Comment #2 on issue 112 by robhamerling: Memory allocation for
compiler 'internal' variables
http://code.google.com/p/jallib/issues/detail?id=112
{continuation of previous comment}
Joep and I had a private discussion about the problem and I think below is a
reasonable summary of the analysis. The problem can be solved for all
PICs, but only
partly for the 16F73 and 16F74.
This is how the device files will specify _pic_accum and _pic_isr_w:
1. For all PICs _pic_accum and _pic_isr_w will be declared 'shared' and
allocated in
(the highest location of) shared memory, when available.
2. For PICs with only 1 memory bank there is no distiction between shared
and
unshared memory: no problem.
3. For PICs with multiple banks and both shared and unshared memory
_pic_accum and
_pic_isr_w will be allocated in shared memory. No further special measures
are needed.
4. For PICs with only shared memory _pic_accum and _pic_isr_w will be
declared in
shared memory, the rest of shared memory in (bank 0) will be declared as
unshared
memory. Note this is a compiler requirement: the current compiler allocated
user data
only in unshared memory.
5. For PICs with multiple banks and no shared memory _pic_accum and
_pic_isr_w will
be allocated in the highest location of bank 0 and memory in the other
banks with the
same relative offset (low order 7 address bits) will be reserved (made
unavailable
for user programs).
This is only partly a solution: it will probably work for _pic_isr_w, but
probably
not for _pic_accum for calculations with multi-byte variables which are
allocated in
different banks. But this involves only the 16F73 and 16F74, and because of
this
restriction a warning message will be issued when using the device files of
these PICs.
Note: Currently the device files have most of these items already
implemented, and
these are also documented in 'devicefiles.html', but repeated here to give
a complete
insight in the problem (for possible future reference).
Only some minor changes in some device files are needed.
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