Topic       : TOS - The Operating System
Author      : 
Version     : tos.hyp (December 19, 2008)
Subject     : Programmieren/Atari
Nodes       : 3010
Index Size  : 93790
HCP-Version : 5
Compiled on : Atari
@charset    : atarist
@lang       : 
@default    : Title
@help       : 
@options    : +g -i -s +x +zz -t4
@width      : 70
View Ref-File12.1.11.11  atariModePossible                                      TOS

This points to a byte indicating whether a switch from the Mac to the 
Atari MMU-mode is possible at that instant (0=forbidden, 1=allowed). 
So long as the Macintosh is running without virtual memory (VM) (also 
without RAM-doubler), the 'modeAtari' call is always possible; however 
if VM is active and also the 'UnlockMemOnMacSide' flag is non-zero, 
then on switching from the MagiC to the Mac screen the memory occupied 
by MM (MagiC Mac) for the TPA will be released for offloading.

However, as soon as the VM management offloads a block, the MMU tables 
calculated by MM are no longer valid and must be calculated anew prior 
to the next switch to the Atari-mode. This is not done automatically, 
however, but only when switching back to the Atari screen. For this 
reason interrupt and other asynchronous routines that are installed 
under MagiC Mac by MacOS functions may not perform Atari-mode switches 
under the described circumstances.

Atari programs that hook into the Atari vectors (e.g. the VBL vector 
at $70) need not take all of this into account, since in cases when 
the MMU tables become invalid the Atari interrupts will not even be 
simulated any more. In cases where one should take further measures 
before or after the status of the flags alters one can install also a 
routine via macAppSwitch that then indicates this event with the 
values 3 and 4 in d0.W).

See also: Program launch and TPA   The MagiC Mac cookie