in assembly as in pascal,c and basic you have the option of using
procedures, to declare a procedure you do :
Proc name "proc" distance (far/near)
endp ; end proc
and to call the proc you type "Call Procname"
the ret must come because you have to return to the exact point
you called the proc from, if you pushed registers and didn't pop them
it might not return to the calling position.
db 10 dup (?)
clearscreen proc near
mov es,ax ; clear the screen, point to screen
rep stosw ; if don't understand go back to learn
ret ; try to erase this and see what happends
LESSON7 - PROCEDURES AND MACROS