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Passing float variables to a C-callable assembly routine

Jack Harris1
Prodigy 180 points

Hello all

A quick question about calling an assembly function from C on a C6713 DSK.

I've already looked at sections 7.3 to 7.5 in http://www.ti.com/lit/ug/spru187o/spru187o.pdf.

I am writing a function to multiply two complex numbers together in assembly, but first I wanted to be able to access two floating point numbers and find the product using the MPYSP instruction.

When I step through my program I can't see the values I thought I passed to my assembly (2.1 and 1.3) in the registers A4 and B4 as expected, I get 1.067869798e+09 and 1.074161254e+09 instead. I don't know what I am doing wrong.

See below for my code.

Many thanks,
Jack

				.global		_cmplx_mult_sp	;ASM function callable from C
_cmplx_mult_sp:	
				;LDW		.D1	A4,A3		; Load first floating point number? - not sure if this is required
				;LDW		.D2	B4,A2		; Load second floating point number?
				;NOP 		4
				MPYSP 		A4,B4,B3		; Multiply the two floating point numbers
				NOP 		5
				.end

Simplified C code:

extern float cmplx_mult_sp(float x, float y);

void main(void)

{

float test = 0.0, a = 2.1, b = 1.3;
test = cmplx_mult_sp(a,b);

}

  • 0
    TI__Guru**** 244710 points

    Have you considered writing linear assembly instead?  That's much easier.  This is documented in the chapter titled Using the Assembly Optimizer in the C6000 compiler manual.

    Jack Harris1 said:
    When I step through my program I can't see the values I thought I passed to my assembly

    How are you looking at the values?  Assembly step (not C step) all the way to the MPYSP instruction.  Does that change things?

    One other note ... You need to return the result in A4.

    Thanks and regards,

    -George

  • 0 in reply to George Mock
    Prodigy 180 points

    Hi George,

    Thanks for reply.

    I've written a linear assembly routine which works, thanks for your help.

    Jack

    ps. I wondered if there was a way of returning the real and imaginary parts of the complex number? The code below only returns the real part.

    					.def	_cmplx_mult_sp;
_cmplx_mult_sp:		.cproc 	a_0,b_0
					.reg 	a,b,c,d,aminusb,res_r,res_i
					LDW *a_0++,a			;Load variables from structure
					LDW *a_0++,b			;
					SUBSP a,b,aminusb		;
					NOP 3
					LDW *b_0++,c			;
					LDW *b_0++,d			;
					;NOP 3
					MPYSP aminusb,d,aminusb	;
					NOP 3
					SUBSP c,d,res_r					;
					MPYSP res_r,a,res_r				;
					NOP 3
					ADDSP c,d,res_i	
					MPYSP res_i,b,res_i	
					NOP 3
					ADDSP res_i,aminusb,res_i		;
					ADDSP res_r,aminusb,res_r		;
					
										
					.return res_r
					.endproc

  • 0 in reply to Jack Harris1
    TI__Guru**** 244710 points

    You don't need any NOP instructions in linear assembly.  The tools schedule the instructions for you.

    Jack Harris1 said:
    I wondered if there was a way of returning the real and imaginary parts of the complex number? The code below only returns the real part.

    A .cproc directive, similar to a function call in C, can only return one argument.  One idea to consider ... Pass in a pointer to a memory location and copy the complex and real parts of the result into that memory location.

    Thanks and regards,

    -George