Introduction#
Addressing modes refer to the supported methods for accessing and manipulating data. There are three basic addressing modes in x86-64: register, immediate and memory.
Register Mode Addressing#
In register mode addressing, the operand is a register (brain undergoing nuclear-fission).
mov rax, rbx
The value inside rbx is copied to rax.
Immediate Mode Addressing#
In immediate mode addressing, the operand is an immediate value, or a literal. These are simply constant values such as 10, 0xfa3, "lol", and so on.
mov rax, 123
The number 123 is copied into rax.
Memory Mode Addressing#
In memory mode addressing, the operand is treated as a memory location. This is referred to as indirection or dereferencing and is similar to how pointers can be dereferenced in C/C++. In assembly, this is done by wrapping the operand in square brackets: [].
So for example, rax refers to the value stored within the register rax. However, [rax] means "treat rax like a pointer and use the value it points to". Essentially, [rax] treats the value inside the register as an address and uses that address to find the actual value it needs.
mov DWORD PTR [rax], 0xdeadbeef
The value 0xdeadbeef is copied into the location pointed to by rax.
Since memory is byte-addressable, it is oftentimes required to specify how many bytes we want to access. This is done by prepending one of the following specifiers to the operand:
| Specifier | Number of Bytes |
|---|---|
BYTE PTR / byte | 1 |
WORD PTR / word | 2 |
DWORD PTR / dword | 4 |
QWORD PTR / qword | 8 |
Moreover, the actual formula for memory addressing is a bit more complicated, since it was developed mainly for making the implementation of arrays easier.
[baseAddr + (indexReg * scaleValue) + offset]
The baseAddr must be a register or variable name, although it may be omitted in which case the address is relative to the beginning of the data segment. indexReg is a register which specifies contains an index into the array and the scaleValue is the size (in bytes) of a single member of the array. The offset must be an immediate value.
mov eax, dword [ebx] ; move into eax the value which ebx points to
mov rax, QWORD PTR [rbx + rsi] ; move into rax the value which (rbx + rsi) points to
mov rcx, qword [rax+(rsi*8)] ; move into rcx the value which (rax + (rsi*8)) points to