Various Problems in C51

question:

     How do I write a C program to echo characters from an 8051 serial interface?

answer:

     The functions _getkey and putchar use the on-chip serial port to perform serial I/O. These subroutines are included in the C51 library, and the source files for each function are located in the \C51\LIB directory. See your C51 user manual for more information about these subroutines.

    The following example program sets up the on-chip serial port, sets the baud rate, and waits for a character (using the _getkey function). When a character is received, it outputs it using the putchar function.

question:

     I want to call a function written in C from my assembly program. How can I do this?

answer:

     The easiest way is to let the C compiler generate the correct assembly code for you.

     Suppose you have a C function called "foo" that takes a single parameter of type unsigned char and returns an unsigned char. In a new C file, write a dummy function, also called "foo". Here is an example:

When the file is compiled, #pragma SRC instructs the C compiler to generate assembly code. The extension of the assembly file is "src".

     If you look at the src file, you will find out how to call the function "foo" from assembly. The file shows the registers and memory addresses used to pass function variables, and the registers and memory addresses used to return values. In addition, it also tells you the correct function naming conventions, which is necessary for interfacing assembly with C.

     Then, you can use the src file as a template to write your own assembly call code. Note that you must include the EXTRN directive in the function, that is:

   EXTRN CODE (_foo)      

question:

     In the C51 compiler manual, there is an example of an assembly function calling a C function. So is there an example of a C program calling an assembly subroutine?

answer:

     It's not in the manual, but it's easy to make one yourself. An ASM subroutine must know how parameters are passed, what the return value is, and the naming conventions for segments. Follow the steps below and you can make one yourself.

1. Write a simple function in C that passes parameters and returns values ​​the same way you would write it in assembly.

2. Use the SRC directive (#PRAGMA SRC at the top of the file) to have the C compiler generate a .SRC file instead of an .OBJ file.

3. Compile the C file. Because the SRC directive is used, a .SRC file is generated. The .SRC file contains the assembly code generated from the C code you wrote.

4. Rename the .SRC file to .A51 file.

5. Edit the .A51 file and insert the assembly code you want to execute in the assembly function body.

For example, the following code

The SRC file is generated when compiling.

question:

     Do you have any examples of mixed C and assembly programming?

answer:

     The following example shows how to mix C and assembly in an 8051 program.

     This example starts with a MAIN C function, which calls an assembly subroutine and then calls a C function.

     The MAIN C module is as follows:

Function a_func is an assembly subroutine:

     Note that the assembler calls the C function c_func:

     The actual code for the assembly module is generated using the SRC pragma directive and the following C source file:

You can download C2ASM2C.ZIP from the Keil website.