falling a word assembly language programming

[org 0x0100]

mov ax, 0xb800
mov es, ax
mov di, 0
mov si,0
nextchar:
mov word [es:di], 0x0720
add di, 2
cmp di, 4000
jne nextchar

cmp si,4000
je end1
add si,81
mov word [es:si], 0x0741
jmp nextchar
end1:
mov ax,0x4c00
int 21h