this looks more like a pc problem than a micro since we don't have standardized scrolling output displays. but if we were to pretend:

assumptions:

(1) the output addressing is assumed to be like a teletype... character prints left to right in sequence and line feeds advance to the next line.

(2) the input is assumbed to be handed to the routine through a register. hardware and some driver a responsible to placing it there before the routine is called. its assumed to be a binary number between 1 and 40. 0 and 40+ are assumbed to be filtered by the input routine and are not tested herein.

variable and constants:

(chr) the fixed printed character '*'
(col) a column counter for each row
(row) a line counter.
(cnt) a binary count

routines:

(getinput) load cnt with a legal number
(putchar) writes the acc character

algorithm:

;assume everything is running
;call (getinput) for square size
;initialize (row) for outside loop
loopline:
;initialize (col) for inside loop
loopchar:
;load character to send
;call putchar;djnz (col) loopchar
;load linefeed to send
;call putchar;djnz (row) loopline;end

;---------------
call getinput ;load r2
mov r0,r2 ;load (row)
loopline:
mov r1,r2 ;load or reload (col)
loopchar:
mov a,#'*' ;character to print
call putchar
djnz r1,loopchar
mov a,#0Ah ;linefeed maybe
call putchar
djnz r0,loopline
;end

duh
;---------------