Re: [avr-gcc-list] Accessing C sram locations in inline asm

[Theodore A. Roth]

Ah, my mistake. The LDS insn is a 32-bit one that needs an indirect
address as an operand. I think the documentation might be wrong about the
label.

I tried this test program:

  #include <inttypes.h>
  #include <avr/io.h>

  uint8_t foo = 10;
  uint8_t bar = 10;

  int main (void)
  {
      asm volatile ("lds __tmp_reg__, foo" :: );
      asm volatile ("lds __tmp_reg__, bar" :: );

      return (0);
  }

And got the following output:

>From my map file (generated with -Wl,-Map,messy.map):

  .data           0x00800060        0x2 load address 0x000000ba
                  0x00800060                PROVIDE (__data_start, .)
   *(.data)
   .data          0x00800060        0x1 messy.o
                  0x00800060                foo
                  0x00800061                bar

>From my list file (generated with avr-objdump -S -D):

  000000a4 <main>:

  uint8_t foo = 10;
  uint8_t bar = 10;

  int main (void)
  {
    a4:   cf ef           ldi r28, 0xFF   ; 255
    a6:   df e0           ldi r29, 0x0F   ; 15
    a8:   de bf           out 0x3e, r29   ; 62
    aa:   cd bf           out 0x3d, r28   ; 61
      asm volatile ("lds __tmp_reg__, foo" :: );
    ac:   00 90 60 00     lds r0, 0x0060
      asm volatile ("lds __tmp_reg__, bar" :: );
    b0:   00 90 61 00     lds r0, 0x0061

      return (0);
  }
    b4:   80 e0           ldi r24, 0x00   ; 0
    b6:   90 e0           ldi r25, 0x00   ; 0
    b8:   0c 94 5c 00     jmp 0xbc

This seems to work fine for me. foo is a global variable at 0x0060 and the
asm statement does what you want.

I wonder if you are having scoping problems?

Ted Roth

On Fri, 6 Sep 2002, Grant Stockly wrote:

:) At 01:21 PM 9/6/2002 -0700, you wrote:
:)
:)
:) >On Fri, 6 Sep 2002, Grant Stockly wrote:
:) >
:) >:) I tried.   asm volatile("lds r24, %1" :: "label" (sram_byte) ); failed no
:) >:) matter what I tried.  (Subtracting the memory start offset, etc.
:) >
:) >How about this:
:) >
:) >   asm volatile("lds r24, %0" :: "r" (sram_byte) );
:) >
:) >sram_byte should end up in a register. Perform a disassemble with
:) >`avr-objdump -D -S' and examine the generated asm. It's usually very
:) >educational as to what the compiler is doing.
:)
:) I'd like to use the C sram locations hard coded.  I have a ~400 asm
:) instruction cycle interrupt routine that needs to run 10,000 times a second
:) and have access to about 30 bytes of storage space.  The routine has to be
:) optimized to run quickly, and run the exact (-+4 instruction cycles) no
:) matter what the result of about 16 conditional branches.  It would be nice
:) and clean to be able to access SRAM by what the .bss segments say.
:)
:)    asm volatile("lds r24, doesnt_exist" ::  ); caused the linker to report
:) an error, so I assume that it will work.  I'm going to code up a test
:) program and see what happens.
:)
:) I'll keep you posted.
:)
:) Grant
:)




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