Necessary to save used registers in assembly ?

Here are a few things to consider:

1. There is no difference between a C function and an assembly function.

2. A called assembler function may use all registers A, B, R0-R7, and DPTR without saving and restoring them.

3. It is ALWAYS the caller's responsibility to reload register contents after a function call.

4. If an assembler function (called from C) takes arguments and/or returns values, it must use the same conventions as the C compiler. Refer to the manual.

5. The regfile is used to perform global register optimizations. Basically, a bipmap of the registers used in each function is built up and used by the C compiler to optimize saving and restoring registers.

Hope this helps.

Jon

Here are a few things to consider:

1. There is no difference between a C function and an assembly function.

2. A called assembler function may use all registers A, B, R0-R7, and DPTR without saving and restoring them.

3. It is ALWAYS the caller's responsibility to reload register contents after a function call.

4. If an assembler function (called from C) takes arguments and/or returns values, it must use the same conventions as the C compiler. Refer to the manual.

5. The regfile is used to perform global register optimizations. Basically, a bipmap of the registers used in each function is built up and used by the C compiler to optimize saving and restoring registers.

Hope this helps.

Jon

Thanks for the reply Jon.

I can therefore assume that the compiler
will restore registers after a call,
even if it is optimizing performance by
using registers. I do not need to
save used registers in an assembly
program ( an ISR is an exception ) since
the caller will assume its registers may have been overwritten.

Thanks again for your reply.
It eliminated my doubt.

Jim

>1. There is no difference between a C function and an assembly function.

>5. The regfile is used to perform
>global register optimizations.
>Basically, a bipmap of the registers
>used in each function is built up and
>used by the C compiler to optimize >saving and restoring registers.

I think you forget about $REGUSE directive.

Sentence (1.) is not true.
Main difference between C and Assembler functions is that C compiler don't know anything about using registers in assembler function.

So, to force compiler apply 'global optimisation' under assembler functions user MUST explicitly declare used registers with $REGUSE directive.

For example:
--- file SWAPBYTE.C ---

#pragma  SRC(SWAPBYTE.a51)


#pragma asm
   $REGUSE _SwapByte(A, R7 )
#pragma endasm

uchar SwapByte(unsigned char Byte)
{
  ACC = Byte;
#pragma asm
  swap a
#pragma endasm
  return( ACC );
}

#pragma asm
   $REGUSE _SwapByte(A, R7 )
#pragma endasm