exxos's Atari, Amiga & retro forum

What's the best way to get the screen base address of the physical screen ?

1) XBIOS function Physbase:
http://toshyp.atari.org/en/Screen_funct ... l#Physbase

2) TOS variable _v_bas_ad:
$00044E|long |Pointer to video RAM (logical screen base) |_v_bas_ad

3) SHIFTER registers:
$FF8201|byte |Video screen memory position (High byte) |R/W
$FF8203|byte |Video screen memory position (Mid byte) |R/W
$FF820D|byte |Video screen memory position (Low byte) |R/W (STe)

_v_bas_ad is the *logical* address, the same that you get by calling Logbase(). It's the one that is used by all VDI drawing operations, as well as by Line-A.

I would prefer using Physbase(), unless you are really short on cycles. Using the hardware addresses may not work with graphics cards (well, directly writing to that memory may not work, either)

Assuming by what you've shown in the other thread: if you do a GEM application, there is probably no reason to deal with the screen address at all.

What do you need it for?

cannot remember why I needed it, but I know I just used logbase to get it.