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-File12.1.11.19  stackLoad                                              TOS

stackLoad activates a new CPU stack (a7).

This is required when one wants to call a routine in the Atari context 
from a Mac context during which an arbitrary CPU stack is active, i.e. 
before a mode-change with 'modeAtari' has been performed. A switch to 
another stack is necessary then, because the memory area that a7 is 
currently using may be blanked out through the switch to Atari-mode, 
which could lead to the CPU freezing (see also the relevant parts of 
'Exceptions under MagiC Mac').

Instead of simply placing the a7 onto the new stack, one should use 
'stackLoad'. Basically this function does the same thing, but it also 
informs the MacOS about this (by setting some global variables). As a 
result the MacOS does not check a7 periodically in the meantime. The 
reason is because otherwise, with certain MacOS versions (prior to 7.5 
and again as of 7.5.3), the MacOS would issue a stack overflow message 
even though this has not occurred.

If virtual memory is active, this call also prevents this stack area 
being offloaded, which could result in the computer freezing in some 
circumstances.

Usage: In a0 one should pass a pointer to a MemArea structure (see 
above). This must contain a pointer to the start (lowest address) of 
the stack as well as its length (respect 'minStackSize'!). The routine 
will destroy the registers a0 and a1 and sets a7 to the value of 
<MemArea.start+MemArea.length>.