C51: Adding C Functions to Existing Assembler Code

Information in this article applies to:

• C251 Version 1 and Later
• C51 Version 5 and Later

QUESTION

I'm using the Keil C51 Tools and have successfully written 8051 programs only in assembly language. Now I need add C code to my assembly program. I want to keep as much as possible of my existing assembly code. Are there some guidelines on how to mix assembly and C code using the Keil C51 compiler? How can I access memory locations from my C programs that I have reserved in assembly code?

ANSWER

The Keil C51 Compiler generates relocatable object files that can interface easily to relocatable assembler programs. The file Keil_v5\C51\ASM\TEMPLATE.A51 shows you how to structure relocatable assembly modules. This file defines several variables, for example:

PUBLIC  data_variable
PUBLIC  xdata_array

data_seg_name   SEGMENT DATA            ; segment for DATA RAM.
                RSEG    data_seg_name   ; switch to this data segment
data_variable:  DS      1               ; reserve 1 Bytes for data_variable

xdata_seg_name  SEGMENT XDATA           ; segment for XDATA RAM
                RSEG    xdata_seg_name  ; switch to this xdata segment
xdata_array:    DS      500             ; reserve 500 Bytes for xdata_array

You access these variables in your C code with:

extern unsigned char  data_variable;
extern unsigned xdata xdata_array[500];

void test (void)  {
  xdata_array[data_variable] = 1;  // write 1 to memory addressed by @xdata_array+data_variable;
}

To reduce data RAM requirements of your application, the BL51/Lx51 Linker/Locater performs call tree analysis of the complete application program. Therefore, the linker needs to be able to distinguish between constants and program code and it must also be able to determine the local data segments for your program code. For this reason you should change the segment naming conventions of your assembler program to the segment names used by the C51 Compiler. Detailed information can be found in Application Note 149: Data Overlaying and Code Banking with A51 Assembler Modules.

For each interrupt function written in assembly, you must create a separate segment for the interrupt vector using CSEG AT.

CSEG AT 03H                    ; EXT0 interrupt vector
             SETB   mybit      ; interrupt function code
             RETI

CSEG AT 0BH                    ; Timer 0 interrupt vector
             LJMP   timer0isr

?BI?timer0?isr_module  SEGMENT BIT OVERLAYABLE ; bit segment
RSEG ?BI?timer0?isr_module
isrbit:            DBIT   1

?PR?timer0?isr_module  SEGMENT CODE  ; program code
RSEG ?PR?timer0?isr_module

timer0isr:                     ; put your program code here
             SETB   isrbit     ; interrupt function code
             RETI

MORE INFORMATION

• Refer to Interfacing C to Assembler in the Cx51 User's Guide.

SEE ALSO

• C51: Mixing C and Assembly
• A51: Calling C Functions From Assembly

Last Reviewed: Thursday, February 25, 2021