SAP computer之program counter

　　Program counter

　　The program is stored in memory with the first instruction at binary address 0000, the second instruction at address 0001, the third at address 0010 and so on.  The program counter, which is part of the control unit, counts from 0000 to 1111. Its job is to send to the memory the address of next instruction.

　　The program counter is reset to 0000 before computer run. When the computer run begins, the program counter sends address 0000 to the memory. The program counter is then incremented to get 0001. After the first instruction is fetched and executed, the program counter sends address 0001 to the memory. Again the program counter is incremented. After the second isntruction is fetched and executed, the program counter sends address 0010 to the memory. In this way, the program counter is keeping track of the next instruction to be fetched and executed.

　　The program counter is like someone pointing at a list of instruction, saying do this first, do this second, etc. This is why the program counter is sometimes called a pointer; it points to an address in memory where something important is being stored.

 library IEEE;
 use ieee.std_logic_1164.all;
 use ieee.std_logic_unsigned.all;
 
 entity PC is
 port
 (
     EP    : in  std_logic;            　　　　--! Active high output enable from PC, or tri-state
     CLR   : in  std_logic;             　　　 --! Active high asynchronous clear
     CLK   : in  std_logic;       　　　　     --! Falling edge clock
     CP    : in  std_logic;            　　　　--! Active high enable PC to count
     Q     : out std_logic_vector( downto ) --! 4-bit PC output
 );
 end PC ;
 
 architecture beh of PC is
 
 signal count : std_logic_vector( downto );
 
 begin
 
 process (CLR,EP,CP,CLK,count)
 begin
     if CLR = '' then
         Q <= "";
         count <= "";
     elsif CP = '' then
          if (CLK'event and CLK = '') then
             if count < "" then
                 count <= count + ;
             else
                 count <= "";
             end if;
         end if;
      end if;
 
     if EP = '' then
         Q <= "ZZZZ";
     else
         Q <= count;
     end if;
 
 end process; 
 
 end beh;

Question: why do not use the following code in process?

 begin
     if EP = '' then
         Q <= "ZZZZ";
     elsif CLR = '' then
         Q <= "";
         count <= "";
     elsif CP = '' then
         if(CLK'event and CLK = '') then
             if count < "" then
                 count <= count + ;
             else
                 count <= "";
             end if;
         end if;
     end if;
 
 end process;

Answer: first code, line 40, Q <= count

Own code for ASIC: use package ieee.numeric_std

 library IEEE;
 use ieee.std_logic_1164.all;
 use ieee.numeric_std.all;

 entity PC is
 port
 (
     EP       : in  std_logic;                     --! Active high output enable from PC, or tri-state
     CLR      : in  std_logic;                     --! Active high asynchronous clear
     CLK      : in  std_logic;                     --! Falling edge clock
     CP       : in  std_logic;                     --! Active high enable PC to count
     Q        : out std_logic_vector( downto )   --! 4-bit PC output
 );
 end PC ;
 
 architecture beh of PC is
 
 signal count : std_logic_vector( downto );
 
 begin
 
 process (CLR,EP,CP,CLK,count)
 begin
     if CLR = '' then
25　　--       Q <= "0000";
         count <= "";
     elsif CP = '' then
          if (CLK'event and CLK = '') then
             if count < "" then
                 count <= std_logic_vector(unsigned(count) + );
             else
                 count <= "";
             end if;
         end if;
      end if;
 
     if EP = '' then
         Q <= "ZZZZ"; -- not good, in ASIC use only std_logic signal state '0', '1'
     else
         Q <= count;
     end if;
 
 end process; 
 
 end beh;

 Question: In ASIC design, why use only std_logic signal states '0', '1'(and 'Z' for FPGA)???