x86 Assembly Basic

File Extension

Captial extension .S represents the assembly language file that still needs to be passed into a preprocessor, i.e. contains include command, denoted as .sx also, while .s represnts the pure assembly code that can be complied into object file (.o file) directly.

Register

Register is the variable of CPU, but other than variable in the programming language, register used specific name to play specific function.

ax, bx, cx, dx and their extention are the most common-used register, which are assigned as the special function respectically:

ax: accumulator register, used to store the middle result of sum or product.
bx: base register, used to store the base address during the memory addressing process.
cx: counter register, as the counter in rep and loop instruction.
dx: remainder register, used to store the remainder of the integer division

ax, bx, cx, dx represent 16-bit register, each of them can be devided to the low bits parts (0~7 bits) and high bits part (8~15 bits), called al and ah for ax and similarly for the rest. The extended 32-bit eax, ebx, ecx, edx are most famous register in modern 32-bit machine, and 64-bit rax, rbx, rcx, rdx.

Instruction

mov

Four types of mov instruction included: movb, movw, movl, movq.

movb: move 1 bytes (8-bit).
movw: move 1 word (2 bytes, 16-bit).
movl: move 1 long, (2 words, 4 bytes, 32-bit)
movq: move quad-words (4 words, 8 bytes ,64-bit).

Four typical usage of mov:

Transfer value between register.
```
movl %eax, %edx;
```
Notice that this operation means %eax->%edx, i.e. %edx=%eax, instead of vice versa.
Assign a constant into the register
```
movl 0x0000ffff, %edx;
```

Unittest For x86 Assembly Basic

What’s the difference between .S file and .s file?
Explain the usage of following register: ax, bx, cx, dx.
Explain the difference among ax, eax, rax.
Give the four types of mov instruction.