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

From: Andy H
Subject: Re: [avr-gcc-list] Why are we using EICALL and EIJMP for AVR256?
Date: Thu, 17 Dec 2009 20:45:39 -0500
User-agent: Thunderbird (Windows/20090812)

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

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


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!)


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


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