1. assembly basics

1.1. memory

1.1.1. ram

Memory can be addressed using some number:

0x00000000000000000000
...
*memory*
...
0xffffffffffffffffffff

1.1.2. binary

Somewhere in memory lies the binary:

0x??? .text
      initialized data
      .bss
      heap
       |
       v
      ...

      ...
       ^
       |
      stack
      cmd line args
0x???

1.1.3. function frames

On the stack:

...

locals          |
base pointer    | line function
ret addr        |
parameters      |

locals          |
base pointer    | rectangle function
ret addr        |
parameters      |

1.2. registers

1.2.1. sizes

  64 |                            32 |            16 |     8 |    0 |
     v                               v               v       v      v
rax: ................................................................
                               eax:  ................................
                                                 ax: ................
                                                  ah ........        
                                                          al ........

r?x: 64 bit e?x: 32 bit ?x: 16 bit ?h: "high" 8 bit ?l: "low" 8 bit

1.2.2. common x86 registers

stack
- rsp (stack pointer)
- rbp (base pointer)

function arguments
- rdi
- rsi
- rdx
- rcx

return value
- rax

intruction pointer
- rip

1.3. stack

1.3.1. push

Stack: ___ ___ ___

push "a"

Stack: _a_ ___ ___

push "b"

Stack: _a_ _b_ ___

push "c"

Stack: _a_ _b_ _c_

1.3.2. pop

Stack: _a_ _b_ _c_
rax: ___
rbx: ___
rcx: ___

pop "rax"

Stack: _a_ _b_ ___
rax: _c_
rbx: ___
rcx: ___

pop "rbx"

Stack: _a_ ___ ___
rax: _c_
rbx: _b_
rcx: ___

pop "rcx"

Stack: ___ ___ ___
rax: _c_
rbx: _b_
rcx: _a_

1.4. syntax

1.4.1. intel

mov rax, rbx

1.4.2. AT&T

mov %rax, %rbx

1.5. Instruction

1.5.1. mov

mov dst, src

values in square brackets dereference pointers, so the following:

mov rbx, [rax]

sould move the value the pointer that is stored in rax points to into rbx.

1.5.2. sub / add

subtrace or add to the provided value

sub rax, 0x100

add rdx, [rax]

1.5.3. ret

1.5.4. leave

1.5.5. call

1.5.6. jmp

jne: jump if not equal jnz: jump if not zero

1.6. Functions

group of instructions
execute function by preparing registers, jumping to code, execution code and returning to original code
function arguments passed depending on arch:
- 32: stack
- 64: registers (rdi, rsi, rdx, rcx, r8, r9)

1.6.1. prolog

push rip
push rbp
mov rbp, rsp

1.6.2. epilog

mov rsp, rbp
pop rbp
pop rip

1.7. Buffer

char str[16];

_ _ _ _
_ _ _ _
_ _ _ _
_ _ _ _

^
| stack grows up

| buffer get's written in the other dir
v

gets(str);

*input 16 "A"*

A A A A
A A A A
A A A A
A A A A
_ _ _ _ <- old rbp
_ _ _ _ <- old rip

*input 4 "B"*

A A A A
A A A A
A A A A
A A A A
B B B B <- old rbp
_ _ _ _ <- old rip