%macro scall 4
  mov rax,%1   ;function number for writing or o/p data
  mov rdi,%2   ;file descriptor id for monitor
  mov rsi,%3   ;starting address of string to be displayed
  mov rdx,%4   ;number of bytes to be displayed
syscall
%endmacro
section .data   ;initialized variable declaration
  menu db 10d,13d,"!!!!MENU!!!!"     ;messages
  db 10d,"1.ENTER A STRING"               
  db 10d,"2.LENGTH OF STRING"
  db 10d,"3.REVERSE STRING"
  db 10d,"4.CHECK STRING IS PALLINDROME OR NOT"
  db 10d,"5.EXIT"
  db 10d,"ENTER YOUR CHOICE:"
  menulen equ $-menu
newline db 10
m1 db 10d,"ENTER STRING:"
l1 equ $-m1

m2 db 10d,"LENGTH OF STRING:"
l2 equ $-m2

m3 db 10d,"REVERSE STRING:"
l3 equ $-m3

m4 db 10d,"STRING IS PALLINDROME!!"
l4 equ $-m4

m5 db 10d,"STRING IS NOT PALLINDROME!!"
l5 equ $-m5

m6 db 10d,"!!!!!STRING OPERATION COMPLETED!!!!!"
l6 equ $-m6

section .bss       ;uninitialized variable declaration
   string resb 50
   string2 resb 50
   len resb 16
   choice resb 16
   answer resb 16
section .text
global _start   ;entry point for program
_start:         ;program code goes here
  scall 1,1,menu,menulen ;display the menu
  scall 0,0,choice,2      ;read the choice from the user
  cmp byte[choice], '1'   
    je enter_string   ;if choice =1 jump to enter_string procedure
  cmp byte[choice], '2'
    je display_length
  cmp byte[choice], '3'
    je string_reverse
  cmp byte[choice], '4'
    je pallindrome
  cmp byte[choice], '5'
    je exitcall

enter_string:
   scall 1,1,m1,l1        ;enter the string
   scall 0,0,string,50        ;read the string
   dec rax                  
   mov [len],rax
   scall 1,1,newline,1
   jmp _start

display_length:
   scall 1,1,m2,l2
   mov rax,[len]
   call display
   jmp _start

string_reverse:
   call stringreverse
   scall 1,1,m3,l3
   scall 1,1,string2,[len]
   scall 1,1,newline,1
   jmp _start
pallindrome:
   call stringreverse
   mov rsi,string
   mov rdi,string2
   mov rcx,[len]
   call loop3
   jmp _start

exitcall:
  scall 1,1,m6,l6
  scall 1,1,newline,1
  mov rax,60
  mov rdi,0
  syscall

display:
  mov rsi,answer+15
  mov rcx,16
loop1:
  mov rdx,0
  mov rbx,16
  div rbx
  cmp dl,09h
  jbe skip1
  add dl,07h
skip1:
  add dl,30H
  mov [rsi],dl
  dec rsi
  dec rcx
  jnz loop1
scall 1,1,answer,16
scall 1,1,newline,1
ret

stringreverse:
  mov rsi,string
  mov rdi,string2
  mov rcx,[len]
  add rdi,rcx
  dec rdi

loop2:
  mov al,[rsi]
  mov [rdi],al
  dec rdi
  inc rsi
  loop loop2
ret
loop3:
  mov al,[rsi]
  mov bl,[rdi]
  inc rsi
  inc rdi
  cmp al,bl
  jne skip2
  loop loop3
scall 1,1,m4,l4
scall 1,1,newline,1
jmp _start

skip2:
scall 1,1,m5,l5
scall 1,1,newline,1
ret