Re: [avr-gcc-list] inline assembler problem

[Ewout Boks]

Theodore A. Roth wrote:
> On Tue, 20 Jan 2004, Ewout Boks wrote:
>
>
> > That is the inline asm problem I asked Joerg about yesterday. It is an
> > unecessary warning since your code looks perfectly allright.
> >
> > I believe that it has been solved in post 3.3.1 versions of avr-gcc, so
> > update the compiler is your solution.
>
>
> The warning are correct and the posted code will not work as he expects.
> The reason it won't work is discussed in this FAQ entry:
>
>   http://jubal.westnet.com/AVR/doc/avr-libc-user-manual/FAQ.html#faq_port_pass
>
>
> > > Since some of you wanted to view the whole testprogram in order to help
> > > me track the error I've struggeling with, I post it here. The compiler
> > > I'm using is still the avr-gcc-3.3.1.
> > > The functions in the code below I did implement in order to have some
> > > simple functions to learn from.
> > >
> > >
> > > #include <inttypes.h>
> > >
> > > #define PORT_A 0x1B
> > > #define DDR_A 0x1A
> > > #define PIN_A 0x19
>
>
> Change these to:
>
> #define PORT_A PORTA
> #define DDR_A DDRA
> #define PIN_A PINA
>
>
> > > uint8_t read(const uint8_t adr) {
>
>
> In order to pass the io reg, it must be done via a pointer, change this
> to:
>
> uint8_t read (volatile uint8_t *adr) {
>
>
> > >  uint8_t p;
> > >  asm volatile ("in %0, %1" : "=r" (p) : "I" (adr));
> > >  return p;
> > > }
>
>
> This is impossible. Read the FAQ entry linked above.

I understand that, in order to access the value at the port address, you 
need to pass the address of the port. The above code is clearly wrong in 
that respect : one should use the 'ld' mnemonic with the port address 
loaded into X.

However, I think the following is done clearly wrong by the compiler: 
(this example comes from the avr-libc FAQ):

7.4.1 GCC asm Statement
Let's start with a simple example of reading a value from port D:

asm("in %0, %1" : "=r" (value) : "I" (PORTD) : );


When I attempt to do this, I first get a complaint from avr-gcc saying 
that :

avrHardware.c: In function `avrInportByte':
avrHardware.c:27: error: parse error before ')' token
gmake: *** [avrHardware.o] Error 1

For some reason, the assembler has trouble with the 3rd colon in the 
statement (no clobber list possible ?). When I remove the 3rd colon, I 
get this:

avrHardware.c: In function `avrInportByte':
avrHardware.c:27: warning: asm operand 1 probably doesn't match constraints
avrHardware.c:27: error: impossible constraint in `asm'
gmake: *** [avrHardware.o] Error 1


Now this I clearly call a compiler deficiency/error. The compiler should 
have no trouble inserting something like:

in r24,0x1b


Ewout



> > > uint8_t fastSBI(const uint8_t port, const uint8_t bit) {
> > >  asm volatile("sbi %0, %1" : : "I" (port), "I" (bit));
> > > }
> > >
> > > int main(void) {
> > >  uint8_t port_a = PORT_A;
> > >  uint8_t ddr_a = DDR_A;
> > >  uint8_t pin = 3;
> > >  uint8_t data;
> > >
> > >  data = read(port_a);
> > >  fastSBI(ddr_a, pin);
> > >
> > >  return 0;
> > > }
> > >
> > > When trying to compile this I get the following errors:
> > > inlasm.c: In function `read':
> > > inlasm.c:11: warning: asm operand 1 probably doesn't match constraints
> > > inlasm.c:11: error: impossible constraint in `asm'
> > > inlasm.c: In function `fastSBI':
> > > inlasm.c:17: warning: asm operand 0 probably doesn't match constraints
> > > inlasm.c:17: warning: asm operand 1 probably doesn't match constraints
>
>
> I don't think that gcc is lying to you about the "impossible
> constraint".
>
> Ted Roth