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Re: [avr-gcc-list] inline ASM newbie problem

From: Joerg Wunsch
Subject: Re: [avr-gcc-list] inline ASM newbie problem
Date: Tue, 11 Oct 2005 19:40:12 +0200 (MET DST)

"Marco Randazzo" <address@hidden> wrote:

> I would like to pass a simple input value to an asm routine but compiler =
> says "error: impossible constraint in `asm'" and I can't understand what =
> I've to do. Here is my code:

>   uint8_t servo_prot_offset;
>     [...]
>   asm volatile (
>    "\n"
>    "PUSH    R18" "\n\t"
>    "PUSH    R24" "\n\t"
>    "PUSH    R25" "\n\t"
>    "        LDI     R24,0x00" "\n\t"
>    "        LDI     R25,0x00" "\n\t"
>    "l1   :  ADIW    R24,0x01" "\n\t"
>    "        LDI     R18,0x06" "\n\t"=20
>    "        CPI     R24,%0  " "\n\t"  <--- HERE I WANT COMPARE R24 WITH =
> THE VALUE OF servo_prot_offset
>    "        CPC     R25,R18 " "\n\t"
>    "        BRCS    l1"       "\n\t"
>    "POP     R25" "\n\t"
>    "POP     R24" "\n\t"
>    "POP     R18" "\n\t"
>   : //output operands
>   : "M" (servo_prot_offset) //input operands
>   );

See the inline assembler documentation (part of avr-libc docs).
Constraint "M" would expect an 8-bit integer constant, but you're
trying to pass an 8-bit variable, yet your CPI instruction would
expect a constant literal again.  Something doesn't fit right

Perhaps you try first expressing your desire in C.  Did you verify the
generated code from C?

Also, don't push/pop your own stuff.  Instead, use the "clobbers"
section of the inline asm statement to tell the compiler which
registers you are destroying, or even better, invent a couple of
scratch variables in the surrounding C code, and pass them down to
your inline asm statement (e.g. using "d" constraints).  Also, you are
always allowed to use __tmp_reg__ as a scratch register.  That way,
you'll give the compiler freedom to arrange the register usage
appropriately, so most likely, no additional push/pop operations will
be required at all.

cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)

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