multiply problem

pfournier · Joined: 30 Sep 2003 Posts: 89

I am using PCWH 3.245
I tried using a simple multiply and found the answer was wrong.

1b9*4e2=6a52, should be 0x86952

I then discovered _mul(a,b). That worked fine.

Here is my demo code.
Why doesn't the simple multiply work right?
Did I do something wrong?

I tried searching the group first but I didn't see this.

Thanks, Pete

pfournier · Joined: 30 Sep 2003 Posts: 89

-oops
I took out some extra code an put in a mistake, here is the right stuff

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

pfournier · Joined: 30 Sep 2003 Posts: 89

Sorry, just change that to int32.

I have a stdint.h file that I use to make int8_t, int16_t and int32_t available cross platform. At one time I wrote c code that ran on a Rabbit processor and a PIC. It avoid the issue of an int being 8 bits on a PIC and 16 bits on a rabbit.
_________________
-Pete

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Fix it by casting one of the operands to 32-bits. See the changes
shown in bold below.

pfournier · Joined: 30 Sep 2003 Posts: 89

Yup, that did it. Didn't need to cast in the _mul() though.
I thought just casting the whole operation would do it.
One of those things I don't do very often, so I never learned that lesson till now.

Thank you.
_________________
-Pete

Ttelmah · Guest

This is the whole 'point' of using _mul. It takes two int_16 values, and multiplies them to produce an int32 result. Quicker and smaller than casting. When you casted the whole command in brackets, the effect was to multiply two int16 values, which therefore used int16 arithmetic, and then convert the result to int32 (which would be the default behaviour since the result was being transferred to an int32).

Best Wishes