09-16-25 Lab 6

Class: CSCE-312

Notes:

gcc hello.c -o hello

• You can use ldb on mac to debug C and C++ code

Registers available in x86 Assembly

+-----------------+----------------+----------------+----------------+
|   64-bit (8B)   |  32-bit (4B)   |  16-bit (2B)   |   8-bit (1B)   |
+-----------------+----------------+----------------+----------------+
| RAX             | EAX            | AX             | AH / AL        |
| RBX             | EBX            | BX             | BH / BL        |
| RCX             | ECX            | CX             | CH / CL        |
| RDX             | EDX            | DX             | DH / DL        |
| RSI             | ESI            | SI             | SIL            |
| RDI             | EDI            | DI             | DIL            |
| RBP             | EBP            | BP             | BPL            |
| RSP             | ESP            | SP             | SPL            |
| R8              | R8D            | R8W            | R8B            |
| R9              | R9D            | R9W            | R9B            |
| R10             | R10D           | R10W           | R10B           |
| R11             | R11D           | R11W           | R11B           |
| R12             | R12D           | R12W           | R12B           |
| R13             | R13D           | R13W           | R13B           |
| R14             | R14D           | R14W           | R14B           |
| R15             | R15D           | R15W           | R15B           |
+-----------------+----------------+----------------+----------------+

Some other registers:

rdi: 
...

Flags:

+--------+----------------------+----------------------------+
| Bit #  | Flag Name            | Description                |
+--------+----------------------+----------------------------+
|   0    | CF  (Carry Flag)     | Set on carry/borrow out of |
|        |                      | most significant bit       |
|   1    |  -- Reserved --      | Always 1 in EFLAGS         |
|   2    | PF  (Parity Flag)    | Set if low 8 bits have     |
|        |                      | even parity                |
|   3    |  -- Reserved --      | Always 0                   |
|   4    | AF  (Aux Carry Flag) | Carry from bit 3 → bit 4   |
|   5    |  -- Reserved --      | Always 0                   |
|   6    | ZF  (Zero Flag)      | Set if result == 0         |
|   7    | SF  (Sign Flag)      | Set if result is negative  |
|   8    | TF  (Trap Flag)      | Enables single-step mode   |
|   9    | IF  (Interrupt Flag) | Enables/disables interrupts|
|  10    | DF  (Direction Flag) | Controls string ops dir    |
|  11    | OF  (Overflow Flag)  | Set if signed overflow     |
|  12    | IOPL (bit 0)         | I/O privilege level (low)  |
|  13    | IOPL (bit 1)         | I/O privilege level (high) |
|  14    | NT  (Nested Task)    | Controls task switching    |
|  15    |  -- Reserved --      | (Was RF on 286, reserved)  |
+--------+----------------------+----------------------------+

Basic instructions in assembly

mov

Usage:

mov rax, rdx

• Reads bytes from register rax, and moves them to register rdx

Variations of mov

2 bytes -> mov
4 bytes -> movD 
8 bytes -> mov@

If we have

mov rax, [rdx]

• Then [rdx] is an address

If we have

mov [rax], rdx

• It goes to that address, reads it, and moves it to register rdx

lea

Usage:

lea rdi, [rbx + 0x10]

• Loads the effective address inside the register

Other basic instructions

add rax, rdx        -> rax = rax + rdx
sub rsp, 0x10       -> rsp = rsp - 0x10
xor
or
and

• Note that the prefix 0x in front of a number just means "hex"

Accessing your Stack

If you want to access your stack you can use the following instructions

push rax
pop rax

• push adds an element to the stack
• pop removes the last value from the stack and stores it inside register rax

jump

Usage:

jmp 0x602010

• It sets your RIP
  • " " Instruction Point (RIP)
• 0x602010 is an address

cal

Usage:

cal test

• This is a function call
• The first thing it does is that it just pushes rsp and the push rbp to the stack so that when we are done with that function it just pops these two things
  • push rsp
  • push rbp
• Where is the stack store?
  • It is inside the Memory
  • Remember registers are inside your CPU
  • Memory is apart from your CPU

ret

Usate:

ret

• A simple function return

cmp

Usage:

cmp rax, rbx
JZ(JNZ) test

• It checks these flags
• Basically a "set flags 1 0"
• It means you probably have "if" statements in your code
• JZ = Jump Zero
• JNZ = Jump Non-Zero