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Re: [avr-libc-dev] rjmp across FLASHEND

From: Bernhard Kuemel
Subject: Re: [avr-libc-dev] rjmp across FLASHEND
Date: Fri, 07 Oct 2011 12:08:29 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20110818 Icedove/3.0.11

On 10/07/2011 11:42 AM, Joerg Wunsch wrote:
>> Should the odd case happen, that the application starts beyond the
>> range of rjmp I have to use jmp in the loader. This makes the loader
>> bigger by one word and the reset rjmp changes by one word. So I have 2
>> different rjmps which I have to consider as sync word. I want to
>> minimize the number of sync words to make syncing most reliable.
> OK, I see.
> The following should be possible to work around the situation:
> #include <avr/io.h>
>         .global main
> main:
>         rjmp    beyond_vectors
>         nop
> /* vectors follow */
>         jmp foo
>         jmp foo
> foo:    rjmp foo
> beyond_vectors:
>         jmp tinyloader
>         .org FLASHEND+1-0x3e
> tinyloader:
>         rjmp    tinyloader

That's not really an option, because the application code is usually
located at beyond_vectors.

> Btw., .org is a "DONT" in a relocation assembler.  Already during the
> days of the venerable Z80, this was avoided.  The correct way would be
> to establish a separate section, and then assign the section start
> address to the linker either within the linker script, or from the
> linker commandline.

Hmm, this is my first asm project and I had never used .org or sections
before. I did it with a section, too, but I want to the bootloader as
userfriendly as possible. Is there a good way to get the bootloader
section start address to the linker script or linker command line
automatically? One way I see would be to extract the size of the
bootloader section (it varies by one word, depending on whether jmp or
rjmp is used in it) form the *.lst file, e.g. with perl, calculate the
section start address so that it ends at flashend and call the linker
with the appropriate command line. I don't like this, though. So I used

.if FLASHEND+1>8*1024
        .warning "jmp"
        .org FLASHEND+1-0x3e
        .warning "rjmp"
        .org FLASHEND+1-0x3c

Easy, simple, automatic. The section variant would have the advantage,
though, that it doesn't program all the unused space skipped by .org
into the uC. Not a big deal, though, because it happens only once when
installing the bootloader.

What's wrong with my usage of .org, btw?


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