Plug and play services

Editor's Notes

• #3: Plug-and-Play is a hardware and software standard for automatically informing software (device drivers) where it can find various pieces of hardware (devices) such as modems, network cards, video cards, etc. What this means is that Plug-and-Play's task is to pair up the physical devices with the software (device drivers) that operates them and form channels of communication from each physical device to its driver. For this to happen, Plug-and-Play assigns the following "bus-resources" to both the drivers and hardware: I/O addresses, IRQs, DMA channels (ISA bus only), and memory regions. This allocation of resources by Plug-and-Play is sometimes referred to as "configuring", but it’s only a low level form of configuring. Plug-and-Play is basically defined as the ability of a computer system to automatically configure new hardware devices. Thus, in a perfect world where Plug-and-Play works exactly as all the claims of manufacturers say, ‘you plug it in, it runs, you’re done”. A dream system where no DIP switches, jumpers, memory allocation, IRQ channel declarations or anything else needs to be acknowledged by the user, other then that he/she placed the hardware device is in there. They need to know nothing about any of the ‘magic’ that happens to make it work. Yet, Plug-and-Play seems to fail at consistently meeting all of these claims. The technology cannot be condemned and thrown out though because of a few failures in its still early life. Plug-and-Play does run into some errors in performance at this time, but the complexity of what it’s trying to perform for the user is very detailed and it can simplify some very difficult tasks for novice computer users. To truly understand this value of Plug-and-Play you must take a detailed look at the steps required in installing new hardware devices and how it performs at these steps.    
• #4: Interrupt requests (IRQ) - An IRQ, also known as a hardware interrupt, is used by the various parts of a computer to get the attention of the CPU. For example, the mouse sends an IRQ every time it is moved to let the CPU know that it's doing something. Before PCI, every hardware component needed a separate IRQ setting. But PCI manages hardware interrupts at the bus bridge, allowing it to use a single system IRQ for multiple PCI devices. Direct memory access (DMA) - This simply means that the device is configured to access system memory without consulting the CPU first. Memory addresses - Many devices are assigned a section of system memory for exclusive use by that device. This ensures that the hardware will have the needed resources to operate properly. Input/Output (I/O) configuration - This setting defines the ports used by the device for receiving and sending information.
• #5: 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. Pnp requirements: 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.
• #6: 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. You open up your computer's case and plug the sound card into an empty PCI slot on the motherboard. You close the computer's case and power up the computer. The system BIOS initiates the PnP BIOS. The PnP BIOS scans the PCI bus for hardware. It does this by sending out a signal to any device connected to the bus, asking the device who it is. The sound card responds by identifying itself. The device ID is sent back across the bus to the BIOS. The PnP BIOS checks the ESCD to see if the configuration data for the sound card is already present. Since the sound card was just installed, there is no existing ESCD record for it. The PnP BIOS assigns IRQ, DMA, memory address and I/O settings to the sound card and saves the data in the ESCD. Windows XP boots up. It checks the ESCD and the PCI bus. The operating system detects that the sound card is a new device and displays a small window telling you that Windows has found new hardware and is determining what it is. In many cases, Windows XP will identify the device, find and load the necessary drivers, and you'll be ready to go. If not, the "Found New Hardware Wizard" will open up. This will direct you to install drivers off of the disc that came with the sound card. Once the driver is installed, the device should be ready for use. Some devices may require that you restart the computer before you can use them. In our example, the sound card is immediately ready for use. You want to capture some audio from an external tape deck that you have plugged into the sound card. You set up the recording software that came with the sound card and begin to record. The audio comes into the sound card via an external audio connector. The sound card converts the analog signal to a digital signal. The digital audio data from the sound card is carried across the PCI bus to the bus controller. The controller determines which device on the PCI device has priority to send data to the CPU. It also checks to see if data is going directly to the CPU or to system memory. Since the sound card is in record mode, the bus controller assigns a high priority to the data coming from it and sends the sound card's data over the bus bridge to the system bus. The system bus saves the data in system memory. Once the recording is complete, you can decide whether the data from the sound card is saved to a hard drive or retained in memory for additional processing.   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.
• #7: When you add a new USB device to your computer system, the USB bus detects the new component and notifies the function driver of the change. The function driver notifies the Plug and Play Manager that a new device has been added to the system. The Plug and Play Manager queries the USB bus to locate the drivers for the current device tree. An Interrupt Request Packet (IRP) is sent from the Plug and Play Manager to the device stack to determine the devices that are currently on the USB bus. When the USB bus completes the IRP, it is sent back through the device stack to the Plug and Play Manager, which determines whether a new device has been added or removed. The Plug and Play Manager obtains information about the component and then begins to configure it. The Plug and Play Manager searches through the registry to determine whether the device has been previously installed on the system. If the device has not previously been attached to the system, the Plug and Play Manager updates the registry with information about the device. The Plug and Play Manager finds and loads the drivers for the device if they are present. The Plug and Play Manager assigns the necessary system resources to the device. The Plug and Play Manager sends an IRP to start the device.