x86 Assembly Language Reference Manual

Addressing modes are represented by the following:

[sreg:][offset][([base][,index][,scale])]

All the items in the square brackets are optional, but at least one is necessary. If you use any of the items inside the parentheses, the parentheses are mandatory.
sreg is a segment register override prefix. It may be any segment register. If a segment override prefix is present, you must follow it by a colon before the offset component of the address. sreg does not represent an address by itself. An address must contain an offset component.
offset is a displacement from a segment base. It may be absolute or relocatable. A label is an example of a relocatable offset. A number is an example of an absolute offset.
base and index can be any 32-bit register. scale is a multiplication factor for the index register field. Its value may be 1, 2, 4, 8 to indicate the number to multiply by. The multiplication then occurs by 1, 2, 4, and 8.

Refer to Intel's 80386 Programmer's Reference Manual for more details on x86 addressing modes.

Following are some examples of addresses:

movl var, %eax: Move the contents of memory location var into %eax.
movl %cs:var, %eax: Move the contents of the memory location var in the code segment into %eax.
movl $var, %eax: Move the address of var into %eax.
movl array_base(%esi), %eax: Add the address of memory location array_base to the contents of %esi to get an address in memory. Move the contents of this address into %eax.
movl (%ebx, %esi, 4), %eax: Multiply the contents of %esi by 4 and add this to the contents of %ebx to produce a memory reference. Move the contents of this memory location into %eax.
movl struct_base(%ebx, %esi, 4), %eax: Multiply the contents of %esi by 4, add this to the contents of %ebx, and add this to the address of struct_base to produce an address. Move the contents of this address into %eax.