strings example added

Author: 0xAX

README.md

@@ -6,8 +6,9 @@ Learning assembly for linux-x64
 Examples
 ==============
 
-  * hello - hello world
-  * sum   - sum of two numbers
-  * stack - sum of command line arguments
+  * hello - hello world;
+  * sum   - sum of two numbers;
+  * stack - sum of command line arguments;
+  * reverse - reverse string.
 
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hello/Makefile

@@ -1,5 +1,3 @@
-
-# check nasm version, must be more than 2.0
 all:
 	nasm -f elf64 -o hello.o hello.asm
 	ld -o hello hello.o

strings/Makefile

@@ -0,0 +1,6 @@
+all:
+	nasm -g -f elf64 -o reverse.o reverse.asm
+	ld -o reverse reverse.o
+
+clean:
+	rm reverse reverse.o

strings/reverse.asm

@@ -0,0 +1,123 @@
+;;
+;; initialized data
+;;
+section .data
+		SYS_WRITE equ 1
+		STD_OUT   equ 1
+		SYS_EXIT  equ 60
+		EXIT_CODE equ 0
+
+		NEW_LINE db 0xa
+		INPUT db "Hello world!"
+
+;;
+;; non initialized data
+;;
+section .bss
+		OUTPUT resb 1
+
+;;
+;; code
+;;
+section .text
+        global _start
+
+;;
+;; main routine
+;;
+_start:
+		;; get addres of INPUT
+		mov rsi, INPUT
+		;; zeroize rcx for counter
+		xor rcx, rcx
+		; df = 0 si++
+		cld
+		; remember place after function call
+		mov rdi, $ + 15
+        ;; get string lengt
+		call calculateStrLength
+		;; write zeros to rax
+		xor rax, rax
+		;; additional counter for reverseStr
+		xor rdi, rdi
+		;; reverse string
+		jmp reverseStr
+
+;;
+;; calculate length of string
+;;
+calculateStrLength:
+		;; check is it end of string
+		cmp byte [rsi], 0
+		;; if yes exit from function
+		je exitFromRoutine
+		;; load byte from rsi to al and inc rsi
+		lodsb
+		;; push symbol to stack
+		push rax
+		;; increase counter
+		inc rcx
+		;; loop again
+		jmp calculateStrLength
+
+;;
+;; back to _start
+;;
+exitFromRoutine:
+		;; push return addres to stack again
+		push rdi
+		;; return to _start
+		ret
+
+;;
+;; reverse string
+;;
+;; 31 in stack
+reverseStr:
+		;; check is it end of string
+		cmp rcx, 0
+		;; if yes print result string
+		je printResult
+		;; get symbol from stack
+		pop rax
+		;; write it to output buffer
+		mov [OUTPUT + rdi], rax
+		;; decrease length counter
+		dec rcx
+		;; increase additional length counter (for write syscall)
+		inc rdi
+		;; loop again
+		jmp reverseStr
+
+;;
+;; Print result string
+;;
+printResult:
+		mov rdx, rdi
+		mov rax, 1
+		mov rdi, 1
+		mov rsi, OUTPUT
+        syscall
+		jmp printNewLine
+
+;;
+;; Print new line
+;;
+printNewLine:
+		mov rax, SYS_WRITE
+		mov rdi, STD_OUT
+		mov rsi, NEW_LINE
+		mov rdx, 1
+		syscall
+		jmp exit
+
+;;
+;; Exit from program
+;;
+exit:
+		;; syscall number
+		mov rax, SYS_EXIT
+		;; exit code
+		mov rdi, EXIT_CODE
+		;; call sys_exit
+		syscall