multiplication problem

[micro_man]

guys its a very simple program.simply multiplying a variable with 100 and store it in another variable.but the result store in the variable is incorrect.
here is the program

[Mike Walne]

In 8 bit maths 3*100 = 44.
Exactly the result you're getting.
What's the problem?

Mike

EDIT Hint 256 + 44 = 300!

[Ttelmah]

Further hint. is 100, the same as 100L in C?.

Do a forum search, you will find hundreds of answers.

Best Wishes

[micro_man]

thanks guys for your replies

are you saying that i can achieve the result by doing something like

b= PRODH*256 + PRODL;


how i search the forum, i mean what should i write in the search bar for this specific problem?

[Mike Walne]

Each of 'a' and '100' is an 8 bit value.

So the compiler has no way of knowing you want a 16 bit result from a*100.

Mike

EDIT

You have to tell the compiler you want a 16bit result.
It's done in 'C', not by playing about with registers.

[micro_man]

I have already initialized result variable b as an int16 bit.Should i also initialize a as an int16 bit?

[Ttelmah]

It has been answered hundreds of times. However:

'C' works by looking at the 'types' of both arguments in an expression, and using the 'higher' type of both.

So if you take int8*int8, it uses int8 arithmetic.
However int8*int16, it uses int16. etc..

Now, 100, is an 'int8'. 'a', also is an int8, hence int8 arithmetic.

You can 'tell' the compiler, that a constant, is int16, by using 'L' (long). So 100, is an int8 constant, while 100L, is an int16. You can also 'tell' the compiler to convert a variable to a higher type before using it with a 'cast'. So

b=(int16)a*100;

tells the compiler to treat 'a' as an int16 before use. While:

b=a*100L;

tells the compiler to treat the '100' as an int16.

Now key is that in most C's the default integer type is an int16, not int8, so problems like this only appear with larger numbers. Also, on C's written for chips with hardware maths units, the unit itself handles 'overflow', and so in many cases the language will propagate the overflow condition and not clip the result. However it happens on all C's at some point or another.....

This is described in the original K&R books, and in most C textbooks, though the latter in particular tend to 'assume' int16 as the smallest maths type. K&R, gives examples with an early PDP, and 12bit as the default maths size.

Best Wishes

[andrewg]

Lookup function "_mul" in the CCS manual...
_________________
Andrew