ASM tutorials

calc_str_len.asm

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+; calc_str_len.asm
+; Prints calculates the length of the string "Hello, world!" and prints to stdout
+; Compile with "nasm -f elf64 calc_str_len.asm && ld calc_str_len.o -o calc_str_len" 
+
+.text:
+	global _start
+
+_start:
+	mov rax, msg      ; move msg to rax register
+	call strlen       ; call strlen subroutine
+
+	mov rax, 1        ; system call for write
+	mov rdi, 1        ; file handle for stdout
+	syscall           ; call kernel
+
+	mov rax, 60       ; system call for exit
+	mov rdi, 0        ; exit code
+	syscall           ; call kernel
+
+
+strlen:
+	push rax          ; push rax to stack
+	mov rbx, rax      ; move rax value (msg variable) to rbx register
+
+calc_str_len:
+	cmp byte [rax], 0 ; check if the pointer of rax equals 0 (string delimeter)
+	jz exit_loop      ; jump to "exit_loop" if zero flag has been set
+	inc rax           ; increment rax (position along string)
+	jmp calc_str_len  ; jump to start of loop
+
+exit_loop:
+	sub rax, rbx      ; subtract rbx from rax to equal to length of bytes between them
+	mov rdx, rax      ; rax will now equal the length of the string
+	pop rsi           ; pop top value on stack to rsi for printing
+	ret
+
+.data:
+	; initialize doubleword msg variable
+	msg db 'Hello, world!',0xa,0x0

fizzbuzz.asm

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+; fizzbuzz.asm
+; Prints numbers 1 to 100, however if the number is a multiple of 3 it will print Fizz,
+; and if the number is a multiple of 5 it will print Buzz. If the number is a multiple
+; of both, it will print FizzBuzz
+; Compile with "nasm -f elf64 fizzbuzz.asm && ld fizzbuzz.o -o fizzbuzz" 
+
+%include 'macros.asm'
+
+.text:
+	global _start
+
+_start:
+	mov rcx,0 ; counter
+	mov rsi,0 ; fizz check
+	mov rdi,0 ; buzz check
+
+loop:
+	inc rcx
+
+check_fizz:
+	mov rdx,0
+	mov rax,rcx
+	mov rbx, 3
+	div rbx
+	mov rsi,rdx
+	cmp rsi,0
+	jne check_buzz
+	printStr f
+
+check_buzz:
+	mov rdx,0
+	mov rax,rcx
+	mov rbx, 5
+	div rbx
+	mov rdi,rdx
+	cmp rdi,0
+	jne check_int
+	printStr b
+
+check_int:
+	cmp rsi,0
+	je cont
+	cmp rdi,0
+	je cont
+	printInt rcx
+
+cont:
+	printStr n 
+	cmp rcx,99
+	jle loop
+	exit 0
+
+.data:
+	; initialize doubleword msg variable
+	f db 'Fizz',0x0
+	b db 'Buzz',0x0
+	n db 0xA,0x0

hello_world.asm

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+; hello_world.asm
+; Prints "Hello, world!" in 64 bit linux assembly written in intel syntax
+; Compile with "nasm -f elf64 hello_world.asm && ld hello_world.o -o hello_world" 
+
+.text:
+	global _start
+
+_start:
+	mov rax, 1   ; system call for write
+	mov rdi, 1   ; file handle for stdout
+	mov rsi, msg ; move msg variable to rsi register for printing
+	mov rdx, 14  ; length of the string
+	syscall      ; call kernel
+
+	mov rax, 60  ; system call for exit
+	mov rdi, 0   ; exit code
+	syscall      ; call kernel
+
+.data:
+	; initialize doubleword msg variable
+	msg db 'Hello, world!',0xa

hello_world_macro.asm

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+; hello_world_macro.asm
+; Prints "Hello, world!" using Assembly macros
+; Compile with "nasm -f elf64 hello_world_macro.asm && ld hello_world_macro.o -o hello_world_macro" 
+
+%include 'macros.asm'
+
+.text:
+	global _start
+
+_start:
+	printStr msg
+	exit 0
+
+.data:
+	; initialize doubleword msg variable
+	msg db 'Hello, world!',0x0

macros.asm

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+; Collection of Assembly macros to make programming in assembly easier
+; Include it with "%include 'macros.asm'
+
+section .data
+	newline db 0xA,0x0
+
+	; Macro to calculate string length and print to stdout
+	%macro printStr 1
+		;; Store previous data
+		push rax
+		push rbx
+		push rcx
+		push rdx
+		push rdi
+		push rsi
+
+		;; Move first arg to rax
+		mov rax, %1
+		;; push rax to stack
+		push rax
+		;; move 0 to rbx for loop counter
+		mov rbx,0
+		;; counts letters
+		%%printLoop:
+			inc rax
+			inc rbx
+			mov cl,[rax]
+			cmp cl,0
+			jne %%printLoop
+			;; sys_write
+			mov rax,1
+			mov rdi,1
+			pop rsi
+			mov rdx,rbx
+			syscall
+
+		;; pop values back to registers
+
+		pop rsi
+		pop rdi
+		pop rdx
+		pop rcx
+		pop rbx
+		pop rax
+
+	%endmacro
+	
+	%macro printStrLF 1
+		push rax
+		mov rax,%1
+		printStr rax
+		printStr newline
+		pop rax
+	%endmacro
+
+	%macro printInt 1
+		push    rax
+		push    rcx
+		push    rdx
+		push    rsi
+
+		mov rax,%1
+		mov     rcx, 0
+ 
+		%%divideLoop:
+			inc     rcx
+			mov     rdx, 0
+			mov     rsi, 10
+			idiv    rsi
+			add     rdx, 48
+			push    rdx
+			cmp     rax, 0
+			jnz     %%divideLoop
+	 
+		%%printLoop:
+			dec     rcx
+			mov     rax, rsp
+			printStr rax
+			pop     rax
+			cmp     rcx, 0
+			jnz     %%printLoop
+		
+		pop     rsi
+		pop     rdx
+		pop     rcx
+		pop     rax
+
+	%endmacro
+
+	%macro printIntLF 1
+		push rax
+		mov rax, %1
+		printInt rax
+		printStr newline
+		pop rax
+	%endmacro
+
+	%macro exit 1
+		mov rax,60
+		mov rdi,%1
+		syscall
+	%endmacro
+