Error: Cannot drive signal 'data' of mode IN

Hi,

I am new user to ModelSim and writing VHDL code..

I've got the follwing msg when I compiled a vhdl code

"Error controller.vhd(64): Cannot drive signal 'data' of mode IN."

First, I want to to know where could I find a place to read about compiler error messages ..

Second, I didn't understand what this error means?

I have the follwing declarations in my code

constant instruction_size :integer:=10;

constant operation_size :integer:=4;

subtype command_word is unsigned(operation_size-1 downto 0);

subtype program_word is unsigned(instruction_size-1 downto 0);

data : in program_word; -- in the entity of the controller

--the follwing in the structure of the controller

alias command: command_word is data (instruction_size-1 downto 6);

signal instr : program_word ;

type state_type is (S0, S1, S2, S3, S4, S5, S6, S7, S8);

signal state: state_type;

Then I have this piece of code

case state is

when S2 => instr<=data; -- Load instruction

state <= S3;

when S3 => command <= instr (instruction_size-1 downto 6);

when I compile this file I got the above mentioned error msg on the last line of code above, i.e " when S3 => ...."

I am looking forward to get some help with this problem..

Thank you in advance