syntax-error - 我的 VHDL 代码出错，但我似乎无法弄清楚原因

Question

我为显示的状态图编写了一个 VHDL 代码(哎呀，因为我是新用户，所以无法发布图像)。但是，当我编译它时，它说有错误:
第 16 行: process(clk) -- 解析时检测到语法错误
第 21 行:else -- 解析时检测到语法错误
第 23 行:如果结束； -- 解析时检测到语法错误。

这是我的代码:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
use IEEE.std_logic_unsigned.ALL;
entity memory_controller is
port(clk: in std_logic;
 reset: in std_logic;
 bus_id: in std_logic_vector(7 downto 0);
 read_write, burst: in std_logic;
 oe, we, addr_1, addr_2: out std_logic
 );
end memory_controller;
architecture behavioral of memory_controller is
type statetype is (idle, decision, wr, rd1, rd2, rd3, rd4);
signal present_state, next_state : statetype;
process(clk) [LINE 16]
begin
if (rising_edge(clk)) then
    if (reset ='0') then
        present_state <= next_state;  
    else [LINE 21]
        present_state <= idle;   
    end if; [LINE 23]
end if;
end process;  
process(present_state, read_write, ready, burst)
begin
case present_state is
when idle => 
    oe => '0'; we=> '0'; addr_1=> '0'; addr_2=> '0';
if(bus_id = "11110011") then
    next_state <= decision;
else
    next_state <= idle;
end if;
when decision =>
    if (read_write = '1')
        then next_state <= rd1;
    else next_state <= wr;
end if;
when wr =>
we = '1';
    if (ready = '1')
then next_state <= idle;
else
next_state <= wr;
end if;
when rd1 =>
oe = '1';
addr_1 = addr_1 + '1';
addr_2 = addr_2 + '1';
if(ready = '0') then
next_state <= rd1;
if(burst = '0') then
next_state <= idle;
else next_state <= rd2;
end if;
when rd2 =>
oe = '1';
addr_1 = addr_1 + '1';
 addr_2 = addr_2 + '1';
if(ready = '1') then
next_state => rd3;
else
next_state => rd2;
end if;
when rd3 =>
oe = '1';
addr_1 = addr_1 + '1';
 addr_2 = addr_2 + '1';
if(ready = '1') then 
next_state => rd4;
else
next_state => rd3;
when rd4 =>
oe = '1';
addr_1 = addr_1 + '1';
addr_2 = addr_2 + '1';
if(ready = '1')
 then next_state => idle;
else next_state => rd4;
end if;
end case;
end process;
end behavioral;

语法完全正确，我不明白为什么这是一个错误。有什么问题？

此外，我想在 ready =0、burst =0 和 ready = 0 和 burst = 1 的情况下使用断言语句，但我不太确定如何在主代码中实现它们。

我已经突出显示了第 16、21 和 23 行。

任何帮助都会很棒。

Answer 1

VHDL 模块的形式通常是:

entity MODULENAME is
  <Port description>
end MODULENAME;

architecture behavioral of MODULENAME is
  <signal declarations and similar>
begin
  <synchronous and combinatorial logic statements>
end architecture behavioral;

你缺少的是 begin在你的信号声明之后。也就是改变

architecture behavioral of memory_controller is
  type statetype is (idle, decision, wr, rd1, rd2, rd3, rd4);
  signal present_state, next_state : statetype;

  process(clk) [LINE 16]
  begin
    if (rising_edge(clk)) then

到:

architecture behavioral of memory_controller is
  type statetype is (idle, decision, wr, rd1, rd2, rd3, rd4);
  signal present_state, next_state : statetype;

begin

  process(clk) [LINE 16]
  begin
    if (rising_edge(clk)) then