Additional circuit within a circuit - if then else

[Altera_Forum]

Hello, 

 I am a beginner to VHDL and currently have an assignment to complete. It is as follows:   

• A digital circuit has two 4-bit inputs a

  I have written the architecture correct for the subtractor part. This fully compiles. When I try to write the subcircuit for the If Then Else part about the 'the circuit has a single bit output t that is to be high if the result of the subtraction is zero' part I struggle.   This is my code so far  entity ComparatorSubtractor4Bit is port (  a,b : in integer range 0 to 15;  borrowin : in integer range 0 to 1;  sub : out integer range 0 to 15;  t : out integer range 0 to 1); end ComparatorSubtractor4Bit;  architecture Adder of ComparatorSubtractor4Bit is begin  sub<= a - b - borrowin; -- note borrow out will be sub[4]  process (a,b)  begin   if sub<= 0 then t<=1;  else t<=0;  end if;  end process; end Adder;  I have written a sub If Then Else code on a previous assignment which involved the inputs being compared, however it seems to have a problem when I try to If Then Else the output.   Any help would be greatly appreciated  Sam

[Altera_Forum]

Thje error message seems self-explanatory to me, isn't it? 

  --- Quote Start ---  Error (10309): VHDL Interface Declaration error in test.vhd(17): interface object "sub" of mode out cannot be read. Change object mode to buffer.  --- Quote End ---  

[Altera_Forum]

--- Quote Start ---  Thje error message seems self-explanatory to me, isn't it?  --- Quote End ---    No, I don't really know what the error messages mean. Like I say I'm just a beginner. Could you explain further?

[Altera_Forum]

You cannot read (eg. check the value of) an output port. Either create a temporary internal signal or change the mode from "out" to "buffer". 

 sub : buffer integer range 0 to 15;