OK Erik,
I think I can see the mechanism now.
The crt[tn13].o seems to be complied from avr-libc-1.4.6/crt1/gcrt1.S
which has:
==========================================================================
#ifndef __AVR_ASM_ONLY__
.weak __stack
/* By default, malloc() uses the current value of the stack pointer
minus __malloc_margin as the highest available address.
In some applications with external SRAM, the stack can be below
the data section (in the internal SRAM - faster), and __heap_end
should be set to the highest address available for malloc(). */
.weak __heap_end
.set __heap_end, 0
.section .init2,"ax",@progbits
clr __zero_reg__
out _SFR_IO_ADDR(SREG), __zero_reg__
ldi r28,lo8(__stack)
#ifdef SPH
ldi r29,hi8(__stack)
out _SFR_IO_ADDR(SPH), r29
#endif
out _SFR_IO_ADDR(SPL), r28
==========================================================================
This means that the __stack value is determined at the time of compiling
avr-libc. It is marked as weak, so I can override it's value in my
source file (tested). It's a shame that not all the .init[x] sections
aren't marked as weak so they could be individually overwritten (.init0 is).
Either I live with the (admitted small - but on these parts every byte
counts when you're up against the end stops) penalty of some redundant
code or I modify my avr-libc and recompile as a non-standard toolchain.
I checked the latest release and the code here is still the same.
Anyway, many thanks for your comments Erik, I wish you good health,
Robert
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