2.asm

.model small
.stack 100h

.data
inv db 'Invalid input$'
new_line db 0ah,0dh,"$"
space db 20h,"$"

.code
main proc
   ; connect code segment and data segment
   mov ax, @data
   mov ds, ax

   ; input label
   mov ah, 1
   int 21h
   mov bl, al

   ; new line
   mov ah, 9
   lea dx, new_line
   int 21h

   ; check if it is a number
   cmp bl, '9'
   jg Invalid
   cmp bl, '0'
   jl Invalid

   sub bl, 48

   mov ch, 1     ; ch for checking bound of Loop2
   Loop1:
   cmp bl, 0     ; bl for checking bound of loop1
   je End_Label
   mov cl, 1

   Loop2:
   mov ah,2
   mov dl, cl
   add dl, 48
   int 21h       ; print number

   mov ah, 9
   lea dx, space ; print space
   int 21h

   inc cl
   cmp cl, ch    ; if cl > ch then break
   jg End_loop2:
   jmp Loop2

   End_loop2:
   mov ah, 9
   lea dx, new_line  ; print a new line
   int 21h
   inc ch
   dec bl            ; decrement bl
   jmp Loop1

   ; check if it is a valid input
   Invalid:
   mov ah,9
   lea dx, inv
   int 21h

   End_Label:

   main endp
end main