Plug and Play

http://baike.baidu.com/view/33701.htm

即插即用

编辑

 

PNP是Plug-and-Play（即插即用）的缩写。它的作用是自动配置（低层）计算机中的板卡和其他设备，然后告诉对应的设备都做了什么。PnP的任务是把物理设备和软件（设备驱动程序）相配合，并操作设备，在每个设备和它的驱动程序之间建立通信信道。换种说法，PnP分配下列资源给设备和硬件：I/O地址、IRQ、DMA通道和内存段。

 

1解析编辑

PnP是即插即用的意思。

在PnP技术出现之前，中断和I/O端口的分配是由人手工进行的，您想要这块声卡占用中断5，就找一个小跳线在卡上标着中断5的针脚上一插。这样的操作需要用户了解中断和I/O端口的知识，并且能够自己分配中断地址而不发生冲突，对普通用户提出这样的要求是不切实际的。

PnP技术就是用来解决这个问题的，PnP技术将自动找到一个不冲突的中断和I/O地址分配给外部设备，而完全不需要人工干预。但是如果您读懂了上面关于中断冲突的那一部分，您就应该了解，在中断资源非常紧张的今天，即使是PnP技术，也不一定能找到一个合适的中断分配给您刚刚插入的设备，所以尽量释放那些没有必要的中断，对PnP正常工作也是很有帮助的。

有些PnP冲突来源于主板的设计。许多主板上有一个AGP插槽、五个PCI插槽和两个ISA插槽，而其中的AGP插槽一般是和一个PCI插槽共用一个中断的，也就是这两个槽的中断可以是合理的任何值，但必须是相同的，当您在AGP槽上插了显示卡，如果您还在同中断的PCI槽上插了一块声卡的话，就一定会产生中断冲突。对大多数五个PCI插槽的主板而言，第一和第五插槽是和AGP槽共用中断的，第四插槽是和USB共用中断的，虽然可以在BIOS设置中强制给每个PCI槽位分配中断，但这对于大多数用户来说都是一个比较困难的事情，而且由于第二和第三插槽没有和其它设备共用中断，所以，在大多数情况下，显卡插在AGP槽上，声卡和其它PCI插卡，比如网卡，应该插在第二和第三PCI插槽上，也就是从AGP插槽向ISA插槽方向数的第二和第三PCI插槽上，这样一般都不用改变BIOS中的PnP设置，就能让这些卡正常工作。[1]

2应用编辑

即插即用是一个用于自动处理PC机硬件设备安装的工业标准，由Intel和Microsoft联合制定。通常，当您需要安装新的硬件时，往往要考虑到该设备所使用的DMA和IRQ资源，以避免设备之间因竞争而出现冲突，甚至导致机器无法正常工作。

有了“即插即用”(PnP)，它使得硬件设备的安装大大简化，您无须再做跳线，也不必使用软件配置程序，但是您所安装的新硬件必须是符合PnP规范的，否则是行不通的。

即插即用代表着接口技术的主要进展。但它不是一个全新的概念。它是MCA与EISA接口设计的关键特性，但是MCA和EISA有限的吸引力使得它没有成为行业标准。因此，主流PC用户仍然为I/O地址，DMA通道以及IRQ的设置担忧。早期基于PCI的系统也使用了一种PnP配置的方式，但由于没有提供PCI插卡和ISA插卡冲突的管理措施，许多用户仍然为一些配置问题而烦恼。但即插即用规范可用于基于ISA、PCI、SCSI、IDE和PCMCIA的系统中，所有新计算机的购买者不用再担心硬件设置了。

为了使即插即用正常工作，需要以下的部件：

·即插即用硬件。

·即插即用BIOS。

·即插即用操作系统。

这些部件都要求兼容即插即用，意味着它们要服从即插即用规范。

1．硬件部件

硬件部件包括计算机系统与适配卡。这并不意味着在即插即用系统中不能使用较老的ISA适配卡。可以使用这些插卡，实际上，即插即用BIOS自动围绕存在的遗留部件重新指定即插即用兼容插卡的设置。另外，许多后期的ISA插卡也可以转换到即插即用模式下。即插即用适配卡同系统BIOS和操作系统通信来传播关于所需系统资源的信息。然后，BIOS和操作系统解决冲突（如果有的话）并通知适配卡应当使用哪些特定资源。适配卡便改变其自身的配置以使用特定的资源。

2．BIOS部件

BIOS部件意味着多数较老PC机的使用者需要升级他们的BIOS，或者购买新的具有PnP　BIOS的机器。BIOS要成为兼容的，必须支持13个附加的系统功能调用，它们可以被即插即用系统的操作系统部件使用。PnP BIOS规范由Compaq、Intel和Phoenix Technologies共同发展。

BIOS的PnP特性通过一个扩展的POST实现。BIOS负责鉴别、隔离和配置PnP适配卡。BIOS通过以下的步骤实现这些任务：

1．禁用所有主板和适配卡上配置的设备。

2．鉴别所有PnP PCI或ISA设备。

3．为端口、IRQ、DMA及存储器生成一个最初的资源分配图。

4．启用I/O设备。

5．扫描ISA设备的ROM。

6．配置最初的载人程序设备，用于后来启动系统。

7．通过通知分配了哪些资源来启用可配置的设备。

8．开始载人启动程序。

9．将控制传递给操作系统。

3．操作系统部件

操作系统部件可以通过多数新系统实现，如Windows 9x/Me/2000/XP。有时，系统供应商为其指定的硬件提供了对操作系统的扩展，如在笔记本系统中更是如此。如果系统需要，要保证这些扩展已经安装在系统中。

通知用户不能由BIOS解决的冲突是操作系统的责任。依靠使用操作系统的经验，用户可以手工设置冲突的插卡，或者关闭系统并在插卡上设置开关。当系统重启后，系统检测保持的（或新的）冲突，这些都提示用户要注意。通过这样的过程，可以解决所有的系统冲突。[2]

3由来编辑

对于许多PC用户而言，如果需要为你的PC增加一些新的功能，比如欣赏CD唱盘、看VCD影碟、收发传真等等，那么就得为你的PC机增加一些新的设备，如声卡、CD-ROM、MPEG硬件解压卡、FAX/MODEM传真卡等一系列新设备，通常而言，安装新设备的过程为：

1．按照所购买设备的说明来设置开关和跳线。

2．正确地将新设备按规定的要求插入主板扩展槽中。　3．安装随新设备所配的驱动程序。

4．重新引导系统。

然后，你很有可能得花费大量的时间和精力来处理安装过程中所出现的种种问题。比如手工改变设备的开关或跳线，跟踪CONFIG。SYS文件以解决内部驻留程序的冲突等等。原因在于一台PC机至少有一个或者多个总线设备（BUSDEVICE），而多个总线设备则有可能共享相同的系统总线（SYSTEMBUS），于是这就不可避免地产生硬件资源上的冲突。轻者是新安装的设备无法使用或系统无法启动，严重的话就会导致系统的崩溃。怎样去解决这个问题？人们希望系统应该能提供对底层硬件资源包括IRQ（中断请求）、I/O（输入/输出）端口、DMA（直接内存存取）通道以及内容等的智能管理能力，免除用户因安装新的硬件设备而带来的烦恼。

随着微软推出WINDOWS95/98，它也宣传了WINDOWS95/98为用户提供的底层硬件资源的智能管理能力，即WINDOWS95/98具有即插即用（PNP：PlugandPlay）的功能，由此即插即用才为人们广泛重视。

即插即用这个概念很久以前就已被提出，它的标准则始于几种洞庭湖的解决PC硬件配置问题的方案，其中包括IBM的微通道总线以及COMPAQ公司在EISA（扩展标准工业系统总线）上的不懈努力。

而欲使系统在易于使用上有一个飞跃，那么它就必须解决多个总线设备共享系统总线时所带来的系统底层资源的分配和再分配问题。MICROSFOT同INTEL和COMPAQ的结盟促使其在1993年以后相继颂了即插即用的规范，包括其他总线类型如ISA、EISA、PCMCIA、PCI、VESA以及SCSI等，但是就现阶段而言，完全意义上的即插即用恐怕还需要一段时间才能够进一步完善。

MICROSFOT在WINDOWSFORWORKGROUPS和WINDOWSNT上就已开始尝试一些设备检测和配置方面的试验。在WINDOWSFORWORKGROUPS中，操作系统可自动检测出视频适配卡、鼠标、键盘及网络适配卡等的类型。WINDOWSNT则更进一步，它可以检测出SCSI设备及其他一些硬件，到了WINDOWS95/98，则几乎可以实现全自动的安装和系统的动态重配置。

尽管WINDOWS95不可能对所有旧设备都能正确地识别，甚至有时也会求助于用户，但就即插即用本身而言，这已经是一个真正的飞跃了。

在BIOS中“PNP/PCICONFIGRATION”中有一条“PNPOSInstalled”一项，应该设为“Yes”，让您的Windows能够得到BIOS的PnP设置。[2]

 

Legacy Plug and Play

http://en.wikipedia.org/wiki/Legacy_Plug_and_Play

The term Legacy Plug and Play,^[1] also shortened to PnP,^[2] describes a series of specifications and Microsoft Windows features geared towards operating system configuration of devices. The standards were primarily aimed at the IBM PC standard bus, later dubbed ISA. Related specifications are also defined for the common external or specialist busses commonly attached via ISA at the time of development, including RS-232 and parallel port devices.

As a Windows feature, Plug and Play refers to operating system functionality that supports connectivity, configuration and management with native plug and play devices.^[3]Originally considered part of the same feature set as the specifications, Plug and Play in this context refers primarily to the responsibilities and interfaces associated with Windows driver development.^[4]

Plug and Play allows for detection of devices without user intervention, and occasionally for minor configuration of device resources, such as I/O ports and device memory maps. PnP is a specific set of standards, not be confused with the generic term plug and play, which describes any hardware specification that alleviates the need for user configuration of device resources.^[5]

Contents

  [hide] 

• 1 Overview
• 2 Specifications
• 3 Requirements
• 4 Hardware identification
• 5 See also
• 6 References
• 7 External links

Overview[edit]

The Plug and Play standard requires configuration of devices to be handled by system firmware, which then provides details of resources allocations to the operating system. The process is invoked at boot time. When the computer is first turned on, compatible devices are identified and assigned non-conflicting addresses and interrupt request numbers.

The term Plug and Play was first used by Microsoft in reference to their Windows 95 product. Other operating systems, such as AmigaOS Autoconfig and the Mac OS NuBussystem, had already supported such features for some time (under various names, or no name^[6]), but the term plug and play gradually became universal due to worldwide acceptance of Windows.^{[citation needed]}

Typically, non-PnP devices need to be identified in the computer's BIOS setup so that the PnP system will not reassign those devices. Problems in the interactions between legacy non-PnP devices and the PnP system can cause it to fail, leading to this technology having historically been referred to as "plug and pray".^[2]

Specifications[edit]

Legacy Plug and Play was defined in Microsoft and Intel specifications, which proposed changes to legacy hardware, as well as the BIOS to support operating system-bound discovery of devices. These roles were later assumed by the ACPI standard,^[1] which also moves support for power management and configuration into the operating system, as opposed to the firmware as previously required by the "Plug and Play BIOS" and APM specifications. The following standards compose what Microsoft describe as Legacy Plug and Play, as opposed to native Plug-and-Play specifications such as PCI and USB.

• Plug and Play BIOS Specification
• Plug and Play ISA Specification
• Plug and Play Design Specification for IEEE 1394
• Plug and Play External COM Device Specification
• Plug and Play Parallel Port Device Specification
• Plug and Play SCSI Specification
• Legacy Plug and Play Guidelines

Apart from the Plug and Play BIOS Specification, all standards are still supported by Microsoft.^[1] However, support for them as opposed to Advanced Configuration and Power Interface will be removed in future version of Windows.^[7]

Requirements[edit]

A PnP-compatible computer must meet three requirements:

1. The OS must be PnP-compatible.
2. The BIOS must support PnP.
3. The device to be installed must be a PnP device.

Hardware identification[edit]

Plug-and-play hardware typically also requires some sort of ID code that it can supply, in order for the computer software to correctly identify it.

This ID code system was not integrated into the early Industry Standard Architecture (ISA) hardware common in PCs when Plug and Play was first introduced. ISA Plug and Play caused some of the greatest difficulties that made PnP initially very unreliable. This led to the derisive term "Plug and Pray", since I/O addresses and IRQ lines were often set incorrectly in the early days. Later computer buses like MCA, EISA and PCI (which was becoming the industry standard at that time) integrated this functionality.

Finally, the operating system of the computer needs to be able to handle these changes. Typically, this means looking for interrupts from the bus saying that the configuration has changed, and then reading the information from the bus to locate what happened. Older bus designs often required the entire system to be read in order to locate these changes, which can be time consuming for lots of devices. More modern designs use some sort of system to either reduce or eliminate this "hunt"; for example, USB uses a hub system for this purpose.

When the change is located, the OS then examines the information in the device to figure out what it is. It then has to load up the appropriate device drivers in order to make it work. In the past, this was an all-or-nothing affair, but modern operating systems often include the ability to find the proper driver on the Internet and install it automatically.

Plug and play

http://en.wikipedia.org/wiki/Plug_and_play

In computing, a plug and play device or computer bus, is one with a specification that facilitates the discovery of a hardware component in a system without the need for physical device configuration or user intervention in resolving resource conflicts.^[1][2]

Plug and play devices can be due to boot-time assignment of device resources and to hotplug systems such as USB and IEEE 1394 (FireWire).^[3]

Contents

  [hide] 

• 1 History of device configuration
• 2 First attempts at self-configuration
  • 2.1 NuBus
  • 2.2 Zorro bus
  • 2.3 Micro-Channel Architecture
• 3 ISA and PCI self-configuration
• 4 Plug and Play
• 5 Current plug and play interfaces
• 6 See also
• 7 References
• 8 External links

History of device configuration[edit]

IBM 402 Accounting Machine control panel wiring. This control panel was labeled "profit & loss summary".

In the beginnings of data processing technology, the hardware was just a collection of modules, and the functions of those modules had to be linked to accommodate different calculating operations. This linking was usually done by connecting some wires between modules and disconnecting others. For many mechanical data processing machines, such as the IBM punched card accounting machines, their calculating operations were directed by the use of a quick-swap control panel wired to route signals between module sockets.

As general purpose computing devices developed, these connections and disconnections were instead used to specify locations in the system address space where an expansion device should appear, in order for the device to be accessible by the central processing unit. If two or more of the same type of device were installed in one computer, it would be necessary to assign the second device to a separate, non-overlapping region of the system address space so that both could be accessible at the same time.

Apple II serial interface card that required cutting and soldering to reconfigure. The user would cut the wire trace between the >< cones at X1 and X3 and solder the <> cones together at X2 and X4. Located vertically near center of board.

Some early microcomputing devices such as the Apple II sometimes required the end-user to physically cut some wires and solder others together to make configuration changes. The changes were intended to be largely permanent for the life of the hardware.

As computers became more accessible to the general public, the need developed for more frequent changes to be made by computer users unskilled with using soldering irons. Rather than cutting and soldering connections, configuration was accomplished by jumpers orDIP switches.

Left: Jumper blocks of various sizes,Right: a slide-style DIP switch with 8 switches

However, the process of configuring devices manually using jumpers or DIP switches could be quite difficult, and there was usually no forgiveness for technical inexperience. Incorrect settings could render either the whole system or just the expansion device completely or partially inoperable. Some settings such as for interrupts (IRQ) or direct memory access (DMA) required knowledge of what configuration had been previously manually assigned to other devices, as well as what settings the main system itself may be using. The system might still seem to work properly with an incorrect setting, until the IRQ or DMA is actually needed and the entire system suddenly freezes and must be reset.

First attempts at self-configuration[edit]

As computing devices spread further out into the general population, there was ever greater pressure developing to automate this configuration process, and relieve the end-user from the hassle and complexity of having to manually assign these many complex settings.

NuBus[edit]

A NuBus expansion card without jumpers or DIP switches.

In 1984, the NuBus architecture was developed by the Massachusetts Institute of Technology (MIT) as a platform agnostic peripheral interface that fully automated device configuration. The specification was sufficiently intelligent that it could work with both big endian and little endian computer platforms that had previously been mutually incompatible. However, this agnostic approach increased interfacing complexity and required support chips on every device which in the 1980s was expensive to do, so the technology did not gain widespread support.

Zorro bus[edit]

In 1984, Commodore developed the AutoConfig protocol and the Zorro expansion bus for its Amiga line of expandable computers. The first public appearance was in the CES computer show at Las Vegas in 1985, with the so-called "Lorraine" prototype. Like NuBus, Zorro devices had absolutely no jumpers or DIP switches. The Zorro architecture did not spread to general computing use outside of the Amiga product line, but was eventually upgraded for the later iteration of Amiga computers.

Micro-Channel Architecture[edit]

An MCA expansion card without jumpers or DIP switches.

In 1987, IBM released an update to the IBM PC known as the Personal System/2 line of computers using the Micro Channel Architecture. The PS/2 was capable of totally automatic self-configuration. Every piece of expansion hardware was issued with a floppy disk containing a special file used to auto-configure the hardware to work with the computer. The user would install the device, turn on the computer, load the configuration information from the disk, and the hardware automatically assigned interrupts, DMA, and so forth.

However, the disks posed a problem if they were damaged or lost, as the only options at the time to obtain replacements was via postal mail or IBM's dialup BBS service. Without the disks, any new hardware would be completely useless and the computer would not boot at all until the unconfigured device was removed.

Microchannel did not gain widespread support, because IBM wanted to exclude clone manufacturers from this next generation computing platform. Anyone developing for MCA had to sign non-disclosure agreements and pay royalties to IBM for each device sold, putting a price premium on MCA devices. End-users and the clone manufacturers revolted against IBM and developed their own open standards bus, known as EISA. Consequently, MCA receded in use except in IBM's mainframes.

ISA and PCI self-configuration[edit]

In time, many Industry Standard Architecture (ISA) cards incorporated, through proprietary and varied techniques, hardware to self-configure or to provide for software configuration; often, the card came with a configuration program on disk that could automatically set the software-configurable (but not itself self-configuring) hardware. Some cards had both jumpers and software-configuration, with some settings controlled by each; this compromise reduced the number of jumpers that had to be set, while avoiding great expense for certain settings, e.g. nonvolatile registers for a base address setting. The problems of required jumpers continued on, but slowly diminished as more and more devices, both ISA and other types, included extra self-configuration hardware. However, these efforts still did not solve the problem of making sure the end-user has the appropriate software driver for the hardware.

ISA PnP or (legacy) Plug & Play ISA was a plug-n-play system that used a combination of modifications to hardware, the system BIOS, and operating system software to automatically manage resource allocations. It was superseded by the PCI bus during the mid-1990s.

Plug and Play[edit]

Main article: Legacy Plug and Play

In 1995, Microsoft released Windows 95, which tried to fully automate device detection and configuration as much as possible, but could still fall back to manual settings if necessary. During the initial install process of Windows 95, it would attempt to automatically detect all devices installed in the system. Since full auto-detection of everything was a new process without full industry support, the detection process constantly wrote to a progress tracking log file during the detection process. In the event that device probing would fail and the system would freeze, the end-user could reboot the computer, restart the detection process, and the installer would use the tracking log to skip past the point that caused the previous freeze.

At the time, there could be a mix of devices in a system, some capable of automatic configuration, and some still using fully manual settings using jumpers and DIP switches. The old world of DOS still lurked underneath Windows 95, and systems could be configured to load devices three different ways:

• through Windows 95 device manager drivers only
• using DOS drivers loaded in the CONFIG.SYS and AUTOEXEC.BAT configuration files
• using both DOS drivers and Windows 95 device manager drivers together

Microsoft could not assert full control over all device settings, so configuration files could include a mix of driver entries inserted by the Windows 95 automatic configuration process, and could also include driver entries inserted or modified manually by the computer users themselves. The Windows 95 device manager also could offer users a choice of several semi-automatic configurations to try to free up resources for devices that still needed manual configuration.

An example of an ISA interface card with extremely limited interrupt selection options, a common problem on PC ISA interfaces.
Kouwell KW-524J dual serial, dual parallel port, 8-bit ISA, manufactured in 1992:
* Serial 1: IRQ 3/4/9
* Serial 2: IRQ 3/4/9
* Parallel 1: IRQ 5/7
* Parallel 2: IRQ 5/7
(There is no technical limitation that 3,4,5,7,9 could not all be selectable choices for each port.)

Also, although some later ISA devices were capable of automatic configuration, it was common for PC ISA expansion cards to limit themselves to a very small number of choices for interrupt request lines. For example, a network interface might limit itself to only interrupts 3, 7, and 10, while a sound card might limit itself to interrupts 5, 7, and 12. This results in few configuration choices if some of those interrupts are already used by some other device.

The hardware of PC computers additionally limited device expansion options because interrupts could not be shared, and some multifunction expansion cards would use multiple interrupts for different card functions, such as a dual serial port card requiring a separate interrupt for each serial port.

Because of this complex operating environment, the autodetection process sometimes produced incorrect results, especially in systems with large numbers of expansion devices. This led to device conflicts within Windows 95, resulting in devices which were supposed to be fully self-configuring failing to work. The unreliability of the device installation process led to that Plug and Play was sometimes referred to as Plug and Pray.

Up until about 2000, PC computers could still be purchased with a mix of ISA and PCI slots, so it was still possible that manual ISA device configuration might be necessary. But with successive releases of new operating systems like Windows 2000 and Windows XP, Microsoft had sufficient clout to say that drivers would not be provided for older devices that did not support auto-detection. In some cases, the user was forced to purchase new expansion devices or a whole new system to support the next operating system release.

Current plug and play interfaces[edit]

Several completely automated computer interfaces are currently used, each of which requires no device configuration by the computer user, and the only task of the user is to install software for the self-configuring devices.

• IEEE 1394 (FireWire)
• PCI, Mini PCI
• PCI Express, Mini PCI Express
• PCMCIA, PC Card, ExpressCard
• USB

For most of these interfaces, very little technical information is available to the end-user about the performance of the interface. Although both FireWire and USB have bandwidth that must be shared by all devices, most modern operating systems are unable to monitor and report the amount of bandwidth being used or available, or report activity on what devices are currently using the interface.

 

 

 PCGuide Plug and Play

 

Plug and Play

The large variety of different cards that can be added to PCs to expand their capabilities is both a blessing and a curse. As you can see from the other sections that have discussed system resources, configuring the system and dealing with resource conflicts is part of the curse of having so many different non-standard devices on the market. Dealing with these issues can be a tremendously confusing, difficult and time-consuming task. In fact, many users have stated that this is the single most frustrating part of owning and maintaining a PC, or of upgrading the PC's hardware.

In an attempt to resolve this ongoing problem, the Plug and Play (also called PnP) specification was developed by Microsoft with cooperation from Intel and many other hardware manufacturers. The goal of Plug and Play is to create a computer whose hardware and software work together to automatically configure devices and assign resources, to allow for hardware changes and additions without the need for large-scale resource assignment tweaking. As the name suggests, the goal is to be able to just plug in a new device and immediately be able to use it, without complicated setup maneuvers.

A form of Plug and Play was actually first made available on the EISA and MCA buses many years ago. For several reasons, however, neither of these buses caught on and became popular. PnP hit the mainstream in 1995 with the release of Windows 95 and PC hardware designed to work with it.

Requirements for Plug and Play

Automatically detecting and configuring hardware and software is not a simple task. To perform this work, cooperation is required from several hardware and software areas. The four "partners" that must be Plug and Play compliant in order for it to work properly are:

• System Hardware: The hardware on your system, through the system chipset and system bus controllers, must be capable of handling PnP devices. For modern PCI-based systems this is built in, as PCI was designed with PnP in mind. Most PCI-based systems also support PnP on their ISA bus, with special circuitry to link the two together and share resource information. Older PCs with ISA-only or VL-bus system buses generally do not support Plug and Play.
• Peripheral Hardware: The devices that you are adding into the system must themselves be PnP compatible. PnP is now supported for a wide variety of devices, from modems and network cards inside the box to printers and even monitors outside it. These devices must be PnP-aware so that they are capable of identifying themselves when requested, and able to accept resource assignments from the system when they are made.
• The System BIOS: The system BIOS plays a key role in making Plug and Play work. Routines built into the BIOS perform the actual work of collecting information about the different devices and determining what should use which resources. The BIOS also communicates this information to the operating system, which uses it to configure its drivers and other software to make the devices work correctly. In many cases older PCs that have an outdated BIOS but otherwise have support for PnP in hardware (PCI-based Pentiums produced between 1993 and 1995 are the prime candidates) can be made PnP-compliant through a BIOS upgrade.
• The Operating System: Finally, the operating system must be designed to work with the BIOS (and thus indirectly, with the hardware as well). The operating system sets up any low-level software (such as device drivers) that are necessary for the device to be used by applications. It also communicates with the user, notifying him or her of changes to the configuration, and allows changes to be made to resource settings if necessary. Currently, the only mainstream operating system with full PnP support is Windows 95.

As you can see, you need a lot for Plug and Play to work, and this is why the vast majority of older systems (pre-1996) do not properly support this standard.

Plug and Play Operation

Most of the actual work involved in making Plug and Play function is performed by the system BIOS during the boot process. At the appropriate step of the boot process, the BIOS will follow a special procedure to determine and configure the Plug and Play devices in your system. Here is a rough layout of the steps that the BIOS follows at boot time when managing a PCI-based Plug and Play system:

1. Create a resource table of the available IRQs, DMA channels and I/O addresses, excluding any that are reserved for system devices.
2. Search for and identify PnP and non-PnP devices on the PCI and ISA buses.
3. Load the last known system configuration from the ESCD area stored in non-volatile memory.
4. Compare the current configuration to the last known configuration. If they are unchanged, continue with the boot; this part of the boot process ends and the rest of the bootup continues from here.
5. If the configuration is new, begin system reconfiguration. Start with the resource table by eliminating any resources being used by non-PnP devices.
6. Check the BIOS settings to see if any additional system resources have been reserved for use by non-PnP devices and eliminate any of these from the resource table.
7. Assign resources to PnP cards from the resources remaining in the resource table, and inform the devices of their new assignments.
8. Update the ESCD area by saving to it the new system configuration. Most BIOSes will print a message when this happens like "Updating ESCD ... Successful".
9. Continue with the boot.

Tip: See the section on PCI / PnP in the BIOS area, which describes the BIOS settings that affect how PnP works in a PCI system.

Extended System Configuration Data (ESCD)

If the BIOS were to assign resources to each PnP device on every boot, two problems would result. First, it would take time to do something that it has already done before, each boot, for no purpose. After all, most people change their system hardware relatively infrequently. Second and more importantly, it is possible that the BIOS might not always make the same decision when deciding how to allocate resources, and you might find them changing even when the hardware remains unchanged.

ESCD is designed to overcome these problems. The ESCD area is a special part of your BIOS's CMOS memory, where BIOS settings are held. This area of memory is used to hold configuration information for the hardware in your system. At boot time the BIOS checks this area of memory and if no changes have occurred since the last bootup, it knows it doesn't need to configure anything and skips that portion of the boot process.

ESCD is also used as a communications link between the BIOS and the operating system. Both use the ESCD area to read the current status of the hardware and to record changes. Windows 95 reads the ESCD to see if hardware has been changed and react accordingly. Windows 95 also allows users to override Plug and Play resource assignments by manually changing resources in the Device Manager. This information is recorded in the ESCD area so the BIOS knows about the change at the next boot and doesn't try to change the assignment back again.

The ESCD information is stored in a non-volatile CMOS memory area, the same way that standard BIOS settings are stored.

Note: Some (relatively rare) systems using Windows 95 can exhibit strange behavior that is caused by incompatibility between how Windows 95 and the BIOS are using ESCD. This can cause an "Updating ESCD" message to appear each and every time the system is booted, instead of only when the hardware is changed. See here for more details.

Plug and Play and Non-Plug-and-Play Devices

Devices that do not support the PnP standard can be used in a PnP system, but they present special problems. These are called legacy devices, which is geekspeak for "old hardware we have to keep using even though it doesn't have the capabilities we wish it did". :^) They make resource assignment much more difficult because they cannot be automatically configured by the BIOS.

Generally, the BIOS deals with non-PnP devices by ignoring them. It simply considers them as "part of the scenery" and avoids any resources they are using. There is usually no problem using these devices with PnP, but using too many non-PnP devices can make it more difficult for PnP to work, due to the large number of resources that it is not allowed to touch.

"Plug and Pray" :^)

This amusing sarcastic name for Plug and Play has become all too commonly heard these days. It refers to the large number of problems associated with getting Plug and Play to work on many systems. It's odd to consider--wasn't the whole point of Plug and Play to make it easier to configure systems? It is, but unfortunately PnP falls short of its lofty goal in many cases.

When you use PnP, you are essentially turning over control of system configuration to the PC. The problem is a common one in computers: the computer isn't as smart as the human, or more specifically, the computer isn't as "resourceful" (no pun intended. :^) ). Computers are not nearly as good as humans at realizing things like this: "Well, if I put the modem at that value and the printer there, I will have a conflict. But I can fix that by changing the assignment for the sound card, moving the modem over here, and putting the printer there". The system can take care of the simple situations, but can become confused by more complicated ones. The use of multiple "legacy" ISA devices can exacerbate this. Generally, the more complex your setup, the more likely you will need to manual "tweak" whatever PnP comes up with by default.

The biggest problems with Plug and Play revolve around its apparent "stubbornness". At times, the BIOS and operating system seem determined to put a device at a location where you do not want it. For example, you may have a modem that you want at COM3 and IRQ5, but the BIOS may decide to put it at COM4 and IRQ3, conflicting with the COM2 serial port. This can get quite aggravating to deal with. Also, some people just prefer the feeling of being "in control" that they lose when PnP is used. (I must admit to being one of these people, oftentimes.)

The problems with PnP are less common now than they were in the first year that it was announced. As with any new technology--especially one that is as complex as PnP and that involves so many parts of the system--it takes time to iron the bugs out. Most systems today work quite well with PnP. In most cases problems with PnP are due to incorrect system configuration, manual overrides of PnP devices through the Windows 95 Device Manager, or incorrect BIOS settings.

Plug-and-Play-HOWTO

http://www.tldp.org/HOWTO/Plug-and-Play-HOWTO.html