Re: [avr-gcc-list] Why are we using EICALL and EIJMP for AVR256?

[Andy H]

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 
>
>
>
> _______________________________________________
> AVR-GCC-list mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/avr-gcc-list