Assembly Experiments

helloworld.asm

; /Usr/local/bin/nasm -f macho64 64.asm && ld -macosx_version_min 10.7.0 -lSystem -o 64 64.o && ./64

BITS 64

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; The .data section is for storing and naming constants.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
section .data

msg:    db      "Hello, world!", 10
.len:   equ     $ - msg             ;$ refers to the address of this constant, so $ - msg is the length of message

othermsg: db    "Goodbye, world!", 10
.len:   equ     $ - othermsg

callmsg: db     "Call, world!", 10
.len:   equ     $ - callmsg

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;The .text section is for the actual code.
;(I assume .text refers to source code being text)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
section .text
global start   ;global is used to tell the kernel where to enter the program.
;OSX expects this to be called "start"

;You can put underscores in numbers to make them easier to read.  They are ignored by nasm.
some_method:
        mov    rax, 0x200_0004
        mov    rdi, 1
        mov    rsi, callmsg
        mov    rdx, callmsg.len
        syscall
        inc
        ret

start:
        mov     rax, 1
        cmp     rax, 1
        jne     printgoodbye

printhello:
        call    some_method
        mov     rax, 0x200_0004 ;The number for the syscall "write": user_ssize_t write(int fd, user_addr_t cbuf, user_size_t nbyte);
        mov     rdi, 1          ;First param for write is the file descripter.  1 is stdout
        mov     rsi, msg        ;Second param is a pointer to the msg
        mov     rdx, msg.len    ;Third param is the length of the message
        syscall
        jmp     terminate

printgoodbye:
        call    some_method
        mov     rax, 0x200_0004
        mov     rdi, 1
        mov     rsi, othermsg
        mov     rdx, othermsg.len
        syscall
        jmp     terminate

terminate:
        call    some_method
        mov     rax, 0x200_0001 ;The number for the syscall "exit": void exit(int rval);
        mov     rdi, 0         ; First param is the return code
        syscall

printargs.asm

BITS 64

section .data
EQUAL_EACH:                equ  1000b
stdout:                    equ  1
no_arguments:              equ  1
no_arguments_msg:          db   "No arguments", 10
.len:                      equ  $-no_arguments_msg

format:                    db "Number of arguments to this function: %d", 10, 0
lenFormat:                 db "Length of string: %d", 10, 0

section .text
global start
extern _printf

writeMsg:
  push  rsi
  push  rdi

  mov   rax, 0x200_0004
  mov   rdi, stdout
  pop   rsi
  pop   rdx

  syscall
  ret

; ==== strlen ====
strlen_sse42:
  ; ecx = string
  mov rax, -16
  mov rdx, rcx
  pxor xmm0, xmm0

STRLEN_LOOP:
    add rax, 16
    PcmpIstrI xmm0, [rdx + rax], EQUAL_EACH
    jnz STRLEN_LOOP

  add rax, rcx
  ret

start:
  mov  r12, [rsp]
  mov  r13, rsp

  mov  rdi, format                          ; set up registers for c func call, printf. printf arg 1, format
  mov  rsi, r12                             ; printf arg 2, the integer currently at the top of the stack (argc)
  and  rsp, -16                             ; align the stack on 16 bit boundary before making a call to printf, required in mac os x
  call  _printf                             ; call printf
  pop r8                                    ; ignore return value

  cmp  r12, no_arguments                    ; if no arguments, do nothing
  je   terminate_no_arguments
  xor  rbx, rbx

process_arguments:
  inc  rbx
  mov  rcx, [r13 + 8 * rbx]
  call strlen_sse42

  mov  rdi, lenFormat
  mov  rsi, rcx
  and  rsp, -16
  call _printf

check_exit:
  cmp  rbx, r12
  jne  process_arguments

  jmp  terminate

terminate_no_arguments:
  mov   rdi, no_arguments_msg
  mov   rsi, no_arguments_msg.len
  call  writeMsg
  jmp   terminate

terminate:
  mov   rax, 0x200_0001
  mov   rdi, 0
  syscall