T    This means that the C code long x   array 5   is actually equivalent to  long x     array   5    because C will automatically multiply the 5 by sizeof long    So in the context of this StackOverflow question  how is this all relevant  It means that when I say  the address array   i   sizeof long    I do not mean for  array   i   sizeof long   to be interpreted as a C expression  I am doing the multiplication by sizeof long  myself in order to make my answer more explicit  but understand that due to that  this expression should not be read as C  Just as normal math that uses C syntax    2  Side note  because all lea does is arithmetic operations  its arguments don t actually have to refer to valid addresses  For this reason  it s often used to perform pure arithmetic on values that may not be intended to be dereferenced  For instance  cc with -O2 optimization translates  long f long x      return x   5      into the following  irrelevant lines removed    f    leaq   rdi   rdi  4    rax    set  rax to  rdi    rdi   4   ret

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If you only specify a literal  there is no difference  LEA has more abilities  though  and you can read about them here   http   www oopweb com Assembly Documents ArtOfAssembly Volume Chapter 6 CH06-1 html HEADING1-136

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LEA  Load Effective Address  is a shift-and-add instruction  It was added to 8086 because hardware is there to decode and calculate adressing modes

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The difference is subtle but important   The MOV instruction is a  MOVe  effectively a copy of the address that the TABLE-ADDR label stands for   The LEA instruction is a  Load Effective Address  which is an indirected instruction  which means that TABLE-ADDR points to a memory location at which the address to load is found    Effectively using LEA is equivalent to using pointers in languages such as C  as such it is a powerful instruction

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MOV  can do same thing as LEA  label   but MOV instruction contain the effective address inside the instruction itself as an immediate constant  calculated in advance by the assembler   LEA uses PC-relative to calculate the effective address during the execution of the instruction

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In NASM syntax   mov eax  var          lea eax   var      i e  mov r32  imm32 lea eax   var 16      mov eax  var 16 lea eax   eax 4       shl eax  2          but without setting flags   In MASM syntax  use OFFSET var to get a mov-immediate instead of a load

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Lets understand this  with a example   mov eax   ebx  and  lea eax   ebx  Suppose value in ebx is 0x400000  Then mov will go to address 0x400000 and copy 4 byte of data present their to eax register Whereas lea will copy the address 0x400000 into eax  So  after the execution of each instruction value of eax in each case will be  assuming at memory 0x400000 contain is 30    eax   30          in case of mov  eax   0x400000    in case of lea  For definition mov copy the data from rm32 to destination  mov dest rm32  and lea load effective address  will copy the address to destination  mov dest rm32

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The instruction MOV reg addr means read a variable stored at address addr into register reg  The instruction LEA reg addr means read the address  not the variable stored at the address  into register reg   Another form of the MOV instruction is MOV reg immdata which means read the immediate data  i e  constant  immdata into register reg  Note that if the addr in LEA reg addr is just a constant  i e  a fixed offset  then that LEA instruction is essentially exactly the same as an equivalent MOV reg immdata instruction that loads the same constant as immediate data

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LEA means Load Effective Address MOV means Load Value   In short  LEA loads a pointer to the item you re addressing whereas MOV loads the actual value at that address   The purpose of LEA is to allow one to perform a non-trivial address calculation and store the result  for later usage   LEA ax   BP SI 5    Compute address of value  MOV ax   BP SI 5    Load value at that address   Where there are just constants involved  MOV  through the assembler s constant calculations  can sometimes appear to overlap with the simplest cases of usage of LEA  Its useful if you have a multi-part calculation with multiple base addresses etc

[assembly] What is the difference between MOV and LEA?

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