You misunderstand.
I said Wiki is misleading. Not that your solution for bootloader is
wrong. For example:
"So in fact *eicall* instruction with current avr-gcc cannot call a
function in upper half flash."
Whereas avr-gcc can indeed call functions anywhere.
It is not possible to handle EIND inside gcc (before gas and linker).
Pointer are only 16 bits -so there is nowhere to put a seed
associate with each pointer.
If it were possible within gcc I assume seeding would be slightly faster
since JMP is 3 cycles and STS is 2. As you
discovered, EIND would need to be saved by ISR or interrupts disabled
while jump occurs - thus overall unlikely to win.
I do not quite understand why bootloader does not use its own trampoline
in lower memory. As this would leave EIND=0 and permit
simplification to IJMP/ICALL - thus leaving EICALL/EIND free for
assembler or more exotic usage.
Kindest regards
Andy
Dusan Ferbas wrote:
Hi Andy,
believe me, we tested, debugged and it is running now in a lot of
devices for several months.
Did anyone tested if seeding the EIND before every eicall is slower
and with bigger code size ?
(in comparison to trampolines)
Even if I admit using trampolines in a bootloader code (we are
counting every byte),
you have a problem, because 1st, only the bootloader is flashed
and also trampolines differ for every application. The bootloader is
same.
BL is linked with LDFLAGS =
-Wl,-Map=$(TARGET).map,--cref,--defsym,__stack=$(StackAddr) \
-nostartfiles -nodefaultlibs -Ttext=0x01F800
Dusan
At 01:30 18.12.2009, Andrew Hutchinson wrote:
...
For 256 devices avr-gcc uses linker trampolines to allow 16 bit
function pointers to reach any function located anywhere in address
map.
The EIND is set to zero by gcc. (Trampolines are located in lower
memory)
When a function pointer is assigned the address of function (in
upper memory), a full sized jump to the function is added to
trampoline table.
The function pointer gets the 16 bit address of this entry.
When the jump occurs - using EICALL, it will jump to trampoline using
16 bit address and EIND set to zero. The trampoline JMP will then
take it to the function.
If the function is in lower memory, the trampoline is not created by
linker (though I have not checked that!)
Andy
Dusan Ferbas wrote:
Hi, your case is, that a bootloader (BL) uses trampolines.
I described a case in this list (June 2008 and Mar 2009), where an
application calls a BL, where eicall resides.
There is a wiki page about it:
http://www.ethernut.de/nutwiki/Nut/ATmega256x
Dusan
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