org     100h
        jmp Start
function_print:
        mov     ah, $09
        int     21h
        ret

function_println:
        call    function_print
        mov     dx, LF
        call    function_print
        ret

function_print_LF:
        mov     ah, $09                 ;printing LF
        mov     dx, LF
        int     21h
        ret

function_read:
        mov     ah, $0A                 ;reading string
        int     21h
        ret

function_read_cbuf:
        mov     ax, $0C0A
        int     21h
        ret

function_await_input:
        mov     ax, $0C08
        int     21h
        test    al, al
        jnz     @F
        mov     ah, $08
        int     21h
@@:
        ret

Start:
        mov     dx, promptMessage       ;displaying prompt
        call    function_println

        mov     dx, string
        call    function_read_cbuf

        call    function_print_LF

CheckLength:
        CMP     [string + 1], 4
        jge     CheckEquals
        mov     dx, lengthMessage
        call    function_println

        call    function_await_input
        ret

CheckEquals:
        mov     al, [string + 5]
        CMP     [string + 2], al
        je      CheckAdditional1
        mov     dx, equalityMessage
        call    function_println

        call    function_await_input
        ret

CheckAdditional1:
        CMP     [string + 4], '0'
        jge     @F
        mov     dx, nanMessage
        call    function_println

        call    function_await_input
        ret
@@:
        CMP     [string + 4], '9'
        jle     CheckAdditional2
        mov     dx, nanMessage
        call    function_println

        call    function_await_input
        ret

CheckAdditional2:
        mov     eax, 0
        mov     al, [string + 1]
        mov     bl, [string + eax]
        CMP     bl, '0'
        jge     @F
        mov     dx, nanMessage
        call    function_println

        call    function_await_input
        ret
@@:
        mov     eax, 0
        mov     al, [string + 1]
        mov     bl, [string + eax]
        CMP     bl, '9'
        jle     Finish
        mov     dx, nanMessage
        call    function_println

        call    function_await_input
        ret
Finish:
        mov     dx, correctMessage
        call    function_println

        call    function_await_input
        ret

promptMessage   db 'Please enter string$'
lengthMessage   db 'Your string''s length does not belong to range [4, 8]$'
equalityMessage db 'Symbols 1 and 4 must be equal$'
nanMessage      db 'Additional requirements were not met$'
correctMessage  db 'String is correct$'
string          db 9, 0, 9 dup('$')
LF              db 10, '$'