What does the LEAL assembly instruction do?

LEA (load effective address) just computes the address of the operand, it does not actually dereference it. Most of the time, it’s just doing a calculation like a combined multiply-and-add for, say, array indexing.

In this case, it’s doing a simple numeric subtraction: leal -4(%ebp), %eax just assigns to the %eax register the value of %ebp - 4. It’s equivalent to a single sub instruction, except a sub requires the destination to be the same as one of the sources.

The movl instruction, in contrast, accesses the memory location at %ebp - 4 and stores that value into %eax.

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