🚀 go-pugleaf

got a question that's been bugging me for a couple days...  I wish to write a
custom-character (from assembly-language program) to a *specific* screen
location (column 80 of line 25, since that line doesn't scroll normally...).
the char is defined thusly --

char	db	{a byte}
	db	{a byte}

etcetera, for 9 bytes (coresponding to the 9 scanlines in a character)

the char is then displayed via this code fragment --

(assume -- ax holds segment address for the desired color-plane and di holds
the offset of the character mask -- 'char' as described above)

	mov	es,ax
	mov	sx,9		; 9 scanlines
	mov	si,79		; row 1, col 80
g_v_l:	mov	ax,es:[si]	; read a row from chosen screen location
	mov	bx,cs:[di]	; read a row from character mask
	xor	ax,bx		; invert 'em
	mov	es:[si],ax	; write 'em to the screen
	add	si,128		; next row of screen
	inc	di		; next row of char mask
	loop	g_v_l

okay, my problem is -- how do I go about determining the 'start point' to get
row 25, col 80 of the screen?  I interpret all the information that exists to
tell me that I should use 9*24*128+79 to get me to the 80th column in row 25.
*but* it doesn't work... what am I missing?  incidentally -- the arithmetic
above is based upon _9_ scanlines per char, _24_ character rows, _128_ bytes
per character rows and _79_ character-positions to 'skip' in the first scanline
of character row 25 (see the example code, the '79' in mov si,79 *does* put me
at row 1, col 80)...  I feel I *must* be missing something quite basic...

Walt K.