; hello_mmap.asm
%define O_RDONLY 0 
%define PROT_READ 0x1
%define MAP_PRIVATE 0x2
%define SYS_WRITE 1
%define SYS_OPEN 2
%define SYS_CLOSE 3
%define SYS_FSTAT 5
%define SYS_MMAP 9
%define SYS_MUNMAP 11
%define SYS_EXIT 60
%define FD_STDOUT 1
%define FSTAT_SIZE 144
%define FSTAT_FSIZE_OFFSET 48

global print_file
global print_string

section .data
    fstat_buff: db "hello", 0

section .text

; use exit system call to shut down correctly
exit:
    mov  rax, SYS_EXIT
    xor  rdi, rdi
    syscall

; These functions are used to print a null terminated string
; rdi holds a string pointer
print_string:
    push rdi
    call string_length
    pop  rsi
    mov  rdx, rax 
    mov  rax, SYS_WRITE
    mov  rdi, FD_STDOUT
    syscall
    ret

string_length:
    xor  rax, rax
.loop:
    cmp  byte [rdi+rax], 0
    je   .end 
    inc  rax
    jmp .loop 
.end:
    ret

; This function is used to print a substring with given length
; rdi holds a string pointer
; rsi holds a substring length
print_substring:
    mov  rdx, rsi 
    mov  rsi, rdi
    mov  rax, SYS_WRITE
    mov  rdi, FD_STDOUT
    syscall
    ret

; rdi - file name
print_file:
    push r12
    push r13
    ; Вызовите open и откройте fname в режиме read only.
    mov  rax, SYS_OPEN
    mov  rsi, O_RDONLY    ; Open file read only
    xor rdx, rdx          ; We are not creating a file
    ; so this argument has no meaning
    syscall
    ; rax holds the opened file descriptor now
    mov r12, rax ; save fd to r12

    ; get stat of file
    mov rdi, r12
    mov rsi, fstat_buff
    mov rax, SYS_FSTAT
    syscall

    ; get size of file in from its stats
    mov r13, fstat_buff
    add r13, FSTAT_FSIZE_OFFSET
    mov r13, [r13]
    ; now rsi stores the files size

    ; Вызовите mmap c правильными аргументами
    ; Дайте операционной системе самой выбрать, куда отобразить файл
    ; Размер области возьмите в размер страницы 
    ; Область не должна быть общей для нескольких процессов 
    ; и должна выделяться только для чтения.
    xor rdi, rdi
    mov r8, r12
    xor r9, r9
    mov rsi, r13
    mov rdx, PROT_READ
    mov r10, MAP_PRIVATE
    mov rax, SYS_MMAP
    syscall
    ; now rax stores the mapped file address

    ; с помощью print_substring теперь можно вывести его содержимое
    push rax ; save mapped file address
    mov rdi, rax
    mov rsi, r13
    call print_substring

    ; unmap mapped file
    pop rdi ; retrieve mapped file address from stack
    mov rsi, r13
    mov rax, SYS_MUNMAP
    syscall

    ; close file
    mov rdi, r12
    mov rax, SYS_CLOSE
    syscall

    pop r13
    pop r12
    ret