Netwide Assembler sample programs
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This is a collection of sample programs for the Netwide Assembler.
A "Hello, world!" program for the DOS operating system:
section .text
org 0x100
mov ah, 0x9
mov dx, hello
int 0x21
mov ax, 0x4c00
int 0x21
section .data
hello: db 'Hello, world!', 13, 10, '$'
An equivalent program for Linux:
global _start
section .text
_start:
mov eax, 4 ; write
mov ebx, 1 ; stdout
mov ecx, msg
mov edx, msg.len
int 0x80 ; write(stdout, msg, strlen(msg));
xor eax, msg.len ; invert return value from write()
xchg eax, ebx ; value for exit()
mov eax, 1 ; exit
int 0x80 ; exit(...)
section .data
msg: db "Hello, world!", 10
.len: equ $ - msg
An example of a similar program for Microsoft Windows:
global _main
extern _MessageBoxA@16
extern _ExitProcess@4
section code use32 class=code
_main:
push dword 0 ; UINT uType = MB_OK
push dword title ; LPCSTR lpCaption
push dword banner ; LPCSTR lpText
push dword 0 ; HWND hWnd = NULL
call _MessageBoxA@16
push dword 0 ; UINT uExitCode
call _ExitProcess@4
section data use32 class=data
banner: db 'Hello, world!', 0
title: db 'Hello', 0
A 64-bit program for Apple OS X that inputs a keystroke and shows it on the screen:
global _start
section .data
query_string: db "Enter a character: "
query_string_len: equ $ - query_string
out_string: db "You have input: "
out_string_len: equ $ - out_string
section .bss
in_char: resw 4
section .text
_start:
mov rax, 0x2000004 ; put the write-system-call-code into register rax
mov rdi, 1 ; tell kernel to use stdout
mov rsi, query_string ; rsi is where the kernel expects to find the address of the message
mov rdx, query_string_len ; and rdx is where the kernel expects to find the length of the message
syscall
; read in the character
mov rax, 0x2000003 ; read system call
mov rdi, 0 ; stdin
mov rsi, in_char ; address for storage, declared in section .bss
mov rdx, 2 ; get 2 bytes from the kernel's buffer (one for the carriage return)
syscall
; show user the output
mov rax, 0x2000004 ; write system call
mov rdi, 1 ; stdout
mov rsi, out_string
mov rdx, out_string_len
syscall
mov rax, 0x2000004 ; write system call
mov rdi, 1 ; stdout
mov rsi, in_char
mov rdx, 2 ; the second byte is to apply the carriage return expected in the string
syscall
; exit system call
mov rax, 0x2000001 ; exit system call
xor rdi, rdi
syscall
section .data
vuid db "BC220425429", 0 ; Provided VUID: "BC220425429" numeric_array times 10 db 0 ; Array to store numeric digits largest_digit db 0 ; Variable to store the largest digit updated_array times 10 db 0 ; Array to store updated VUID temp_array times 10 db 0 ; Temporary array for sorting
section .text global _start
_start:
; Step 1: Store the numeric part of VUID in an array mov esi, vuid ; Point to the VUID string mov ecx, 10 ; Loop counter for 10 digits mov ebx, numeric_array ; Point to the numeric_array
extract_digits:
mov al, [esi] ; Load character from VUID cmp al, 0 ; Check if end of string je end_extract ; If end of string, exit loop
cmp al, '0' ; Check if character is a digit jl next_char ; If not a digit, skip to next character cmp al, '9' jg next_char
sub al, '0' ; Convert character to numeric value mov [ebx], al ; Store numeric value in array inc ebx ; Move to next element in array
next_char:
inc esi ; Move to next character in VUID loop extract_digits ; Repeat until all 10 digits are extracted
end_extract:
; Step 2: Find the largest numeric digit mov ebx, numeric_array ; Point to the numeric_array mov al, [ebx] ; Load the first digit mov [largest_digit], al ; Assume it as the largest digit
mov ecx, 9 ; Loop counter for remaining digits inc ebx ; Move to the next digit
compare_digits:
mov al, [ebx] ; Load the digit cmp al, [largest_digit] ; Compare with largest digit found so far jle skip_update ; If less than or equal, skip update mov [largest_digit], al ; Update largest digit
skip_update:
inc ebx ; Move to the next digit loop compare_digits ; Repeat until all digits are compared
; Step 3: Subtract each numeric digit from the largest digit mov ebx, numeric_array ; Point to the numeric_array mov edi, updated_array ; Point to the updated_array
subtract_digits:
mov al, [ebx] ; Load numeric digit sub al, [largest_digit] ; Subtract from largest digit mov [edi], al ; Store the result in updated_array inc ebx ; Move to the next digit inc edi ; Move to the next element in updated_array loop subtract_digits ; Repeat for all digits
; Step 4: Sort and store updated VUID in ascending order ; (You can implement any sorting algorithm here, like bubble sort or insertion sort)
; Sort updated_array using bubble sort mov ecx, 10 ; Number of elements dec ecx ; Outer loop counter
outer_loop:
mov ebx, updated_array ; Reset pointer to the beginning of the array mov edx, 9 ; Inner loop counter
inner_loop:
mov al, [ebx] ; Load current element mov ah, [ebx + 1] ; Load next element cmp al, ah ; Compare current and next element jg swap_elements ; If current element > next element, swap inc ebx ; Move to the next element dec edx ; Decrement inner loop counter jnz inner_loop ; Repeat inner loop until edx = 0 loop outer_loop ; Repeat outer loop until ecx = 0
; Store sorted array back to updated_array mov ebx, updated_array ; Point to the updated_array mov edi, temp_array ; Point to the temporary array for sorting mov ecx, 10 ; Loop counter rep movsb ; Copy the sorted array back to updated_array
; Display the sorted array mov eax, 4 ; Syscall number for sys_write mov ebx, 1 ; File descriptor (stdout) mov ecx, updated_array ; Pointer to the array mov edx, 10 ; Number of bytes to write int 0x80 ; Call kernel
; Exit the program mov eax, 1 ; Syscall number for sys_exit xor ebx, ebx ; Exit code 0 int 0x80 ; Call kernel
swap_elements:
; Swap elements in updated_array mov al, [ebx] ; Move current element to al mov ah, [ebx + 1] ; Move next element to ah mov [ebx], ah ; Move next element to current position mov [ebx + 1], al ; Move current element to next position jmp inner_loop ; Continue inner loop