Keil error C129

This is what the comments say before the defines

// The Ez-USB FX2 registers are defined here. We use FX2regs.h for register
// address allocation by using "#define ALLOCATE_EXTERN".
// When using "#define ALLOCATE_EXTERN", you get (for instance):
// xdata volatile BYTE OUT7BUF[64]   _at_   0x7B40;
// Such lines are created from FX2.h by using the preprocessor.
// Incidently, these lines will not generate any space in the resulting hex
// file; they just bind the symbols to the addresses for compilation.
// You just need to put "#define ALLOCATE_EXTERN" in your main program file;
// i.e. fw.c or a stand-alone C source file.
// Without "#define ALLOCATE_EXTERN", you just get the external reference:
// extern xdata volatile BYTE OUT7BUF[64]   ;//   0x7B40;
// This uses the concatenation operator "##" to insert a comment "//"
// to cut off the end of the line, "_at_   0x7B40;", which is not wanted.

This is what the comments say before the defines

// The Ez-USB FX2 registers are defined here. We use FX2regs.h for register
// address allocation by using "#define ALLOCATE_EXTERN".
// When using "#define ALLOCATE_EXTERN", you get (for instance):
// xdata volatile BYTE OUT7BUF[64]   _at_   0x7B40;
// Such lines are created from FX2.h by using the preprocessor.
// Incidently, these lines will not generate any space in the resulting hex
// file; they just bind the symbols to the addresses for compilation.
// You just need to put "#define ALLOCATE_EXTERN" in your main program file;
// i.e. fw.c or a stand-alone C source file.
// Without "#define ALLOCATE_EXTERN", you just get the external reference:
// extern xdata volatile BYTE OUT7BUF[64]   ;//   0x7B40;
// This uses the concatenation operator "##" to insert a comment "//"
// to cut off the end of the line, "_at_   0x7B40;", which is not wanted.

Where does it say that after preprocessor expansion of the _AT_ macro, the syntax will still be valid?

I have no idea. I am just getting started in C.
What exactly is wrong here? The #define _AT_ ?

Try using the "PREPRINT" compiler directive and examine the preprocessor output.

http://www.keil.com/support/man/docs/c51/c51_cm_ifile.htm

well, I need more help.

I tried using preprint in debug.
FX2REGS.H PREPRINT

Is that correct?
I get "error34: undefined identifier"

I have no idea. I am just getting started in C.
You need to read a bit more before you start banging the keys. There are many good books about C, my favorite is Kochan: programming in ANSI C

If, as you say, you are just getting started in C, get your feet wet writing code for the PC and then, when you have the basics down pat, have a go at the micros

Erik

Let me rephrase: I am getting started in embedded C.
I am in no way an expert on C. But I have coded in C on PC, not Microcontrollers. All my uC was in assembly.

"I tried using preprint in debug.
FX2REGS.H PREPRINT"

Compile the C source file(s) that include FX2REGS.H.

I get the same error "*** error 34: undefined identifier"

Do you actually understand what the 'C' preprocessor is, and what it does?

If not, it's time to go back to the 'C' textbook; this is standard stuff - nothing peculiar to C51.