Vhdl operations

[cally_cally]

hello, i'm new here and i'm asking you how can i create an arithmetic unit that operates addition, substraction, multiplication and division. I have numbers on 4 bits and the result will be maximum on 8 bits. I've tried using "+" , "-" and "*" operators but I get the error:

Operator "+" is not defined for such operands.

I use the following syntax:

when "00" => y <= A + B;

Thanks in advance!

Cally

 

[eliascm]

You do not give the type of your operands. "+" is not defined for standard_logic_vector. There are several ways around this. One is to use Unsigned type from the ieee numeric_std library.

 

[cally_cally]

how should i declare the 2 operands and what library must i use?

Thanks for your quick response.

 

[joris]

You have A and B in scope, so they must be declared somewhere: as signals or variables, or inputs to the entity.