C51: IN-LINE ASM GIVES COMPILER WARNINGS

---

Information in this article applies to:

• C51 All Versions

---

QUESTION

I have a problem with following in-line assembly code that I created from an old, existing assembly function:

unsigned char PutChar_I2C(unsigned char ch) {
#pragma asm
      SETB    MDE
      MOV     A,R7     ; get ch in ACC
      .
      .
      .
      MOV     R7,#1    ; return value 1
      CLR     MCO
#pragma endasm
}

This code generated these warnings:

warning C280: 'ch': unreferenced local variable
warning C173: missing return-expression

The compiler generates this warning despite the fact I used the variable 'ch' (passed in R7). Also, why is there a warning for a missing return-expression since I set the return value in R7?

ANSWER

There are two solutions to your problem. Since the complete function is just assembly, you should leave it in a separate assembler module and just add the proper segment naming conventions. Then you may call the function from C as shown below:

Assembler Module:

?PR?_PutChar_I2C?FILE   SEGMENT CODE
RSEG ?PR?_PutChar_I2C?FILE
PUBLIC _PutChar_I2C
_PutChar_I2C:
      SETB    MDE
      MOV     A,R7     ; get ch in ACC
      .
      .
      .
      MOV     R7,#1    ; return value 1
      CLR     MCO
      RET

Function Call from C:

extern unsigned char PutChar_I2C(unsigned char ch);

void main (void)  {
  val = PutChar_I2C (1);
}

Another solution is to add dummy statements to the C source file as shown below:

unsigned char PutChar_I2C(unsigned char ch) {

// dummy assignment to avoid warning
// 'unreferenced local variable'
  ch = ch;
#pragma asm
      SETB    MDE
      MOV     A,R7     ; get ch in ACC
      .
      .
      .
      MOV     R7,#1    ; return 1
      CLR     MCO
#pragma endasm

// dummy return to avoid warning
// 'missing return-expression'
  return (ch);
}

The C51 compiler does not generate warnings. However, carefully review the generated code for these statements, and confirm that the variable 'ch' is correctly assigned to the parameter passing register R7 for the return value.

MORE INFORMATION

• Refer to Interfacing C to Assembler in the Cx51 User's Guide.
• Refer to Application Note 149 Data Overlaying and Code Banking with A51 Assembler Modules

SEE ALSO

• C51: CALLING ASSEMBLY ROUTINES FROM C
• C51: MIXING C AND ASSEMBLY
• C51: GETTING INLINE ASSEMBLY TO WORK

FORUM THREADS

The following Discussion Forum threads may provide information related to this topic.

• Is there a way to suspend warning C280 ?

Last Reviewed: Friday, July 15, 2005

---

Did this article provide the answer you needed?
Yes No Not Sure