8x8 Multiply with 16-bit Result

[BadBrainBoy]

Hi All,

The default when doing int8 * int8 with a hardware multiplier PIC is to keep only the LSB of the result (PRODL). This is C standard, I suppose, but not very useful.

If you want the full 16-bit result from int8 * int8, you can use this (written for the 18F2220):
=========================================
#byte PRODH = 0xff4 // HW multiply result MSB
#byte PRODL = 0xff3 // HW multiply result LSB
// MULT macro
// Performs 8x8 hardware multiply of x and y and stores the 16-bit
// result in xy
#define MULT(x,y,xy) \
#asm \
movf x,w \
mulwf y \
#endasm \
xy = MAKE16(PRODH,PRODL);
=========================================
Now just write MULT(arg1,arg2,result) when you want to multiply two int8 variables named arg1 and arg2 and store the product in int16 result.

This is very efficient, it compiles to only 4 machine instructions with 6 instruction cycles. It is set up for straight unsigned multiplication; you may fiddle with it for signed.

Have fun!

[sd211]

[Jimmy]

I tried c = _mul(arg1,arg2);. It's not working properly. I use PIC16F917 and simulate with MPLAB. The main function is like this.

void main()
{
int8 varA=50,varB=100;
long int c;

c = _mul(varA,varB);
while(TRUE);
}

in the watch window show c = 34835 insteal to show c = 5000.

I try many value, but always wrong result.
Is there anyone can help on this? thanks.

[Mircea]