一、前言
在从0-1探索RISCV指令集的路上，我们用百元不到的NANO 9K开发板一步步的实现：
１）最小的内核架构
２）取值，译码和执行的过程（电路实现ISA指令集）
３）存储空间的实现（寄存器，RAM, ROM的寻址）和数据搬移
４） 外设的电路实现
至此，我们有了一个自己用数字电路实现的CPU精简内核，而且实现了RISCV的ISA指令集的一部分，虽然是很小的一部分！
看看我们之前的软件编写方式，
１）用什么语言？首先要用机器码二进制来实现，汇编都不行。
２）软件怎么装载和运行？只能是写死到ROM里，没法更新，下载？更不用想了。
所以这个简洁到简陋的"MCU"和我们平常用的8051，STM32相比，缺什么？
1）编译器和反编译器。 编译器用来把C或者汇编编译成机器语言（二进制）然后可以加载执行；
2）支持交互编程的IDE集成开发环境，最好能一键下载，最好不需要专用的烧录器，比如串口下载。
MCU是工具芯片，能否用起来，用的好，依赖于开发者生态， 很多性能优异的MCU设计出来推广的不好都是因为开发环境“反人类”， 工程师/开发者“不喜欢”，从而制约了发展，越没人用越无法迭代， 越迭代慢越没人用，从而陷入僵局。
反观市场上的“当红炸子鸡”，除了芯片本身的性价比外，简单易用，稳定可靠的IDE环境，便宜易获得的下载工具都为人津津乐道，前有AVR/MCS51, 后有ESP32, STM32，所以设计MCU一定要有关注开发环境和融入生态的思路和提前规划， 这些就是本次内容要解决的问题！
二、解决方案
导入简洁而不简单的IDE , ARDUINO开发环境， 实现开发环境的闭环。
常见的生态：
8位机： 8051/STM8+IAR, STM32+KEIL......还有像STC以及其他厂商推出的各类IDE,或者基于eclipse\codeblock\vscode扩展的各种插件。
如果我们平时玩过电子制作，对arduino不会模式，经常用来做电子积木，逻辑简单而功能强大，生态极为丰富，只要是成熟的外设，不管是温湿度传感器，陀螺仪，气压计，还是OLED屏，SPI屏，甚至蓝牙，wifi都不在话下，不但有硬件生态，还有丰富的软件生态，各种库几乎能满足平常电子制作甚至专业快速开发的一切需求。
先看看我们的成果：
在arduino里导入了我们的精简内核（用nano 9k的板子实现的软核），并且支持直接一键编译，串口下载，是不是完美解决了我们上面的困扰。 当然，由于内核实现的指令集过于简单，这个软核的能编写的应用软件很局限，但至少也可以把流水灯点起来，bink，blink.......
分步骤实现：
1）修改内核，加载到板子上；形成我们自己的"MCU开发板"
硬件设计的增量----内核在前面项目基础上，增加：
- 模块--HWLoader实现
- 模块--双口RAM实现
- A口-HWLoader
- B口-core
- core修改TOP层引用，增加了端口调用，MUX端口选择
- core修改地址译码电路
综合，下载，这里注意要设置下systemverilog语言支持，不然综合会报错。
报错信息：
设置位置： project->configuration
2）配置arduino开发环境；
包括必要的三件套： 硬件支持包+环境配置文件，编译器，下载工具软件
环境配置：
获取压缩包arduino_riscvgcc_tools ” riscv.rar下载，解压到Arduino安装目录的hardware子目录下（如“C:\Arduino\hardware\”）
工具链配置（编译+下载）：
tools下的gcc.rar解压。得到两个工具链的两个要素： 编译器gcc+下载器
查看配置文件： C:\arduino-1.8.19\hardware\riscv\tangnano9k\boards.txt
```
TANG.name=TANG Nano9k Board (Gaoyun GW1NR)
TANG.platform=riscv-gcc
TANG.build.board=_BOARD_TANG_NANO9K_
TANG.build.extra_flags = -march=rv32i -mabi=ilp32
TANG.compiler.c.extra_flags=
TANG.compiler.cpp.extra_flags=
TANG.ld.extra_flags =
TANG.upload.maximum_size=4096
TANG.upload.speed=115200
TANG.upload.tool=Reindeer_upload
TANG.build.mcu=tiny
TANG.build.core=riscv
TANG.build.variant=generic
```platform.txt
```
name=Lanzhoo RISC-V (TANG Nano9k)
compiler.path = {runtime.platform.path}/tools/gcc/bin/
compiler.define= -DIDE=Arduino "-I{compiler.path}../riscv-none-elf/include"
build.extra_flags =
build.flags =
compiler.c.cmd = riscv-none-elf-gcc
compiler.c.flags = -W -c --specs=nosys.specs -O1 -fdata-sections -ffunction-sections -fno-exceptions -fno-unwind-tables
compiler.c.extra_flags =
compiler.elf.flags =
compiler.elf.cmd = riscv-none-elf-gcc
compiler.ld.cmd = riscv-none-elf-gcc
compiler.ld.flags= -Wl,--gc-sections -static -lm --specs=nosys.specs
compiler.ld.extra_flags=
compiler.ldscript=Reindeer.ld
compiler.ar.cmd=riscv-none-elf-ar
compiler.ar.flags=rcs
compiler.objcopy.cmd=riscv-none-elf-objcopy
compiler.elf2hex.cmd=riscv-none-elf-objcopy
compiler.size.path = {runtime.platform.path}/tools/gcc/bin/
compiler.size.cmd = riscv-none-elf-size
build.variant=generic
build.ldscript.path={build.variant.path}
core.header=Arduino.h
compiler.c.elf.extra_flags= -I{build.core.path}
compiler.S.extra_flags=
compiler.cpp.extra_flags=
compiler.ar.extra_flags=
compiler.objcopy.eep.flags = -O binary
compiler.objcopy.eep.extra_flags=
compiler.elf2hex.flags = -O ihex
compiler.elf2hex.extra_flags=
recipe.c.o.pattern="{compiler.path}{compiler.c.cmd}" {build.flags} {compiler.c.flags} {compiler.define} {compiler.c.extra_flags} {build.extra_flags} -I{build.path}/sketch {includes} "{source_file}" -o "{object_file}"
recipe.cpp.o.pattern="{compiler.path}{compiler.c.cmd}" {build.flags} {compiler.c.flags} {compiler.define} {compiler.c.extra_flags} {build.extra_flags} -I{build.path}/sketch {includes} "{source_file}" -o "{object_file}"
recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"
recipe.c.combine.pattern="{compiler.path}{compiler.ld.cmd}" -T {build.ldscript.path}/{compiler.ldscript} {build.flags} {compiler.ld.flags} {build.extra_flags} -o "{build.path}/{build.project_name}.elf" {object_files} "{build.path}/{archive_file}"
recipe.objcopy.hex.pattern="{compiler.path}{compiler.elf2hex.cmd}" {compiler.elf2hex.flags} {compiler.elf2hex.extra_flags} "{build.path}/{build.project_name}.elf" "{build.path}/{build.project_name}.hex"
recipe.output.tmp_file={build.project_name}.hex
recipe.output.save_file={build.project_name}.{build.variant}.hex
recipe.size.pattern="{compiler.size.path}/{compiler.size.cmd}" "{build.path}/{build.project_name}.hex"
recipe.size.regex=Total\s+([0-9]+).*
# up loader
tools.Reindeer_upload.cmd=reindeer_config
tools.Reindeer_upload.path={runtime.platform.path}/tools
tools.Reindeer_upload.upload.params.verbose=
tools.Reindeer_upload.upload.params.quiet=
tools.Reindeer_upload.upload.protocol=UART
tools.Reindeer_upload.upload.pattern="{path}/{cmd}" --reset --run --port={serial.port} --baud=115200 "--image={build.path}/{build.project_name}.hex"
```
链接link配置脚本， 路径： C:\arduino-1.8.19\hardware\riscv\tangnano9k\variants\generic\Reindeer.ld
内容：
/* Script for -z combreloc: combine and sort reloc sections */
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Copying and distribution of this script, with or without modification,
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OUTPUT_FORMAT("elf32-littleriscv", "elf32-littleriscv",
"elf32-littleriscv")
OUTPUT_ARCH(riscv)
ENTRY(_start)
/*
SEARCH_DIR("=/Host/Work/riscv-none-gcc-8.1.0-2/install/win64/riscv-none-gcc/riscv-none-embed/lib");
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3）编写主程序代码和头文件
main.cpp
```
#include "Arduino.h"
int main(){
setup();
while (1) {
loop();
}
return 0;
}
```头文件arduino.h：
```
#ifndef _ARDUINO_H
#define _ARDUINO_H
#ifdef __cplusplus
extern "C" {
#endif
extern void setup();
extern void loop();
#ifdef __cplusplus
}
#endif
#endif
```
（注：这里涉及.c和.cpp中函数相互调用的链接问题。如果Arduino.h中不加“#ifdef __cplusplus”相关语句，则main.c中的main()在链接时找不到.ino.cpp中实现的setup()和loop()。（把main.c改名为main.cpp就可以））
4）编写arduino 应用，查看编译下载日志， 熟悉工作流程
C:\arduino-1.8.19\arduino-builder -dump-prefs -logger=machine -hardware C:\arduino-1.8.19\hardware -hardware C:\Users\hy\AppData\Local\Arduino15\packages -tools C:\arduino-1.8.19\tools-builder -tools C:\arduino-1.8.19\hardware\tools\avr -tools C:\Users\hy\AppData\Local\Arduino15\packages -built-in-libraries C:\arduino-1.8.19\libraries -libraries C:\Users\hy\Documents\Arduino\libraries -fqbn=riscv:tangnano9k:TANG -vid-pid=0403_6010 -ide-version=10819 -build-path C:\Users\hy\AppData\Local\Temp\arduino_build_815823 -warnings=none -build-cache C:\Users\hy\AppData\Local\Temp\arduino_cache_966892 -prefs=build.warn_data_percentage=75 -verbose C:\Gowin\TangNano9k_Tutorial\04_arduino_step01\arduino01\arduino01.ino
C:\arduino-1.8.19\arduino-builder -compile -logger=machine -hardware C:\arduino-1.8.19\hardware -hardware C:\Users\hy\AppData\Local\Arduino15\packages -tools C:\arduino-1.8.19\tools-builder -tools C:\arduino-1.8.19\hardware\tools\avr -tools C:\Users\hy\AppData\Local\Arduino15\packages -built-in-libraries C:\arduino-1.8.19\libraries -libraries C:\Users\hy\Documents\Arduino\libraries -fqbn=riscv:tangnano9k:TANG -vid-pid=0403_6010 -ide-version=10819 -build-path C:\Users\hy\AppData\Local\Temp\arduino_build_815823 -warnings=none -build-cache C:\Users\hy\AppData\Local\Temp\arduino_cache_966892 -prefs=build.warn_data_percentage=75 -verbose C:\Gowin\TangNano9k_Tutorial\04_arduino_step01\arduino01\arduino01.ino
---------------------------------------------
Using board 'TANG' from platform in folder: C:\arduino-1.8.19\hardware\riscv\tangnano9k
Using core 'riscv' from platform in folder: C:\arduino-1.8.19\hardware\riscv\tangnano9k
-------------------------------------------------
Detecting libraries used...
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-gcc" -W -c --specs=nosys.specs -O1 -fdata-sections -ffunction-sections -fno-exceptions -fno-unwind-tables -DIDE=Arduino "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/../riscv-none-elf/include" -march=rv32i -mabi=ilp32 "-IC:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/sketch" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\cores\\riscv" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\variants\\generic" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\sketch\\arduino01.ino.cpp" -o nul
----------------------------------------------------
Generating function prototypes...
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-gcc" -W -c --specs=nosys.specs -O1 -fdata-sections -ffunction-sections -fno-exceptions -fno-unwind-tables -DIDE=Arduino "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/../riscv-none-elf/include" -march=rv32i -mabi=ilp32 "-IC:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/sketch" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\cores\\riscv" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\variants\\generic" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\sketch\\arduino01.ino.cpp" -o "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\preproc\\ctags_target_for_gcc_minus_e.cpp"
"C:\\arduino-1.8.19\\tools-builder\\ctags\\5.8-arduino11/ctags" -u --language-force=c++ -f - --c++-kinds=svpf --fields=KSTtzns --line-directives "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\preproc\\ctags_target_for_gcc_minus_e.cpp"
----------------------------------------------------
正在编译项目...
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-gcc" -W -c --specs=nosys.specs -O1 -fdata-sections -ffunction-sections -fno-exceptions -fno-unwind-tables -DIDE=Arduino "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/../riscv-none-elf/include" -march=rv32i -mabi=ilp32 "-IC:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/sketch" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\cores\\riscv" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\variants\\generic" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\sketch\\arduino01.ino.cpp" -o "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\sketch\\arduino01.ino.cpp.o"
Compiling libraries...
Compiling core...
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-gcc" -W -c --specs=nosys.specs -O1 -fdata-sections -ffunction-sections -fno-exceptions -fno-unwind-tables -DIDE=Arduino "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/../riscv-none-elf/include" -march=rv32i -mabi=ilp32 "-IC:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/sketch" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\cores\\riscv" "-IC:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\variants\\generic" "C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\cores\\riscv\\main.c" -o "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\core\\main.c.o"
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-ar" rcs "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\core\\core.a" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\core\\main.c.o"
Archiving built core (caching) in: C:\Users\hy\AppData\Local\Temp\arduino_cache_966892\core\core_riscv_tangnano9k_TANG_e2890981838dae9eb3ff54af2f115652.a
--------------------------------------------
Linking everything together...
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-gcc" -T "C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k\\variants\\generic/Reindeer.ld" -Wl,--gc-sections -static -lm --specs=nosys.specs -march=rv32i -mabi=ilp32 -o "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/arduino01.ino.elf" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823\\sketch\\arduino01.ino.cpp.o" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/core\\core.a"
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin/riscv-none-elf-objcopy" -O ihex "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/arduino01.ino.elf" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/arduino01.ino.hex"
"C:\\arduino-1.8.19\\hardware\\riscv\\tangnano9k/tools/gcc/bin//riscv-none-elf-size" "C:\\Users\\hy\\AppData\\Local\\Temp\\arduino_build_815823/arduino01.ino.hex"
项目使用了 0 字节，占用了 (0%) 程序存储空间。最大为 4096 字节。
------------------------------------------------------------------
C:\arduino-1.8.19\hardware\riscv\tangnano9k/tools/reindeer_config --reset --run --port=COM53 --baud=115200 --image=C:\Users\hy\AppData\Local\Temp\arduino_build_815823/arduino01.ino.hex
===============================================================================
# Copyright (c) 2019, PulseRain Technology LLC
# Reindeer Configuration Utility, Version 2.1
===============================================================================
baud_rate = 115200
com_port = COM53
toolchain = riscv-none-embed-
===============================================================================
Reseting CPU ...
Loading C:\Users\hy\AppData\Local\Temp\arduino_build_815823/arduino01.ino.hex
===================> start the CPU, entry point = 0x80002000
生成的.hex文件将用于烧写。
生成的.elf文件可以通过riscv-none-elf-objdump.exe反编译为汇编。
5）通过反编译，深入探索
文件路径： C:\Users\hy\AppData\Local\Temp\arduino_build_815823
反编译命令：
-D arduino01.ino.elf > code.txt
得到反编译的汇编代码：
-D arduino01.ino.elf > code.txt
arduino01.ino.elf: file format elf32-littleriscv
Disassembly of section .text:
80002000 <_start>:
80002000: 00001137 lui sp,0x1
80002004: 0240006f j 80002028 <main>
_start函数先设置堆栈指针gp也就是sp的值，然后调用若干函数初始化变量，然后跳转main函数。
80002008 <_fini>:
80002008: 0000006f j 80002008 <_fini>
8000200c <setup>:
8000200c: 00008067 ret
80002010 <loop>:
80002010: 00180813 addi a6,a6,1
80002014: 00a00793 li a5,10
80002018: 00000013 nop
8000201c: fff78793 addi a5,a5,-1
80002020: fe079ce3 bnez a5,80002018 <loop+0x8>
80002024: 00008067 ret
80002028 <main>:
80002028: ff010113 addi sp,sp,-16 # ff0 <__global_pointer$+0x3ec>
8000202c: 00112623 sw ra,12(sp)
80002030: fddff0ef jal ra,8000200c <setup>
80002034: fddff0ef jal ra,80002010 <loop>
80002038: ffdff06f j 80002034 <main+0xc>
Disassembly of section .eh_frame:
00000400 <__FRAME_END__>:
400: 0000 .2byte 0x0
...
Disassembly of section .comment:
00000000 <.comment>:
0: 3a434347 .4byte 0x3a434347
4: 2820 .2byte 0x2820
6: 5078 .2byte 0x5078
8: 6361 .2byte 0x6361
a: 4e47206b .4byte 0x4e47206b
e: 2055 .2byte 0x2055
10: 4952 .2byte 0x4952
12: 562d4353 .4byte 0x562d4353
16: 4520 .2byte 0x4520
18: 626d .2byte 0x626d
1a: 6465 .2byte 0x6465
1c: 6564 .2byte 0x6564
1e: 2064 .2byte 0x2064
20: 20434347 .4byte 0x20434347
24: 3878 .2byte 0x3878
26: 5f36 .2byte 0x5f36
28: 3436 .2byte 0x3436
2a: 2029 .2byte 0x2029
2c: 3231 .2byte 0x3231
2e: 312e .2byte 0x312e
30: 302e .2byte 0x302e
...
Disassembly of section .riscv.attributes:
00000000 <.riscv.attributes>:
0: 1b41 .2byte 0x1b41
2: 0000 .2byte 0x0
4: 7200 .2byte 0x7200
6: 7369 .2byte 0x7369
8: 01007663 bgeu zero,a6,14 <__FRAME_END__-0x3ec>
c: 0011 .2byte 0x11
e: 0000 .2byte 0x0
10: 1004 .2byte 0x1004
12: 7205 .2byte 0x7205
14: 3376 .2byte 0x3376
16: 6932 .2byte 0x6932
18: 7032 .2byte 0x7032
1a: 0031 .2byte 0x31
- 其它函数还包括：deregister_tm_clones、register_tm_clones、__do_global_dtors_aux、frame_dummy、atexit、exit、__libc_fini_array、__libc_init_array、memset、__register_exitproc、__call_exitprocs、_exit。
总共代码量是0x052c，约2.4kB。
另外还占用了约2点多kB的数据区。
三、运行结果观察
下载程序前：
Arduino未下载程序前，core运行的程序是rom_data.mi中的指令。
观察led的值：
```
00, 0000
||: （循环）
01, 0100
10, 1000
11, 1000
00, 1011
:||
```
和以下指令（addr[3:2], code[5:2]）是一一对应的：
rom_data.mi
#File_format=Hex
#Address_depth=1024
#Data_width=32
00002783
00178813
01002023
81002823
ff5ff06f
00000000
00000000
00000000
#### 下载程序后的现象
观察led的值：
```
00, 1101
01, 1011
10, 0100
11, 1000
00, 1011
||: （循环）
11, 1001
00, 0100
01, 0100
10, 0100
11, 0100
00, 1000
01, 1001
10, 0100
11, 1000
00, 1011
:||
```
观察反编译的代码
对照反汇编指令，是一致的：
```
80002000 <_start>:
80002000: 00001137 lui sp,0x1
80002004: 0240006f j 80002028 <main>
80002028 <main>:
80002028: ff010113 addi sp,sp,-16 # ff0 <__global_pointer$+0x3ec>
8000202c: 00112623 sw ra,12(sp)
80002030: fddff0ef jal ra,8000200c <_Z5setupv>
||: （循环）
8000200c <_Z5setupv>:
8000200c: 00008067 ret
80002010 <_Z4loopv>:
80002010: 00180813 addi a6,a6,1
80002014: 00a00793 li a5,10
80002018: 00000013 nop
8000201c: fff78793 addi a5,a5,-1
80002020: fe079ce3 bnez a5,80002018 <_Z4loopv+0x8>
80002024: 00008067 ret
80002028 <main>:
80002028: ff010113 addi sp,sp,-16 # ff0 <__global_pointer$+0x3ec>
8000202c: 00112623 sw ra,12(sp)
80002030: fddff0ef jal ra,8000200c <_Z5setupv>
:||
```
由于没有实现ret指令和bnez指令，所以只有jal和j进行了跳转，其它都是顺序执行。
四、小结
通过这样的IDE+TOOLCHAIN组合，我们建立了自研CPU核的小生态闭环
内核实现，外设，指令集，开发工具，编译环境。
没错，商用的MCU也是这些，只是规模和复杂度增加了几个数量级，但是原理基本一致。
