Introduction

In register mode addressing, the operand is a register (brain undergoing nuclear-fission).

mov rax, rbx
Copy

The value inside rbx is copied to rax.

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
Copy

The number 123 is copied into rax.

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
Copy

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:

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]
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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
Copy