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

[Dusan Ferbas]

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