Data Manipulation

Syntax#

• .386: Tells MASM to compile for a minimum x86 chip version of 386.
• .model: Sets memory model to use, see .MODEL.
• .code: Code segment, used for processes such as the main process.
• proc: Declares process.
• ret: used for exiting functions successfully, see Working With Return Values.
• endp: Ends process declaration.
• public: Makes process available to all segments of the program.
• end: Ends program, or if used with a process, such as in ”end main”, makes the process the main method.
• call: Calls process and pushes its opcode onto the stack, see Control Flow.
• ecx: Counter register, see registers.
• ecx: Counter register.
• mul: Multiplies value by eax

Remarks#

mov is used to transfer data between the registers.

Using MOV to manipulate values

Description:

mov copies values of bits from source argument to destination argument.

Common source/destination are registers, usually the fastest way to manipulate values with[in] CPU.

Another important group of source_of/destination_for values is computer memory.

Finally some immediate values may be part of the mov instruction encoding itself, saving time of separate memory access by reading the value together with instruction.

On x86 CPU in 32 and 64 bit mode there are rich possibilities to combine these, especially various memory addressing modes. Generally memory-to-memory copying is out limit (except specialized instructions like MOVSB), and such manipulation requires intermediate storage of values into register[s] first.

Step 1: Set up your project to use MASM, see Executing x86 assembly in Visual Studio 2015
Step 2: Type in this:

.386
.model small
.code

public main
main proc
    mov ecx, 16       ; Move immediate value 16 into ecx
    mov eax, ecx      ; Copy value of ecx into eax
    ret               ; return back to caller
        ; function return value is in eax (16)
main endp
end main

Step 3: Compile and debug.

The program should return value 16.