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Re: Assembly macro with parameters

Subject: Re: Assembly macro with parameters
From: Fabrizio Fazzino <>
Date: Sun, 03 Jul 2005 12:31:00 +0200
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Many thanks Maciej and David,
during these days I've learned a lot about stringification, a feature
that I had never used before.

In any case I didn't have to use this feature... I was just missing the
fact that the opcode evaluation didn't have to happen by declaring an
int variable (in this case the value is computed at runtime) but by the
preprocessor, so I solved my problem this way:

#define NEWOPCODE(base,rd,rs,rt) (base|(rs<<21)|(rt<<16)|(rd<<11))
#define myopcode(rd,rs,rt) asm(".long %0" : : "i" 

and then I call it simply as myopcode(10,8,9).

By the way, is there any quick way of writing a setreg(reg_num,reg_val)
C macro to set the value of a register?
And another one to read the value like a reg2var(reg_num,&result) to put
the value of a register inside my own C variable?
I have written my own versions for both but they have a 32-case switch
statement inside so they are not so efficient...

In any case thanks a lot for your suggestions, I've put acknowledgments
to inside my code!


David Daney wrote:

The arguments to the asm() statement are strings not char*.  They are evaluated 
at compile time not run time.

You will probably have to use the C preprocessor stringification and 
concatination operators ('#' and '##').

David Daney

Maciej W. Rozycki wrote:

 This is untested, but it should be a reasonable starting point:

#define myopcode(rs,rt,rd) do { \
        int opcode_number = 0xC4000000 | (rs<<21) | (rt<<16) | (rd<<11); \
        asm(".word %0" : : "i" (opcode_number)); \
} while (0)

But you may want to add code to tell GCC that these registers are used and how, because otherwise you may have little use of your macro. You'll probably have to investigate the explicit register variable GCC feature and cpp stringification. It should be straightforward though rather boring, so I'm leaving it as an exercise.


   Fabrizio Fazzino -
     Fazzino.IT -

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