Re: [avr-gcc-list] uint8_t SRAM variable for ATmega328p allocated at 0x060 (instead of 0x0100) in avr-gcc 4.6.2, 7.3.2

[Senthil Kumar Selvaraj]

On Wed, Apr 17, 2013 at 08:52:57PM -0400, Kirill Shklovsky wrote:
> Dear avr-gcc-ers:
> 
> It appears that a global SRAM uint8_t variable is placed at address
> 0x060 by avr-gcc versions 4.6.2 and 7.3.2. The avr-gcc compiler was
> compiled on cygwin. For example, given the followign AVR cpp code:
> 
> volatile uint8_t latchingFlag;
> 
> int main() {
> 
>     LED_PORT &= ~(1<<LED_BIT);
>     DDRD = 0xFF;
>     for (;;) {
>         latchingFlag=1;
>         if (latchingFlag==0) {
>             LED_PORT ^= 1<<LED_BIT;    // Toggle the LED
>             _delay_ms(100);
>             latchingFlag = 1;
>         }
>     }
> }
> 
> The disassembly for the conditional looks as follows:
> 
>         if (latchingFlag==0) {
>   a0:    80 91 60 00     lds    r24, 0x0060
>   a4:    81 11           cpse    r24, r1
>   a6:    fa cf           rjmp    .-12         ; 0x9c <main+0xc>
> 
> At the same time, compiling this on avr-gcc 4.7.0 on Ubuntu I get:
> 
> lds r24, 0x0100
> 
> Presumably 0x0100 is correct, since on ATmega 0x0060 is actually
> mapped to the WDTCSR register, if I understand it correctly.  Is this
> a problem with the way I built avr-gcc or is this not an avr-gcc
> issue?
> 

I don't see the problem when I tried compiling the below code with 4.6.2. 
Can you paste the command line you used, with -v and -Wl,--verbose turned on? 
It would be helpful to know what version of binutils was used, as well as the 
build steps.

#include <stdint.h>
volatile uint8_t x;

int main()
{
    x = 1;
    return x;                                                                   
                                                                                
                                                                 
}

When I compile with

/proj/builds/3.4.0/avr/install/bin/avr-gcc -mmcu=atmega328p -v test.c

I get

COLLECT_GCC=/proj/builds/3.4.0/avr/install/bin/avr-gcc
COLLECT_LTO_WRAPPER=/data2/builds/3.4.0/avr/install/bin/../libexec/gcc/avr/4.6.2/lto-wrapper
Target: avr
Configured with: /proj/builds/3.4.0/avr/gcc-4.6.2/configure 
--prefix=/proj/builds/3.4.0/avr/install --target=avr --enable-languages=c,c++ 
--disable-nls --disable-libssp --with-dwarf2
Thread model: single
gcc version 4.6.2 (GCC) 
COLLECT_GCC_OPTIONS='-mmcu=atmega328p' '-v'
 /data2/builds/3.4.0/avr/install/bin/../libexec/gcc/avr/4.6.2/cc1 -quiet -v 
-imultilib avr5 -iprefix 
/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/ test.c -quiet 
-dumpbase test.c -mmcu=atmega328p -auxbase test -version -o /tmp/cctr0ZVY.s
GNU C (GCC) version 4.6.2 (avr)
        compiled by GNU C version 4.1.2 20080704 (Red Hat 4.1.2-52), GMP 
version 4.3.2, MPFR version 2.4.2, MPC version 0.8.2
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C (GCC) version 4.6.2 (avr)
        compiled by GNU C version 4.1.2 20080704 (Red Hat 4.1.2-52), GMP 
version 4.3.2, MPFR version 2.4.2, MPC version 0.8.2
COLLECT_GCC_OPTIONS='-mmcu=atmega328p' '-v'
 
/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/bin/as 
-mmcu=atmega328p -o /tmp/ccCbCXlO.o /tmp/cctr0ZVY.s
COMPILER_PATH=/data2/builds/3.4.0/avr/install/bin/../libexec/gcc/avr/4.6.2/:/data2/builds/3.4.0/avr/install/bin/../libexec/gcc/:/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/bin/
LIBRARY_PATH=/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/avr5/:/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/lib/avr5/:/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/:/data2/builds/3.4.0/avr/install/bin/../lib/gcc/:/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/lib/
COLLECT_GCC_OPTIONS='-mmcu=atmega328p' '-v'
 /data2/builds/3.4.0/avr/install/bin/../libexec/gcc/avr/4.6.2/collect2 -m avr5 
-Tdata 0x800100 
/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/lib/avr5/crtm328p.o
 -L/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/avr5 
-L/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/lib/avr5
 -L/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2 
-L/data2/builds/3.4.0/avr/install/bin/../lib/gcc 
-L/data2/builds/3.4.0/avr/install/bin/../lib/gcc/avr/4.6.2/../../../../avr/lib 
/tmp/ccCbCXlO.o -lgcc -lc -lgcc

The -Tdata 0x800100 tells the linker that the data segment starts from 
0x800100. The disassembly looks like this

000000a6 <main>:
  a6:   cf 93           push    r28
  a8:   df 93           push    r29
  aa:   cd b7           in      r28, 0x3d       ; 61
  ac:   de b7           in      r29, 0x3e       ; 62
  ae:   81 e0           ldi     r24, 0x01       ; 1
  b0:   80 93 00 01     sts     0x0100, r24
  b4:   80 91 00 01     lds     r24, 0x0100
  b8:   88 2f           mov     r24, r24
  ba:   90 e0           ldi     r25, 0x00       ; 0
  bc:   df 91           pop     r29
  be:   cf 91           pop     r28
  c0:   08 95           ret



> Any help would be appreciated.
> 
> Kirill
> 
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