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