Stuart Whelan wrote:
Hi Folks,
I am using gcc 3.4.4 and avr-libc 1.4.4 on win32....
I have been using the CodeVision compiler for a long time,
but have decided to port my projects to gcc-avr.
My application jumps to the bootloader on receipt of jump
to bootloader command, but the jmp I am using seems to
be doing strange things. I cut it done down to the
smallest test:
main()
{
asm volatile("jmp 0x1c00"::);
}
And compiled it with:
avr-gcc -mmcu=atmega16 -o foo.obj foo.c
And the jmp command comes out as:
96: 0c 94 00 0e jmp 0x1c00 <__stack+0x17a1>
Now if I read that correctly, it is jmp'ing to 0e00,
not 1c00.. I notice that 0e00 is 1c00/2...
Jumps are to word addresses. You can't jump to an odd address, so
call/jmp (at the processor level) are in terms of words (not bytes).
When I step through this in the debugger, the program
counter does indeed jump to 0e00, not 1c00.
That's correct.
The GNU toolchain (avr-gcc and avr-as, anyway) uses byte addresses.
The AVR uses word addresses for program memory (which is more than 1
byte wide).
Compare the code generated for the following jump:
typedef void (* funcptr) (void);
goto *(funcptr)0x1c00; // a gcc extension; should do what you want:
a4: e0 e0 ldi r30, 0x00 ; 0
a6: fc e1 ldi r31, 0x1C ; 28
a8: 09 94 ijmp
--
Ned Konz
http://bike-nomad.com
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