|Location||ATARI_REXT (v2.25+), SMSQ/E|
This function returns the base address of the screen linked to the specified channel (default #0), this is normally 131072 on standard QLs but can alter on other resolutions or if dual screen mode is supported. On machines which support higher resolutions, the screen base will only be at the standard address of 131072 if you configure the machine to start up in 512x256 and even here there is no guarantee - see the documentation for the particular QL resolution you are using.
If the specified channel is not open then Invalid Channel ID will be reported. However, if no channel is specified and #0 is not open, then a special window will be opened for #0 on screen, which may destroy what is already on screen.