I am trying to use either MAC or MPY to make a product of two
variables. The problem I have is that the result is always of twice
of the expected. For example, If I multiply 1 by 1, the the result I
got is always 2 other than 1. The following is the code I used;
The C portion:
int multiply(int, int);
int some_global = 4;
int main(void)
{
int a=1, b=1, c=0;
c = multiply(a,b);
some_global = c;
return(0);
}
The assembly portion:
section rtlib
org p:
global Fmultiply
Fmultiply:
clr A
move Y1,X0
mpy Y0,X0,A
move A0,Y0
rts
endsec
Could someone tell me why this is happening.
Thanks for the help
MAC and MPY problem in assembly language
Started by ●May 14, 2005
Reply by ●May 16, 20052005-05-16
The DSP is not a floating point machine, it's fixed point. This is
discussed in the Family Manual as well as several Application Notes...
--
dB --- haoyeshen <haoyeshen@haoy...> wrote:
> I am trying to use either MAC or MPY to make a product of two
> variables. The problem I have is that the result is always of twice
> of the expected. For example, If I multiply 1 by 1, the the result I
> got is always 2 other than 1. The following is the code I used; > The C portion:
>
> int multiply(int, int);
> int some_global = 4;
>
> int main(void)
> {
> int a=1, b=1, c=0;
>
> c = multiply(a,b);
>
> some_global = c;
>
> return(0);
>
> }
>
> The assembly portion:
>
> section rtlib
> org p:
>
> global Fmultiply
>
> Fmultiply:
>
> clr A
> move Y1,X0
> mpy Y0,X0,A
> move A0,Y0
> rts
>
> endsec
>
> Could someone tell me why this is happening. > Thanks for the help >
__________________________________________________
discussed in the Family Manual as well as several Application Notes...
--
dB --- haoyeshen <haoyeshen@haoy...> wrote:
> I am trying to use either MAC or MPY to make a product of two
> variables. The problem I have is that the result is always of twice
> of the expected. For example, If I multiply 1 by 1, the the result I
> got is always 2 other than 1. The following is the code I used; > The C portion:
>
> int multiply(int, int);
> int some_global = 4;
>
> int main(void)
> {
> int a=1, b=1, c=0;
>
> c = multiply(a,b);
>
> some_global = c;
>
> return(0);
>
> }
>
> The assembly portion:
>
> section rtlib
> org p:
>
> global Fmultiply
>
> Fmultiply:
>
> clr A
> move Y1,X0
> mpy Y0,X0,A
> move A0,Y0
> rts
>
> endsec
>
> Could someone tell me why this is happening. > Thanks for the help >
__________________________________________________
Reply by ●May 16, 20052005-05-16
The problem is that MPY is a fixed point factional multiply, while
you are likely looking for the integer multiply result.
These commands differ by a 1 bit shift (i.e. x2) from each other in the
result. Check in the family manual section on the ALU.
Some DSPs may offer both types of multiply instructions, in
which case the extra shift is not required.
Floating point also avoids this minor problem, but for number
crunching, fixed point is gererally the best solution (cheapest, lowest power
consumption).
Howard
-----Original Message-----I am trying to use either MAC or MPY to make a product of two
From: haoyeshen [mailto:h...@yahoo.com]
Sent: Saturday, May 14, 2005 1:11 AM
To: m...@yahoogroups.com
Subject: [motoroladsp] MAC and MPY problem in assembly language
variables. The problem I have is that the result is always of twice
of the expected. For example, If I multiply 1 by 1, the the result I
got is always 2 other than 1. The following is the code I used;The C portion:
int multiply(int, int);
int some_global = 4;
int main(void)
{
int a=1, b=1, c=0;
c = multiply(a,b);
some_global = c;
return(0);
}
The assembly portion:
section rtlib
org p:
global Fmultiply
Fmultiply:
clr A
move Y1,X0
mpy Y0,X0,A
move A0,Y0
rts
endsec
Could someone tell me why this is happening.Thanks for the help
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