在Unix系统下，root用户所在的组称之为wheel：
# ls -alt
total 18776
drwx------ 6 root wheel 512 Oct 7 01:30 .
-rw------- 1 root wheel 40 Oct 6 17:20 .git-credentials
drwxr-xr-x 7 root wheel 512 Oct 5 17:32 Project
-rw-r--r-- 1 root wheel 56 Oct 5 17:28 .gdbinit
......
Wheel来源于big wheel这个俚语，指有强大“威力”或“影响力”的人。随之而来的就是TENEX操作系统发明了wheel bit，一个允许处理器做一些限制性的操作的“特权bit”。Unix操作系统则在上世纪八十年代引入wheel，这就是为什么root用户所在的组称之为wheel的来历。
参考资料：
Wheel (Unix term)。
以下摘自The Little Book of Semaphores：
A semaphore is like an integer, with three differences:
1. When you create the semaphore, you can initialize its value to any integer, but after that the only operations you are allowed to perform are increment (increase by one) and decrement (decrease by one). You cannot read the current value of the semaphore.
2. When a thread decrements the semaphore, if the result is negative, the thread blocks itself and cannot continue until another thread increments the semaphore.
3. When a thread increments the semaphore, if there are other threads waiting, one of the waiting threads gets unblocked.
To say that a thread blocks itself (or simply “blocks”) is to say that it notifies the scheduler that it cannot proceed. The scheduler will prevent the thread from running until an event occurs that causes the thread to become unblocked. In the tradition of mixed metaphors in computer science, unblocking is often called “waking”.
That’s all there is to the definition, but there are some consequences of the definition you might want to think about.
• In general, there is no way to know before a thread decrements a semaphore whether it will block or not (in specific cases you might be able to prove that it will or will not).
• After a thread increments a semaphore and another thread gets woken up, both threads continue running concurrently. There is no way to know which thread, if either, will continue immediately.
• When you signal a semaphore, you don’t necessarily know whether another thread is waiting, so the number of unblocked threads may be zero or one.
Finally, you might want to think about what the value of the semaphore means. If the value is positive, then it represents the number of threads that can decrement without blocking. If it is negative, then it represents the number of threads that have blocked and are waiting. If the value is zero, it means there are no threads waiting, but if a thread tries to decrement, it will block.
semaphore的初始值为1，通常就用来构建mutex。
本文是RESTful Web Services: A Tutorial的笔记：
REST stands for Representational State Transfer, which is an architectural style for networked hypermedia applications, it is primarily used to build Web services that are lightweight, maintainable, and scalable. A service based on REST is called a RESTful service. REST is not dependent on any protocol, but almost every RESTful service uses HTTP as its underlying protocol.
尽管REST本身不依赖任何协议，但是事实上RESTful服务基本都使用HTTP协议。
RESTful服务本身的特性：
Every system uses resources. These resources can be pictures, video files, Web pages, business information, or anything that can be represented in a computer-based system. The purpose of a service is to provide a window to its clients so that they can access these resources. Service architects and developers want this service to be easy to implement, maintainable, extensible, and scalable. A RESTful design promises that and more. In general, RESTful services should have following properties and features, which I’ll describe in detail:
Representations
Messages
URIs
Uniform interface
Stateless
Links between resources
Caching
具体来说：
Representations：使用JSON或XML；
Messages，URIs和Uniform interface：使用HTTP协议；
Stateless：很重要，不同的request之间不能有依赖。
这篇笔记来自于stackoverflow的一篇帖子，答案如下：
From the documentation(http://www.kernel.org/doc/Documentation/devices.txt):
/dev/tty Current TTY device
/dev/console System console
/dev/tty0 Current virtual console
In the good old days /dev/console was System Administrator console. And TTYs were users' serial devices attached to a server.
Now /dev/console and /dev/tty0 represent current display and usually are the same. You can override it for example by adding console=ttyS0 to grub.conf. After that your /dev/tty0 is a monitor and /dev/console is /dev/ttyS0.
An exercise to show the difference between /dev/tty and /dev/tty0:
Switch to the 2nd console by pressing Ctrl+Alt+F2. Login as root. Type "sleep 5; echo tty0 > /dev/tty0". Press Enter and switch to the 3rd console by pressing Alt+F3.
Now switch back to the 2nd console by pressing Alt+F2. Type "sleep 5; echo tty > /dev/tty", press Enter and switch to the 3rd console.
You can see that "tty" is the console where process starts, and "tty0" is a always current console.
早些时候，/dev/console是系统管理员控制台，而TTYs则代表用户连接服务器的串行设备。而现在，/dev/console和/dev/tty0均指当前的显示设备，并且通常情况下是一样的。你可以修改/dev/console所关联的设备。举个例子，在grub.conf中加入console=ttyS0。则现在，/dev/tty0所关联的是显示器，而dev/console则关联/dev/ttyS0。
/dev/tty是当前进程控制的tty设备，而tty0则是当前的控制台。当你在一个终端执行“sleep 5; echo tty0 > /dev/tty0”命令后，切换到其它终端，则tty0会在你切换后的终端显示。而执行“sleep 5; echo tty > /dev/tty”命令后，无论切换到那个终端，tty始终会在输入命令的终端显示。
BIOS（Basic Input/Output System）是固化在主板芯片上的一段代码。电脑启动时，由BIOS负责启动各个硬件单元，并把“控制权”交给操作系统。BIOS使用MBR（Master Boot Record，存储电脑的分区表）来决定使用哪个操作系统。BIOS为用户提供了一个操作硬件的接口。
UEFI（Unified Extensible Firmware Interface）是BIOS的继任者。它同BIOS的比较如下：
a）16-bit vs 32/64-bit
BIOS只能工作在16-bit处理器模式上，且只能访问1M内存。而UEFI则可工作在32/64-bit处理器模式上，且可访问更大的内存空间。
b）Booting
MBR限制每个磁盘只能有4个分区，且可以boot的磁盘大小限制在2.2TB。而UEFI使用GUID Partition Table，可以访问更大的磁盘。
c）Extensions
UEFI支持老的extensions（比如，ACPI）。
参考资料：
HTG Explains: Learn How UEFI Will Replace Your PC’s BIOS
“Emulation”是软件模拟硬件，即在你当前的host机器（举个例子：X86）上运行另一个SPARC平台的的虚拟机。软件需要把SPARC平台指令转化为X86指令，所以速度很慢。（qemu-system-x86_64）
“Hardware virtualization”是硬件支持虚拟化。即软件直接利用CPU和芯片组等硬件。由于没有指令转化，所以速度很快。当然，host机器和虚拟机是同样的指令集。（qemu-system-x86_64 -enable-kvm）
Linux现在支持多个PCI domain，每个PCI domain管理256个bus（2^8），每个bus上可以挂载32（2^5）个设备，每个设备最多支持8（2^3）个function。同一PCI bus上的设备共享memory location和I/O port的地址空间，而configuration register则不是。每个PCI设备的configuration register占256个字节。
执行“lspci -D”命令显示当前系统的PCI设备信息。格式为：domain:bus:dev:func：
[root@localhost ~]# lspci -D
0000:00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
0000:00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
0000:00:01.1 IDE interface: Intel Corporation 82371AB/EB/MB PIIX4 IDE (rev 01)
0000:00:02.0 VGA compatible controller: InnoTek Systemberatung GmbH VirtualBox Graphics Adapter
0000:00:03.0 Ethernet controller: Intel Corporation 82540EM Gigabit Ethernet Controller (rev 02)
0000:00:04.0 System peripheral: InnoTek Systemberatung GmbH VirtualBox Guest Service
0000:00:05.0 Multimedia audio controller: Intel Corporation 82801AA AC'97 Audio Controller (rev 01)
0000:00:06.0 USB controller: Apple Inc. KeyLargo/Intrepid USB
0000:00:07.0 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 08)
0000:00:0b.0 USB controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB2 EHCI Controller
0000:00:0d.0 SATA controller: Intel Corporation 82801HM/HEM (ICH8M/ICH8M-E) SATA Controller [AHCI mode] (rev 02)“Intel VT-d（以下简称VT-d）”代表“Intel Virtualization Technology for Directed I/O”。“VT（Virtualization Technology）”泛指Intel所有的虚拟化技术，而“VT-d”则是虚拟化技术解决方案中的一种。VT-d的整体思想就是用硬件方式支持隔离和限制对设备的访问。
VT-d有以下主要功能：
a）分配I/O设备：这个功能允许管理员根据需求，灵活地为虚拟机分配I/O设备。
b）DMA remapping：支持针对虚拟机DMA访问的地址转换。
c）Interrupt remapping：支持虚拟机对设备中断的路由和隔离。
d）可靠性功能：记录关于DMA和Interrupt的错误访问。
参考资料：
Understanding VT-d: Intel Virtualization Technology for Directed I/O。
在计算机领域，IOMMU（Input/Output Memory Management Unit）是一个内存管理单元（Memory Management Unit），它的作用是连接DMA-capable I/O总线（Direct Memory Access-capable I/O Bus）和主存（main memory）。传统的内存管理单元会把CPU访问的虚拟地址转化成实际的物理地址。而IOMMU则是把设备（device）访问的虚拟地址转化成物理地址。为了防止设备错误地访问内存，有些IOMMU还提供了访问内存保护机制。参考下图：
IOMMU的一个重要用途是在虚拟化技术（virtualization）：虚拟机上运行的操作系统（guest OS）通常不知道它所访问的host-physical内存地址。如果要进行DMA操作，就有可能破坏内存，因为实际的硬件（hardware）不知道guest-physical和host-physical内存地址之间的映射关系。IOMMU根据guest-physical和host-physical内存地址之间的转换表（translation table），re-mapping硬件访问的地址，就可以解决这个问题。
另外，在AMD的VIRTUALIZING IO THROUGH THE IO MEMORY MANAGEMENT UNIT (IOMMU)文档中，也有一个更全面的总结图：
参考资料：
IOMMU。
DMA（Direct Memory Access）是指在现代计算机系统上，外接设备可以不用CPU干预，直接把数据传输到内存的技术。
DMA控制器（controller）是一种特殊的硬件，它用来管理数据传输和总线仲裁。当要发起数据传输时，它会发一个申请使用系统总线的DMA请求信号给CPU，CPU完成当前操作后，就会让出系统总线，同时会发一个DMA确认信号给DMA控制器。接下来，DMA控制器接管系统总线，开始数据传输。数据传输完毕后，DMA控制器会通知CPU重新接管总线。
正常情况下，CPU全权负责内存的读写操作，而DMA技术可以把CPU解放出来，这将使计算机性能得到显著改善。
参考资料：
DMA (Direct Memory Access)。