USB System Architecture

Table 1-1: Typical Legacy Interrupt Lines Used by Standard Devices

I/O Addresses

I/O address conflicts are also quite common in the PC environment. Note that peripheral devices usually require a block of I/O address locations to report status information and to issue commands to the device. While it’s true that x86 processors have the ability to access 64KB of I/O address locations (more than enough for all peripheral devices), legacy ISA expansion cards typically decode only 10 of the 16 address lines available. This yields a maximum 1KB block of address space that is usable by ISA expansion devices. Furthermore, the limited decode creates the well known aliasing effect that renders the upper 768 bytes of each aligned 1KB of I/O space unusable by other devices. See MindShare’s ISA System Architecture book, published by Addison-Wesley, for details.

Non-shareable Interfaces

Standard PC peripheral interfaces (e.g., serial and parallel connections) support the attachment of a single device. Since only one peripheral device can be attached at any given time, the flexibility of such connections is minimized. This limitation frequently leads to the costly decision of building an expansion card that plugs into an expansion bus (e.g., ISA or PCI) to create an attachment point for a new peripheral design.

End User Concerns

End users are faced with a variety of problems when connecting peripherals to their PCs. These concerns include:

•Too many connector/cable types

•System must be shut down to attach most peripherals

•System must be restarted to install/load software

•Cost

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Chapter 1: Design Goals of USB

Cable Crazed

Dedicated cables are required for the mouse, keyboard, printer, external modem, Zip drive, plotter, etc., most of which are completely different. Figure 1-2 on page 17 illustrates the backplane of a typical PC before USB. The variety of different connectors and cables required to connect particular peripheral devices is inconvenient and confusing.

Figure 1-2: Connectors at Backplane

Installation and Configuration of Expansion Cards

When peripherals are purchased, many of them require the installation of expansion cards. This, of course, may involved removing the cover of the PC, setting the switches and jumpers to configure the card, inserting the card, and replacing the cover. The trouble only begins there. Once the system is powered up the software for this device may have to be installed from diskette, which can also be a frustrating process for novice and experienced user alike.

No Hot Attachment of Peripherals

When most legacy I/O devices are attached to the system, they will not work without first restarting the system. Restarting the system is required so that the new peripheral can be detected by software. In the process, system resources must be selected and assigned to the new device (e.g., I/O space, IRQ line, and DMA channel) in order for it to work correctly and to ensure that the resource selected is not already being used by another device in the system.

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