🚀 go-pugleaf

>>  >        mov     ax, 320                 <<<-- change this to mov bx, y
>>  >        mul     y       ; AX = y * 320  <<<-- and this to add bx, bx
>>       then set ds, si for the table lookup by:  lds si, table

>>        then load ax with screen addr by          mov ax, ds:[si+bx]

>The way I usually do these things is:

>    mov    ah, y
>    sub    al, al
>    mov    bx, ax
>    shr    bx, 1
>    shr    bx, 1
>    add    ax, bx

>I remember comparing that with the table method, and this was faster,
>although I may be wrong. It probably also depends on what machine you're
>using.

Actually what you have written above won't even work.  The AX needs
to be shifted four times right, before being added to the BX.  Also
what if y is greater than 256?  

the following code should work, if you want to use shifts and y<256:

	mov	ah, y
	sub	al, al
	mov	bx, ax
	shr	bx, 1
	shr	bx, 1
	mov	ax, bx
	shr	ax, 1
	shr	ax, 1
	add	bx, ax	

however the table lookup is much more flexible and faster, the only
possible argument against the table look up is that it requires 400 bytes