---
title: Embedded System Final
---

* Mid term questions are skipped
* Progress: 1,2,3,4,5,6,7,8,9,10,11,12,13 (total 13)
# 1. Device drivers are broadly classified into two basic categories: character devices and block devices. Please individually describe them. (10%)
## Appeared in
99, 99_2, 101, 102, 104, 105_1, 105_2, 106, 108
## Derivation
* Please simply describe the Device Driver concepts. (10%) 103_2
* 驅動程式，允許High-Level的軟體與硬體溝通的程式，建立了一個硬體與硬體，或硬體與軟體溝通的介面，讓device可以交換資料。
## Answer
### Character devices
一個序列的資料流，以不定長度的字元來傳送資料

* Character devices are accessed as stream of data.
* e.g. Serial port

### block devices
以一個block為單位，在可定址的隨機位址可讀取或寫入資料
* Read and write blocks of data from random locations on addressable medium.
* e,g, disks
# 2. Please refer to below example code of a Linux driver and then describe the functionalities of Section 1 and Section 2.

static int my_major = 0;
…
If (my_major) {
	// Section 1
	dev = MKDEV(my_major, my_minor);
	result = register_chrdev_region(dev, nr_devs, “mydev”);
} else {
	// Section 2
	result = alloc_chrdev_region(&dev, my_minor, nr_devs, “mydev”);
	my_major = MAJOR(dev);
}
If (result < 0){
	printk(KERN_WARNING “mydev: can’t get major %d\n”, my_major);
	return result;
}

## Appeared in
103, 105_1, 106, 108
## Answer
Section 1:
* pre assigned
* if my_major!=0, register device by using the assigned ***mymajor, myminor***

Section 2:
* dynamic alloc (動態配置號碼)
* else, dynamically register device
# 3.What is the Proc File System? Please describe the functionalities of /proc file system. (10%)
## Appeared in
101, 103_2, 105_1, 105_2, 106, 108
## Answer
一個虛擬的檔案系統，實現Linux核心空間與使用者空間的通訊
* Interface that allows the kernel to communicate information about each running process on a Linux system.
* Communication between kernel space and user space
# 4. What is Memory Technology Devices (MTD) subsystem? (10%)
## Appeared in
99, 99_2, 101, 102, 103, 103_2, 104, 105, 106, 108
## Answer
Application和driver中間的虛擬層。讓Application用相同的指令對不同的flash driver做相同的事，且可以設定欲使用的driver型號。
* A type of device file to interact with flash memory.
* The MTD layer architecture enables the separation of the lowlevel device complexities from the higher-layer data organization and storage formats that use memory devices.

# 5. Why the engineers usually mount a NFS to develop an embedded system? Please point out the advantages of NFS for building an embedded system. (10%)
## Appeared in
99_2, 101, 104, 105_1, 105_2, 106, 108
## Answer

使用NFS可以把程式透過網路的方式將資料儲存在其他地方，如要使用或修改的時候一樣在透過網路的方式進行，讓儲存空間不在受限於硬體儲存空間。
可以直接在NFS上的系統安裝其他工具程式直接進行編譯，可以增加程式開發與效能的速度。

* Network File System
* NFS enables you to export a directory on an NFS server and mount that directory on a remote client machine as if it were a local file system. In other words, access file on a server machine like it is a local file system.
* developer can have access to a huge number of files, libraries, tools, and utilities during development and debugging, even if the target embedded system is resource constrained.
# 6. Please list the advantages of cramfs. (10%)
## Appeared in
102, 104, 105_1, 106, 108
## Answer
cramfs是專門針對快閃記憶體設計的唯讀壓縮的檔案系統，不是將檔案系統中的所有內容解壓到ROM中，而是系統需要訪問某個位置的資料時再將其解壓縮到ROM中，節省ROM的儲存空間，使用簡單，加載容易，速度快
* A read-only ,compressed file system. Simple and small, ideal for boot ROMS
# 7. What is the BusyBox in an embedded system? How to add a new command into the embedded system except the commands that BusyBox provides. (10%)
## Appeared in
99, 99_2, 102,103, 103_2, 104, 105, 106, 108
## Answer
把常用的指令和參數壓縮成一個檔案
* BusyBox provides compact replacements for many traditional fullblown utilities found on most desktop and embedded Linux distributions.

1.在Busybox中勾選欲增加的指令
2.將欲增加的指令Source Code編譯後放在\bin中

* add a new command
* 1. cross compile c then copy to bin
* 2. 修改busybox設定

# 8. Please describe the functionalities and their advantages of DHCP server and NFS server. (10%)
## Appeared in
103
## Answer
#### DHCP
DHCP server: 給內部網絡或網絡服務供應商自動分配IP位址、主機名、DNS伺服器、域名，方便管理。在子網移動中不需要重新設置IP，自動獲取就行。

* A DHCP server controls the IP address assignments for IP subnets for which it has been configured, and for DHCP or BOOTP clients that have been configured to participate. A DHCP server listens for requests from a DHCP client (such as your target board), and assigns addresses and other pertinent information to the client as part of the boot process.

#### NFS
NFS server: 讓不同的機器、不同的作業系統透過網路可以彼此分享個別的檔案。將常用的數據存放在一台服務器，可以通過網路訪問，減少本地端磁碟空間的使用。
* Network File System
* Your root file system is not size-restricted by your board's own limited resources, such as Flash memory.
* Changes made to your application files during development are immediately available to your target system.
* You can debug and boot your kernel before developing and debugging your root file
system.
# 9 Fig. 1 shows a typical DHCP exchange, please explain the each step (4 steps) in Fig. 1. (10%)
## Appeared in
103_2, 105_1
## Answer
1. Client要求租用IP位址
* The sequence starts with the client (target) transmitting a broadcast frame attempting to discover a DHCP server. This is shown by the DHCPDISCOVER message shown.
2. DHCP提供可租用的IP位址
* The server responds (if it has been so configured and enabled) by offering an IP address for the client. This is evidenced by the DHCPOFFER message.
3. Client確認DHCP的IP租約
* The client then responds by testing this IP address locally. The testing includes sending the DHCPREQUEST packet to the DHCP server, as shown.
4. DHCP Server同意IP租約
* Finally, the server responds by acknowledging the IP address assignment to the client, thus completing the automatic target configuration.
# 10. What advantages of Ext3 File System compared to Ext2 File System? How to convert an Ext2 File System to the Ext3 File System? (10%)
## Appeared in
103, 105_1, 106, 108
## Answer
### Advantage
Ext3加入了日誌檔用來記錄之前做了哪些事情，提高檔案系統的完整性，避免意外當機對檔案系統的破壞。
* Availability: By journaling, after an unexpected power failure or system crash, file system check is not necessary
* Data complete: Journaling can prevent the data damage.
* speed: ext3's journaling optimizes hard drive head motion.
* Easy to convert: Don't have to reformat to migrate from ext2 to ext3
### Convert
先將要轉換的磁碟先mount上去，再輸入"tune2fs -j 磁碟名稱"，就會產生一個日誌檔
* mount ext2 file system
`mount /dev/sdb1 /mnt/flash`
* create journal
`tune2fs -j /dev/sdb1`
# 11 How to build a simple EXT2 file system and modify its content (add and delete directories, make device nodes, and so on).
## Appeared in
105_1, 106, 108
## Answer
在一個一般的檔案中使用Linux loopback device建立一個EXT2 file system image，建立好之後我們就能在這個檔案裡擁有一個 EXT2 file system，如此一來就能對這個一般的檔案像對block device進行操作和調整。

### Create file image
* mke2fs
### make directory
*　mkdir
### remove directory
* rmdir or rm
### make device node
* mknod
# 12 Please describe the functionalities of below module utilities. (10%)
(1) insmod
(2) lsmod
(3) modprobe
(4) depmod
(5) rmmod
## Appeared in
106, 108
## Answer
### insmod
載入模組到kernel
* insert a module into a running kernel.
### lsmod
列出目前已經載入的driver
* displays a formatted list of the modules that are inserted into the kernel
### modprobe
Linux kernel新增或刪除模組
*和insmod不同的是此指令可以根據模組相依性載入
* discover this relationship and load the dependent modules in the proper order.
### depmod
查看這支driver是否與其他driver有相依性，就是A driver要能動必須先安裝B driver，生成modules.dep檔案和對映檔案。
* When modprobe is executed, it searches for a file called modules.dep in the same location where the modules are installed
### rmmod
刪除kernel中一個正在執行的模組
* It simply removes a module from a running kernel
# 13 Please list the advantages of JFFS2. (10%)
## Appeared in
99
## Answer
1. 支援 NAND flash memory 裝置。
2. 提供Hard links support，JFFS只能用soft link。
3. 提供zlib、rubin、rtime壓縮。
4. 比JFFS有更好的效能，透過演算法去收集垃圾資料將其清除。
* Support for NAND flash devices
* Hard links
* Compression
* Better performance: JFFS treated the disk as a purelycircular log. This generated a great deal of unnecessary I/O. The garbage collection algorithm in JFFS2 makes this mostly unnecessary.

# 14 What is the Initial RAM Disk? What is the purpose of Initial RAM Disk?
## Answer
* (參考某題的答案)Initial ramdisk is a Linux kernel feature to mount an early root file system to perform certain startup-related system initialization and configuration
* The initial RAM disk is a small self-contained root file system that usually contains directives to load specific device drivers before the completion of the boot cycle.
* 中文解釋：初始RAM磁盤（Initial RAM Disk）是Linux系統中的一個臨時根文件系統，用於在系統引導過程中掛載以支持兩階段引導過程。它包含各種可執行文件和驅動程序，允許掛載真正的根文件系統，然後卸載initrd RAM磁盤並釋放其內存。在許多嵌入式Linux系統中，initrd是最終的根文件系統。初始RAM磁盤（initrd）和initramfs是Linux系統中加載臨時根文件系統的兩種方法。它們常用於在實際的根文件系統可以掛載之前進行準備工作。當使用initrd時，系統通常按以下步驟啟動：引導加載程序加載內核和初始RAM磁盤，內核將initrd轉換為“正常”的RAM磁盤，並釋放initrd使用的內存。如果根設備不是/dev/ram0，則會遵循舊的（已棄用的）change_root過程。初始RAM磁盤的目的是在系統引導過程中提供一個臨時的根文件系統，以便進行各種準備工作，例如**加載驅動程序、設置設備、配置網絡**等。它允許在實際的根文件系統被掛載之前執行必要的操作，從而確保系統能夠成功引導並進入正常運行狀態。一旦真正的根文件系統準備就緒，初始RAM磁盤就會被卸載，系統將轉移到真正的根文件系統上進行操作。總結起來，**初始RAM磁盤是Linux系統中的一個臨時根文件系統，用於在引導過程中提供必要的準備工作，以確保系統能夠成功引導並進入正常運行狀態**。

###### tags: `Embedded system` `CSnote`