[Fwd: Re: [avr-gcc-list] in-line assembler]

[Robert von Knobloch]

Hi Wouter,

thanks for responding.
This is not quite what I want, but your comment about the sts is
interesting.
I want to achieve this:

uint8_t        rambyte;
uint16_t ram_address;

void get_ram_byte(void)
{
    asm ("push R28\n\t"
    "push R29\n\t"

    "lds r29, ram_address + 1\n\t"
    "lds r28, ram_address\n\t"
    "ld __tmp_reg__, Y\n\t"
    "sts rambyte, __tmp_reg__\n\t"

    "pop R29\n\t"
    "pop R28"::);
}

Which works but completely ignores the inline-assembler stuff which
would do automatic register saving etc.
I want to pass a uint16_t address as the argument and get a uint8-t byte
returned (which is the contents of the address in ram.
Here is the sts that I want to achieve - namely storing the __tmp_reg__
contents into the variable rambyte. Of course this should become the
return value (must I store this in a variable first or can the compiler
know to return this in whatever the usual register is for a uint8_t () ?.

Btw your example compiled OK but did not assemble.

Robert

Hi,

I don't know what you intended to do but I guess this is more like it 
(read the value and write it back):
I personally prefer the %[] construction.

uint8_t get_ram_byte(uint16_t ram_address)
{
     uint8_t    byte;

     asm  ("ld  %[reg] , %[adr]"  "\n\t"
           "sts %[adr] , %[reg]"  "\n\t"
     : [reg] "=&r" (byte)
     : [adr] "e"   (ram_address));
     return byte;
}

You're construction of "sts %0, __tmp_reg__" is not correct. GCC is 
trying to feed sts r24 as first argument, which is invalid.
You are feeding him the uninitialized variable 'byte'. Which is also 
allocate to R24.

HTH,

Wouter


Robert von Knobloch schreef:
> Hello,
> I've been trying to decipher the intricacies of in-line assembler (using
> the Inline Assembler Cookbook as my guide).
> 
> I have a very simple application that I cannot seem to realise.
> 
> I want a C function that will return the contents of the RAM address
> that I give it as argument.
> 
> My assembler-based function looks like this:
> 
> file is hex.c
> =====
> uint8_t get_ram_byte(uint16_t ram_address)
> {
>     uint8_t    byte;
> 
>     asm  ("ld __tmp_reg__, %a1"    "\n\t"
>     "sts %0, __tmp_reg__"    "\n\t"
>     : "=&r" (byte) : "e" (ram_address));
>     return byte;
> }
> 
> and is called from
> 
>             rambyte = get_ram_byte(i );
>             u_hex8out(rambyte);         // Print byte as 8-bit hex.
> 
> Trying to compile this results in "~/Monitor/hex.c:5: undefined
> reference to `r24' "
> If I comment out the line "sts %0, __tmp_reg__"    "\n\t" then it
> compiles and I see that the parameter is passed in R24,25, copied to
> R30,31[Z] and the value is read into R0 [__tmp_reg__].
> I cannot see what is wrong with the sts command or why R24 is mentioned.
> 
> Can anybody help me ?
> 
> Many thanks,
> 
> Robert von Knobloch.
> 
> 
> 
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