home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
EnigmA Amiga Run 1996 February
/
EnigmA AMIGA RUN 04 (1996)(G.R. Edizioni)(IT)[!][issue 1996-02][Skylink CD III].iso
/
earcd
/
util1
/
vavail.lha
/
vAvail
/
VMProgGuideline
< prev
Wrap
Text File
|
1995-05-23
|
4KB
|
108 lines
$Date: 95/05/23 10:22:30 $
If you are writing programs, which use a lot of memory, such as ray
tracers, compilers,... the following guidelines will enable you to
take advantage of virtual memory in a safe way:
*********************************************************************
Don't access virtual memory inside forbidden/disabled sections. The
following example shows you, why this is necessary:
/* This example should read the names of all currently waiting tasks */
UBYTE *buffer;
UBYTE *buffer_ptr;
struct Task *tmpTask;
/* The following request is allocated as VM */
if ((buffer = AllocMem (SOME_SIZE, MEMF_ANY)) == NULL)
return (FALSE);
buffer_ptr = buffer;
Forbid ();
for (tmpTask = (struct Task*)SysBase->TaskWait.lh_Head;
tmpTask->tc_Node.ln_Succ != NULL;
tmpTask = tmpTask->tc_Node.ln_Succ)
{
strcpy (buffer_ptr, tmpTask->tc_Node.ln_Name); /* can cause page-fault */
buffer_ptr += strlen (tmpTask->tc_Node.ln_Name) + 1);
}
Permit ();
The preceding example might produce a page-fault while copying a task
name to the internal buffer. A page-fault results in task switching
and subsequently the ln_Next pointer is invalid, when the page-fault
has completed. A crash may result.
*********************************************************************
Don't allocate messages, IORequests, task structures etc. on the
stack, if you are not absolutely sure that your stack is in public
memory.
E.g., if you allocate an IORequest on your stack, which might be in
virtual memory, and you send this IORequest to the timer.device, the
machine might crash, because the timer.device accesses its requests
from an interrupt routine. Page-faults while in supervisor mode are
deadly.
*********************************************************************
Don't access virtual memory from supervisor mode. This includes
interrupt and trap-handling routines. Because the task producing the
page-fault is put into the wait state, a page-fault may only be
invoked by a task using its standard user stack, not the
(system-global) supervisor stack.
*********************************************************************
Don't allocate or free non-public memory from inside a
forbidden/disabled section. Requests inside forbidden/disabled
sections are simply not granted virtual memory, freeing virtual memory
is postponed until that section is over. It does not crash the
machine, it simply isn't recommended.
*********************************************************************
Memory allocated with the MEMF_PUBLIC flag cleared should not be
accessed by tasks not belonging to your program. I.e. don't read
from a file using a buffer of virtual memory. You never know in which
way another task or program accesses your buffer. It might access it
from within an interrupt.
*********************************************************************
Localize your data. This means, if you are writing a program, which
consists of several passes, try to put the data needed for each pass
together in address space. This greatly reduces the page-fault rate.
*********************************************************************
If you need to change your stack pointer, use a MEMF_PUBLIC area. The
code below could be used (StackSwap() is in ROMS >= 2.0).
if (newstack = AllocMem(__stack, MEMF_PUBLIC))
{
stackStruct.stk_Lower = newstack;
stackStruct.stk_Upper = newstack + __stack;
stackStruct.stk_Pointer = stackStruct.stk_Upper;
StackSwap(&stackStruct);
}
...
(In cleanup routine:)
...
if (newstack)
{
StackSwap(&stackStruct);
FreeMem(newstack, __stack);
}
(code by Jim Cooper)
*********************************************************************
Apart from this, always follow Commodore's programming guideline.