TASM duplicate byte on buffer

Question

I can't find flaw in my algorithm. Basically I want that get_byte would deal with a thing when whole buffer is analysed, new buffer would be loaded. Like, for example, if I have set my buffer length to 3 and I call get_byte 5 times, I get output "12245" with my file content being "123456789 ....". If I increase my buffer size to more than 5 (9, for example), output is 12345. Please help me find out why this happens. Thank you. Code is below

proc get_byte
  mov ah, read_buffer_length   ; ah = read_buffer_length
  cmp read_position, ah        ; comparing current position and ah
  jl @@less                    ; if read_position < read_buffer_length, then we jump to @@less part
  @@read_input_buffer:         
    call read_input_buffer

    ; if we have read 0 bytes, we are done
    cmp read_buffer_length, 0
    je @@done

    ; if not, we renew buffer information
    mov read_position, 1       ; setting position to 1
    mov si, offset read_buffer ; showing to read position
    inc current_position       ; for other purposes
    ret

  @@less:                  ; we dont have to reread buffer, it wokrs okay
    mov al, byte ptr[si]   ; putting al byte from buffer
    inc si                 ; going to next byte
    inc read_position      ; to know when to renew buffer
    inc current_position   ; for other purposes
  ret

  ; no more to do, stopping loop
  @@done:
    mov stop_prog_loop, 1
    ret
endp

proc read_input_buffer
  ; saving registry values
  push ax
  push bx
  push cx
  push dx

  mov bx, input_handler      ; descriptor number
  mov cx, read_buffer_size   ; how many bytes to read
  mov dx, offset read_buffer ; address of buffer

  mov ah, 3Fh                ; calling dos
  int 21h                    ; calling dos

  mov read_buffer_length, al ; how many symbols current buffer has

  ; giving registers values back
  pop dx
  pop cx
  pop bx
  pop ax

  ret
endp

EDIT: my debugging code. I won't comment it since, if you understand english and basic assembler, it should be clear what happens. Opening/closing files works perfectly.

call open_input_file
call open_output_file
call read_input_buffer

mov read_position, 1       
mov si, offset read_buffer 

call get_byte
int 29h
call get_byte
int 29h
call get_byte
int 29h
call get_byte
int 29h
call get_byte
int 29h

call close_output_file   
call close_input_file   

Answer 1

I don't see where you call get_byte.

Anyway - since you initialize read_position to one (not zero), your

jl @@less

should probably be

jle @@less

after comparing read_position to read_buffer_length.

Answer 2

When you call GET_BYTE and you are at the end of the buffer and there is no conditional jump to @@LESS, the flow continues to @@READ_INPUT_BUFFER and the called subroutine, where you PUSH registers on the stack and then POP them. However, observe that the AL value from the previous CALL to GET_BYTE when you INCreased SI remains unchanged, so you are displaying the same character again. You could fix it by adding: MOV AL, BYTE PTR [SI] just before CALL READ_INPUT_BUFFER in GET_BYTE.