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## 21. Interfacing with assembly routines.

## 21.1 Global registers used for parameter passing.

By default the compiler uses the global registers "DPL,DPH,B,ACC" to pass the first parameter to a routine, the second parameter onwards is either allocated on the stack (for reentrant routines or --stack-auto is used) or in the internal / external ram (depending on the memory model).

### Assembler routine non-reentrant

In the following example the function cfunc calls an assembler routine asm_func, which takes two parameters.

extern int asm_func( unsigned short, unsigned short);

    int c_func (unsigned short i, unsigned short j)
    {
            return
     asm_func(i,j);
    }
    int main()
    {
       return c_func(10,9);
    }

The corresponding assembler function is:-

            .globl _asm_func_PARM_2
            .globl _asm_func
            .area
     OSEG
    _asm_func_PARM_2:       .ds      1
            .area CSEG
    _asm_func:

     mov     a,dpl
            add     a,_asm_func_PARM_2
            mov     dpl,a

     mov     dpl,#0x00
            ret

Note here that the return values are placed in 'dpl' - One byte return value, 'dpl' LSB & 'dph' MSB for two byte values. 'dpl', 'dph' and 'b' for three byte values (generic pointers) and 'dpl','dph','b' & 'acc' for four byte values.

The parameter naming convention is _<function_name>_PARM_<n>, where n is the parameter number starting from 1, and counting from the left. The first parameter is passed in "dpl" for One bye parameter, "dptr" if two bytes, "b,dptr" for three bytes and "acc,b,dptr" for four bytes, the ```varaible name for the second parameter will be _<function_name>_PARM_2.`

Assemble the assembler routine with the following command.

    asx8051 -losg asmfunc.asm

Then compile and link the assembler routine to the C source file with the following command,

    sdcc cfunc.c asmfunc.rel

### Assembler routine is reentrant

In this case the second parameter onwards will be passed on the stack , the parameters are pushed from right to left i.e. after the call the left most parameter will be on the top of the stack. Here is an example.

extern int asm_func( unsigned short, unsigned short);

     int c_func (unsigned short i, unsigned short j) reentrant
    {

     return asm_func(i,j);
    }
    int main()
    {
       return c_func(10,9);

    }

The corresponding assembler routine is.

            .globl _asm_func
    _asm_func:
            push  _bp
            mov  _bp,sp

            mov  r2,dpl
            mov  a,_bp
            clr  c
            add  a,#0xfd

            mov  r0,a
            add  a,#0xfc
            mov  r1,a
            mov
     a,@r0
            add  a,r2
            mov  dpl,a
            mov  dph,#0x00

     mov  sp,_bp
            pop  _bp
            ret

The compiling and linking procedure remains the same, however note the extra entry & exit linkage required for the assembler code, _bp is the stack frame pointer and is used to compute the offset into the stack for parameters and local variables.

## 21.2 With --noregparms option.

When the source is compiled with --noregparms option , space is allocated for each of the parameters passed to a routine.

### Assembler routine non-reentrant.

In the following example the function cfunc calls an assembler routine asm_func, which takes two parameters.

    extern int asm_func( unsigned short, unsigned short);
    int c_func (unsigned short i, unsigned short j)
    {
            return
     asm_func(i,j);
    }
    int main()
    {
       return c_func(10,9);
    }

The corresponding assembler function is:-

            .globl _asm_func_PARM_1
            .globl _asm_func_PARM_2

     .globl _asm_func
            .area OSEG
    _asm_func_PARM_1:       .ds     1
    _asm_func_PARM_2:
     .ds      1
            .area CSEG
    _asm_func:
            mov     a,_asm_func_PARM_1

            add     a,_asm_func_PARM_2
            mov     dpl,a
            mov
     dpl,#0x00
            ret

Note here that the return values are placed in 'dpl' - One byte return value, 'dpl' LSB & 'dph' MSB for two byte values. 'dpl', 'dph' and 'b' for three byte values (generic pointers) and 'dpl','dph','b' & 'acc' for four byte values.

The parameter naming convention is _<function_name>_PARM_<n>, where n is the parameter number starting from 1, and counting from the left. i.e. the ```left-most parameter name will be _<function_name>_PARM_1.`

Assemble the assembler routine with the following command.

    asx8051 -losg asmfunc.asm

Then compile and link the assembler routine to the C source file with the following command,

    sdcc cfunc.c asmfunc.rel

### Assembler routine is reentrant.

In this case the parameters will be passed on the stack , the parameters are pushed from right to left i.e. after the call the left most parameter will be on the top of the stack. Here is an example.

extern int asm_func( unsigned short, unsigned short);

     int c_func (unsigned short i, unsigned short j) reentrant
    {

     return asm_func(i,j);
    }
    int main()
    {
       return c_func(10,9);

    }

The corresponding assembler routine is.

            .globl _asm_func
    _asm_func:
            push  _bp
            mov  _bp,sp

            mov  a,_bp
            clr  c
            add  a,#0xfd
            mov
     r0,a
            mov  a,_bp
            clr  c
            add  a,#0xfc

     mov  r1,a
            mov  a,@r0
            add  a,@r1
            mov  dpl,a

     mov  dph,#0x00
            mov  sp,_bp
            pop  _bp
            ret

The compiling and linking procedure remains the same, however note the extra entry & exit linkage required for the assembler code, _bp is the stack frame pointer and is used to compute the offset into the stack for parameters and local variables.

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