Thank you both for your answers.

I have taken a look at Keil's app notes, but
these documents refer to newer versions of
the compiler than the version I use.

I have tried the evaluation version 6.23 and
it replaces C constants inside inline assembly code. However, its usefulness is not
fully achieved. Since the replacement is done
by the C preprocessor, hexadecimal definitions like "0xNN" (the most usual) are not understood by the assembler, which expects hex values defined as "NNh". Consequently, it is not possible to share hex constants between C code and assembly code.

I don't know the SDCC free C compiler. I would be very pleased if you showed me where to download it.

Thank you again.