| How can I use a DoorsOS libraries in C?

| Assembly and C | Next

Q: | Is there any method to use a DoorsOS library with TIGCC (such as ZipLib) and how to call a routine that uses something other than the stack in the routine for arguments passing?
A: | Good and common question. When an assembly routine uses register passing, the solution is not so obvious. I will give a concrete example. Look at ZipLib, function compress. This is a cite from DoorsOS documentation:
    ; compress ()
    ; Function: compress data
    ; Input: A0   = Pointer to uncompressed data
    ;        A1   = Pointer to where the compressed data
    ;               should be stored
    ;        D0.W = Length of datas which will be compressed
    ; ziplib::compress equ ziplib@0004
So, how to interface this with TIGCC? There is a lot of solutions. One solution is to use an interface function which accepts parameters via stack then to use embeded assembler to call library function:
    void compress (void *src, void *dest, unsigned short len)
    {
      asm ("move.l (%a6,8),%a0
            move.l (%a6,12),%a1
            move.w (%a6,16),%d0
            jsr ziplib__0004");
    }
This works, but maybe it is awkward to know where the parameters are stored on the stack. GNU C has some extensions for interfacing with assembler, so the following solution is more elegant:
    void compress (void *src, void *dest, unsigned short len)
    {
      asm ("move.l %0,%%a0" :: "g"(src));
      asm ("move.l %0,%%a1" :: "g"(dest));
      asm ("move.w %0,%%d0" :: "g"(len));
      asm ("jsr ziplib__0004");
    }
If you don't understand this (which is probably the case if you are not familiar with GNU C extensions), accept this as as-is template. Both solutions have two bad points: first, usage of stack is awkward if you want very fast calling. Second, this interface function will always be inserted in the code (even if not called in the program), so it is not suitable for making universal header files. As GNU C allows interfacing arbitrary C expressions with assembler, and allows building smart safe macros using so-called "statement expressions", the following solution avoids both the stack and "embedding" problem:
    #define compress(src, dest, len) \
      ({ asm ("move.l %0,%%a0" :: "g"(src)); \
         asm ("move.l %0,%%a1" :: "g"(dest)); \
         asm ("move.w %0,%%d0" :: "g"(len)); \
         asm ("jsr ziplib__0004"); })
In any case, you can call function (or macro) "compress" using, for example,
    compress (LCD_MEM, buffer, LCD_SIZE);
etc. The third solution has one drawback: the impossibility of compile-time checking of parameters type. This can be solved using the following construction (don't be afraid by introducing extra variables; the compiler will remove them during the optimization, and it will produce the same code as in previous example, but with type-checking):
    #define compress(src, dest, len) \
      ({ void *__src = (src), *__dest = (dest); \
         unsigned long __len = (len); \
         asm ("move.l %0,%%a0" :: "g"(__src)); \
         asm ("move.l %0,%%a1" :: "g"(__dest)); \
         asm ("move.w %0,%%d0" :: "g"(__len)); \
         asm ("jsr ziplib__0004"); })
There is also fifth, nearly "ideal" solution, using GNU cast constructors which constructs a function during the compilation (look how functions in stdio.h are implemented, etc.). But, I will not present this here, because you will not understand anything if you are not familiar with cast constructors. Note: TIGCC v0.94 and later support explicit register specification.