CCS C Software and Maintenance Offers
FAQFAQ   FAQForum Help   FAQOfficial CCS Support   SearchSearch  RegisterRegister 

ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

CCS does not monitor this forum on a regular basis.

Please do not post bug reports on this forum. Send them to CCS Technical Support

data type conversion - assignments between different lenghts

 
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion
View previous topic :: View next topic  
Author Message
Radomir



Joined: 18 Nov 2005
Posts: 3

View user's profile Send private message

data type conversion - assignments between different lenghts
PostPosted: Fri Nov 18, 2005 2:46 pm     Reply with quote

Hello!

I have recently started using CCS compiler and I don't have much experience with C either.

I need to perform the following calculation:

var1*(var2/const), where:

var1 - 8bit integer
var2 - 8bit integer
const - 8 bit integer
1<var2<const

I assigned a 16bit temp var to hold var1*var2 and afterwards to divide it by const and assign it to the result var.
However, it seems that upper part of the temp var is lost.

Please, help!
Ttelmah
Guest







PostPosted: Fri Nov 18, 2005 3:51 pm     Reply with quote

The key is that the arithmetic used, is defined by the sizes of the operands, not the 'target'. Hence if you multiply an int8, by an int8, the compiler will use int8 arithmetic.
You can force 16bit arithmetic to be used in a number of ways:
1) If you declare 'const' to be a long (for instance 100L), then this will force a switch to using int16 arithmetic
2) If you change one of the values into an int16, before performing the arithmetic, this will also force the switch. This is the 'preferred' way to go, and is standard C.
The arithmetic rules for the conversion, are exactly as written in K&R, but with the 'caveat', that the default 'int' type (which for instance a char will be converted to), is int8, rather than int16, which makes the conversion 'seem' less friendly.

So, use:

var1*((int16)var2/const)

Which will force var2, to be converted to an int16, before the arithmetic is started. Then the division will be done using an int16, and since the result is now an int16, the multiplication will also be done using the larger type.

Best Wishes
Radomir



Joined: 18 Nov 2005
Posts: 3

View user's profile Send private message

PostPosted: Mon Nov 21, 2005 9:38 am     Reply with quote

Ttelmah wrote:
The key is that the arithmetic used, is defined by the sizes of the operands, not the 'target'. Hence if you multiply an int8, by an int8, the compiler will use int8 arithmetic.
You can force 16bit arithmetic to be used in a number of ways:
1) If you declare 'const' to be a long (for instance 100L), then this will force a switch to using int16 arithmetic
2) If you change one of the values into an int16, before performing the arithmetic, this will also force the switch. This is the 'preferred' way to go, and is standard C.
The arithmetic rules for the conversion, are exactly as written in K&R, but with the 'caveat', that the default 'int' type (which for instance a char will be converted to), is int8, rather than int16, which makes the conversion 'seem' less friendly.

So, use:

var1*((int16)var2/const)

Which will force var2, to be converted to an int16, before the arithmetic is started. Then the division will be done using an int16, and since the result is now an int16, the multiplication will also be done using the larger type.

Best Wishes



Thank you very much for the clue!
It looks like I have 16bit data now.
I also found another way of converting 8bit data to 16bit data - make16(v1,v2), and another way of multiplying 8bit vars - mul(v1,v2).

I am still not getting the right results, but I know the path.
Thanks again!
Display posts from previous:   
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group