oster@dewey.soe.berkeley.edu (David Phillip Oster) (12/31/88)
I've been bitten by the system heap versus BufPtr question. Some years
ago, Apple recommended, in a tech note, that if you were going to keep
something large around permananently, that you do it by manipulating
BufPtr.
They wanted you to do this solely because the system heap was limited
in size. The BufPtr technique has the disadvantages that you can't
tell what is already in high memory, and you can never give any of the
memory back to the system. Think about it. This kind of memory allocation
is exactly what the MemoryManager and the System Heap are for.
Just after I'd spent the time to get my move-above-bufPtr-but-leave-stubs-
in-the-system-heap code working, Apple changed things.
They changed the system to grow the system heap as required, and
provided a way for INITs to request the system heap grow before the
INIT is run. (This is all documented in the StartUp Manager section of
I.M. vol5, and works on any Mac with the 128k or 256k ROMs, running
system software Version 5.0 or later.)
In addition, I've been getting complaints on some of my commercial
products that they will not run if certain hardware made by RasterOPs
or Radius is installed in the system. I'll be checking today whether
BufPtr is indeed the problem, but I'd expect some video boards to be
sensitive to the memory map.
So, the absolutely correct way to do it, is do a SysEnvirons() to see
if you are running under a version of the O.S. smart enough to grow the
heap. If you are, just allocate in the system heap. If you are running
on an ancient machine, be prepared to move yourself above BufPtr. I
hope you like testing, because when you have conditional code like this
in a product, you'll need to do a lot of it.
A good compromise to to forget about ancient machines, since their
owners don't buy much software anyway.
--- David Phillip Oster --"When we replace the mouse with a pen,
Arpa: oster@dewey.soe.berkeley.edu --3 button mouse fans will need saxophone
Uucp: {uwvax,decvax}!ucbvax!oster%dewey.soe.berkeley.edu --lessons." - Gasee