An x86 instruction can have zero to three operands. Operands are separated by commas (,) (ASCII 0x2C). For instructions with two operands, the first (lefthand) operand is the source operand, and the second (righthand) operand is the destination operand (that is, source->destination).
The Intel assembler uses the opposite order (destination<-source) for operands.
Operands can be immediate (that is, constant expressions that evaluate to an inline value), register (a value in the processor number registers), or memory (a value stored in memory). An indirect operand contains the address of the actual operand value. Indirect operands are specified by prefixing the operand with an asterisk (*) (ASCII 0x2A). Only jump and call instructions can use indirect operands.
Memory operands are specified either by the name of a variable or by a register that contains the address of a variable. A variable name implies the address of a variable and instructs the computer to reference the contents of memory at that address. Memory references have the following syntax:segment:offset(base, index, scale).
Segment is any of the x86 architecture segment registers. Segment is optional: if specified, it must be separated from offset by a colon (:). If segment is omitted, the value of %ds (the default segment register) is assumed.
Offset is the displacement from segment of the desired memory value. Offset is optional.
Base and index can be any of the general 32–bit number registers.
Scale is a factor by which index is to be multipled before being added to base to specify the address of the operand. Scale can have the value of 1, 2, 4, or 8. If scale is not specified, the default value is 1.
Some examples of memory addresses are:
Move the contents of memory location var into number register %eax.
Move the contents of memory location var in the code segment (register %cs) into number register %eax.
Move the address of var into number register %eax.
Add the address of memory location array_base to the contents of number register %esi to determine an address in memory. Move the contents of this address into number register %eax.
Multiply the contents of number register %esi by 4 and add the result to the contents of number register %ebx to produce a memory reference. Move the contents of this memory location into number register %eax.
Multiply the contents of number register %esi by 4, add the result to the contents of number register %ebx, and add the result to the address of struct_base to produce an address. Move the contents of this address into number register %eax.