>> I am trying to understand relocation types in AVR. To start with,
>> can someone illustrate how R_AVR_16 works in the following case?
>
>I can only guess. Don't know whether there's someone with more
>insight.
Yea...I was hoping if someone who ported binutils to AVR is also here. But I
guess I am quite happy to continue the conversation. It would help me think
aloud.
>
>> As I see it, since the address is 16-bit, 'size' is 1. How do we arrive
>> at 'rightshift' and 'bitsize' entries?
>
>No idea what rightshift is doing, you might get an idea by comparing
>against other relocation types.
Actually the confusion started with the comparisions :-(
Hence I started with a simple relocation. Somewhere in the source code, it is
commented that instruction relocation requires a right shift.
>
>bitsize should be clear: it's a 16-bit relocation. My guess is that
Yeah. 16 bits need to be relocated in this case. Usually all lds/sts require
this. [Similarly for R_AVR_7_PCREL and R_AVR_13_PCREL, 7 and 13 bits of the
'16-bit' instruction need to be relocated. These are issued by branch and rjmp
instructions.]
However, this is a little abstract to me. How exactly we are going to construct
the final address? I'll start with R_AVR_16 type in the given example.
Assembler stores 0x0 in all the lds/sts instructions (disassembly given in the
end) and emits the corresponding relocation type. This forms the input to
linker.
How linker is going to construct the final address of variable 'i'? I know some
theory that we are going shift, add, .... such things. I believe 'HOWTO' comes
into picture then, giving the linker a formula to construct the final address.
It would be good to know how exactly this happens.
>> And how assembler knows to emit
>> R_AVR_16 type?
>
>Probably from the instruction it is connected with. Obviously,
>R_AVR_16 only applies to instructions like STS/LDS etc.
Some generous person in gcc chat pointed out to me fix_new_exp(...) in
avr_operand() of gas/tc-avr.c. I guess I have too many questions.
#program:
long i;
int main()
{
return i++;
}
#Disassembly of section .text:
00000000 <main>:
0: df 93 push r29
2: cf 93 push r28
4: cd b7 in r28, 0x3d ; 61
6: de b7 in r29, 0x3e ; 62
8: 80 91 00 00 lds r24, 0x0000
c: 90 91 00 00 lds r25, 0x0000
10: a0 91 00 00 lds r26, 0x0000
14: b0 91 00 00 lds r27, 0x0000
18: 9c 01 movw r18, r24
1a: 01 96 adiw r24, 0x01 ; 1
1c: a1 1d adc r26, r1
1e: b1 1d adc r27, r1
20: 80 93 00 00 sts 0x0000, r24
24: 90 93 00 00 sts 0x0000, r25
28: a0 93 00 00 sts 0x0000, r26
2c: b0 93 00 00 sts 0x0000, r27
30: c9 01 movw r24, r18
32: cf 91 pop r28
34: df 91 pop r29
36: 08 95 ret
Anitha
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