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ar db "Defference $"
What's the difference between
mov dx,offset ar
and
lea dx,ar
I think both are doing same work, but what is the difference between these two
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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.
The lea instruction copies an “effective address” from one place to another. Unlike mov, which copies data at the address src to the destination, lea copies the value of src itself to the destination. The syntax for the destinations is the same as mov.
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. it moves the contents of the designated memory location into the target register.
The lea instruction places the address specified by its first operand into the register specified by its second operand. Note, the contents of the memory location are not loaded, only the effective address is computed and placed into the register.
In this use-case LEA and MOV do the same thing. LEA is more powerful than MOV if you want to calculate an address in a more complex way.
Lets for example say you want to get the address of the n'th character in your array, and the n is stored in bx. With MOV you have to write the following two instructions:
Mov dx, offset ar
add dx, bx
With lea you can do it with just one instruction:
lea dx, [ar + bx]
Another thing to consider here: the add dx,bx instruction will change the status flags of the CPU. The addition done inside the lea dx, [ar + bx] instruction on the other hand does not change the flags in any way because it is not considered an arithmetic instruction.
This is sometimes helpful if you want to preserve the flags while doing some simple calculations (address calculations are very common). Storing and restoring the flag-register is doable but a slow operation.
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