17.8 How to convert _AX pseudo-registers?

Q: Since DJGPP doesn't recognize Borland-style pseudo-register variables like _AX, how should I port code which uses them to DJGPP?

A: These pseudo-variables are typically used in two different contexts:

• When calling real-mode interrupt services.

  To port such code to DJGPP, use the fields of the __dpmi_regs structure (declared on the dpmi.h header file) to set the register values, and library function __dpmi_int to invoke the interrupt service. For example, consider the following code snippet:

    #include <dos.h>
    void _highcolor (void)
    {
      _AH = 0x10;
      _AL = 0x03;
      _BL = 0;
      geninterrupt (0x10);
    }
  

  Here's one way to express this in DJGPP³⁰:

    #include <dpmi.h>
    void _highcolor (void)
    {
      __dpmi_regs r;
  
      r.h.ah = 0x10;
      r.h.al = 0x03;
      r.h.bl = 0;
      __dpmi_int (0x10, &r);
    }
  

  Please read how to call real-mode interrupt functions, elsewhere in this document, for further details on how call real-mode services from DJGPP programs.

• Immediately before or after an inline assembly code.

  GCC features extensive inline assembly facilities which allow you to assign values to, or read values from registers from the inline assembly code. Since you will have to rewrite your inline assembly code anyway, make it refer directly to your C variables, instead of referencing pseudo-register variables from C. See description of inline assembly, for further info.