Introduction to Assembly
x86-64 assembly
Instructions are the way we tell the CPU what to do. It consists of operator and operands(a list of 0-3 items).
asm
mov rax, rbxCommonly,
operand 1, is the target of the operationoperand 2, is the source of the operation
Registers
In x86-64 there are 16 registers.
rip, register instruction pointer, the only non-general-purpose register
| Register | Higher byte | Lower byte | Lower 2 bytes¹ | Lower 4 bytes² |
|---|---|---|---|---|
| rax | ah | al | ax | eax |
| rcx | ch | cl | cx | ecx |
| rbx | bh | bl | bx | ebx |
| rdx | dh | dl | dx | edx |
| rsp | spl | sp | esp | |
| rsi | sil | si | esi | |
| rdi | dil | di | edi | |
| rbp | bpl | bp | ebp | |
| r8 | r8b | r8w | r8d | |
| r9 | r9b | r9w | r9d | |
| r10 | r10b | r10w | r10d | |
| r11 | r11b | r11w | r11d | |
| r12 | r12b | r12w | r12d | |
| r13 | r13b | r13w | r13d | |
| r14 | r14b | r14w | r14d | |
| r15 | r15b | r15w | r15d |
Hello, World
asm
section .data
; We define the constant `msg`, the string to print.
; We use the `db` (define byte) directive to define
; constants of one or more bytes (1 char = 1 byte).
msg db `Hello, World!\n`
section .text
global _start
_start:
; We call the sys_write syscall to print on the
; screen. Syscalls are invoked with `syscall` and
; they are identified by a number.
;
; The identifier for sys_write is 1. The `syscall`
; intruction will look for the instruction id in the
; register `rax`. So we move 1 in there.
mov rax, 1
; The system call to invoke has the signature:
;
; size_t sys_write(uint fd, const char* buf, size_t count)
; The `syscall` instruction wants the first argument
; in `rdi`. In this case, the first argument is the
; file descriptor of where to write the output.
; We will use 1, which identifies the standard output.
mov rdi, 1
; The second argument is expected to be in `rsi` and
; the signature tells us that it the string we want
; to print. We defined the buffer in the .data section
; (it's `msg`), so we just need to move it to `rsi`
mov rsi, msg
; `rdx` is the register for the third argument.
; We see in the signature that this is the count of
; characters we want to print.
;
; Note: the string is null-terminated, that is, there
; is an ending "ghost" character we need to account for.
mov rdx, 14
; We're now finally issuing the syscall
syscall
; Now the message is printed! Time to exit the program.
; Same as above, we invoke `syscall` with 60, which is
; `exit` and has the following signature:
;
; void exit(int status)
; Once again, `syscall` looks for the identifier in
; `rax`. We move 60, the identifier for `exit`, there
mov rax, 60
; Again, `rdi` is where the first argument is expected
; to be found.
; The first argument is the status code (see signature)
; so we put 0 for a clean exit.
mov rdi, 0
; Issuing the `exit` syscall and cleanly exiting
; the program
syscall