;This program waits for user input. if the user inputs "drawline" a line will be drawn. otherwise the program will print an error.
.model small
.stack 100h
.data 

p = 9                    ;maximum string support: 49 chars
startp = 32000           ;x=0,y=100
line = 256               ;the length of a line is 256 pixels

st_msg db 'Enter your string: ',0dh,0ah,'> $'
in_str db p,0, 8 dup (0dh), '$'
err_msg db 'unknown command$'
dlstr db 'drawline'
strlen db ?

.code

init:  mov ax,@data
       mov ds,ax
       mov es,ax
       
       mov ax,3
       int 10h
       
       lea dx,st_msg      ;prints out st_msg
       mov ah,9           ;
       int 21h            ;
       
       lea dx,in_str      ;reads in in_str
       mov ah,0ah         ;
       int 21h            ;

       call chrfix
       
       mov cx,8           ;check the full length of the word 'drawline' = 8 chars long
       lea si,in_str      ;insert in_str to si
       lea di,dlstr       ;insert dlstr to di
       rep cmpsb          ;run a comparison between the two for 8 chars, if not equal, jump to err
       jne err
       call drawline
       jmp done
       
err:   call newl
       lea dx,err_msg     ;prints out err_msg
       mov ah,9           ;
       int 21h            ;
      
done: 
mov ah,8                  ;will cause the program to hang until the user presses any key
  int 21h
mov ah,4ch                ;return control to OS
  int 21h
;*******************
drawline:                 ;method that draws a line starting at startp
       push cx            ;register backup
       push ax            ;
       push di            ;
       
       mov ax,0013h       ;set vga mode
       int 10h            ;
       
       mov ax,0a000h      ;
       mov ds,ax          ;
       mov di,startp      ;starting location
       mov al,1           ;starting color
       mov cx,line        ;length of line
line:  mov [di],al        ;draw a single pixel
       inc al             ;increase color
       inc di             ;move 1 pixel
       loop line
       
       pop di             ;register restore
       pop ax             ;
       pop cx             ;
   ret 
;*******************
chrfix:push cx            ;method that fixes extra chars that stick to the beginning of a string
       push si            ;register backup
       push ax            ;
       
       mov al,in_str[1]   ;find the length of the string
       mov strlen,al
       mov cl,strlen
       mov ch,0      
       mov si,2           
fix:   mov al,in_str+si   ;shift all characters 2 spaces left and fill in the remaining 2 with '0dh'
       mov in_str+si-2,al 
       inc si             
       loop fix
       sub si,2           
       mov in_str+si,0dh
       mov in_str+si+1,0dh
       
       pop ax             ;register restore
       pop si             ;
       pop cx             ;
   ret
;*******************   
newl:  mov ah,2           ;method that moves the cursor to the next line
       mov dl,10          ;drop 1 line
       int 21h            ;
       mov dl,13          ;go to start of line
       int 21h            ;
   ret    
end