Atari 400 Technical Reference Manual page 42

To operate in split-screen mode, the Screen Editor must ~irst be
opened and then the
Displa~
Handler must be opened using a
separate IOCB (with the split-screen option bit set in AUX1).
CIa Function Descriptions
The
device-speci~ic
characteristics
o~
the standard CIa
~unctions
(described earlier in this section) are detailed below:
OPEN
The device name is
5,
and the handler ignores any device number anl
~ilename speci~icationl i~
included.
The handler supports the ~ollowing options:
AUXl
7
o
+-+-+-+-+-+-+-+-+
:C:S:W:R:
+-+-+-+-+-+-+-+-+
Where: C
=
1 indicates to inhibit screen clear on OPEN.
S
=
1 indicates to set up a split-screen
con~iguration (~or
modes 1 through 8 only).
Rand Ware the direction bits (read and write).
AUX2
7
o
+-+-+-+-+-+-+-+-+
I
mode
+-+-+-+-+-+-+-+-+
Where: mode is the screen mode (0 through 11).
Note:
I~
the screen mode selected is 0, then the AUXl C and
S options are assumed to be O.
You share memory utilization with the
Displa~
Handler
in~ormation.
Sharing is necessary because the Display Handler
dynamically allocates high address memory for use in generating
the screen display, and because di~Terent amounts oT memory are
needed for the different screen modes. Prior to initiating an
OPEN command the variable APPMHI (OOOE] should contain the
highest address oT RAM you need. The Screen handler
will open the screen only
i~
no RAM is needed at or below that
address.
Upon return from a screen OPEN, the variable MEMTOP [02E5J will
contain the address of the last free byte at the end of RAM
memory prior to the screen-required memory.
OPERATING SYSTEM C016555 -- Section 5
57