Added an example to check if a string is palindrome.

Author: gashev

27-palindrome/Makefile

@@ -0,0 +1,6 @@
+all:
+	nasm -f elf64 palindrome.asm -o palindrome.o
+	gcc -nostartfiles palindrome.o -o palindrome
+
+clean:
+	rm -f palindrome.o palindrome

27-palindrome/palindrome.asm

@@ -0,0 +1,144 @@
+extern exit
+extern puts
+extern printf
+
+SECTION .data
+    str: db 'No word, no bond, row on.', 0
+    str_is_palindrome: db '"%s" - is palindrome.', 10, 0
+    str_is_not_palindrome: db '"%s" - is not palindrome.', 10, 0
+
+SECTION .text
+    GLOBAL _start
+
+_start:
+    ; Start string.
+    mov r10, str
+
+    ; Find end of string str.
+    mov r11, str
+
+loop:
+    mov cl, [r11]
+    cmp cl, 0
+    je init_is_palindrome
+
+    inc r11
+    jmp loop
+
+init_is_palindrome:
+    dec r11
+    call is_palindrome
+
+finish:
+    cmp rax, 0
+    je print_is_not_palindrome
+
+    ; Print string is palindrome.
+    mov rsi, str
+    mov rdi, str_is_palindrome
+    mov rax, 0
+    call printf
+    jmp exit_app
+
+print_is_not_palindrome:
+    ; Print string is not palindrome.
+    mov rsi, str
+    mov rdi, str_is_not_palindrome
+    mov rax, 0
+    call printf
+
+exit_app: 
+    ; Exit application.
+    mov rdi, 0
+    call exit
+
+is_palindrome:
+    cmp r10, r11
+    jge finish_palindrome
+
+    ; Check start string is letter.
+    mov dil, [r10]
+    call is_letter
+    cmp rax, 0
+    je inc_left
+
+    ; Check end string is letter.
+    mov dil, [r11]
+    call is_letter
+    cmp rax, 0
+    je dec_right
+
+    ; Compare start of string and end of string.
+    mov cl, [r10]
+    mov dil, cl
+    call lowercase
+    mov cl, al
+ 
+    mov dl, [r11]
+    mov dil, dl
+    call lowercase
+    mov dl, al
+ 
+    cmp cl, dl
+    jne not_palindrome
+ 
+    inc r10
+    dec r11
+    jmp is_palindrome
+
+finish_palindrome:
+    mov rax, 1
+    ret
+
+not_palindrome:
+    mov rax, 0
+    ret
+
+inc_left:
+    inc r10
+    jmp is_palindrome
+
+dec_right:
+    dec r11
+    jmp is_palindrome
+
+; Function to check rdi is letter (a-zA-Z).
+is_letter:
+    ; Check is character between a-z.
+    cmp rdi, 'a'
+    jl not_between_az
+
+    cmp rdi, 'z'
+    jg not_between_az
+
+    mov rax, 1
+    ret
+
+not_between_az:
+    ; Check is character between A-Z.
+    cmp rdi, 'A'
+    jl not_between_AZ
+
+    cmp rdi, 'Z'
+    jg not_between_AZ
+
+    mov rax, 1
+    ret
+
+not_between_AZ:
+    mov rax, 0
+    ret
+
+lowercase:
+    mov al, dil
+
+    cmp dil, 'A'
+    jl lowercase_dont_change
+
+    cmp dil, 'Z'
+    jg lowercase_dont_change
+
+    add al, 'a'
+    sub al, 'A'
+lowercase_dont_change:
+    ret