INTRODUCTION

Processors need a source of input data and a destination for output data. Processors also require storage space for large volumes of intermediate data. Thus the I/O system provides these storage facilities by connecting the processor's memory to the I/O devices. These devices vary widely in latency and bandwidth; thus, a major function of the I/O system is to match their latencies and bandwidths to that of the memory system. I/O devices consist of such diverse items as keyboards, modems, and hard disks. Each of these devices has its own latency and bandwidth requirements that must be satisfied for a properly functioning system.

Blaauw and Brooks (1997) classify I/O devices with the taxonomy shown in modified form in Figure 7.1. Store devices provide ready-access storage space. System resident and database devices are usually in the virtual-address space of the memory, providing low-latency storage space. Archive stores, on the other hand, usually require human intervention when archival information is required or needs to be stored.

Source/sink devices are used as paths to/from the computer to another device and are not generally used for storage of information. Floppy disks and Zip disks can be viewed as either archive or local media. Windows98 can use the Internet Explorer as an access method for both store devices and source/sink devices, blurring the distinction of the taxonomy.